Simulation Analysis of Porthole Die Extrusion Process and Die Structure Modiﬁcations for an Aluminum Proﬁle with High Length–Width Ratio and Small Cavity

materials

Article Simulation Analysis of Porthole Die Extrusion Process and Die Structure Modiﬁcations for an Aluminum Proﬁle with High Length–Width Ratio and Small Cavity

Zhiwen Liu 1,2,* ID , Luoxing Li 2,*, Shikang Li 2, Jie Yi 2 and Guan Wang 2

1 School of Mechanical Engineering, University of South China, Hengyang 421001, China 2 State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha 410082, China; [email protected] (S.L.); [email protected] (J.Y.); [email protected] (G.W.) * Correspondence: [email protected] (Z.L.); [email protected] (L.L.); Tel.: +86-734-857-8031 (Z.L.); +86-731-88-821-571 (L.L.) Received: 26 July 2018; Accepted: 20 August 2018; Published: 23 August 2018

Abstract: The design of a porthole die is one of the key technologies for producing aluminum profiles. For an aluminum profile with high length–width ratio and small cavity, it is difficult to control the metal flow through porthole die with the same velocity to ensure the die’s strength. In the present study, the porthole die extrusion process of aluminum profile with small cavity was simulated using HyperXtrude 13.0 software based on ALE formulation. The simulation results show for the traditional design scheme, the metal flow velocity in porthole die at every stage was severely not uniform. The standard deviation of the velocity (SDV) at the die exit was 19.63 mm/s. The maximum displacement in the small mandrel was 0.0925 mm. Then, aiming at achieving a uniform flow velocity and enough die strength, three kinds of die structure modifications for the porthole die were proposed. After optimization, desired optimization results with SDV of 0.448 mm/s at the die exit and small mandrel deflection were obtained. Moreover, the temperature uniformity on the cross-section of die exit, welding pressure, and die strength were improved greatly. Finally, the optimal porthole die was verified by the real extrusion experiment. A design method for porthole die for aluminum with a high length–width ratio and small cavity was proposed, including sunken port bridges to rearrange the welding chamber in upper die, increasing the entrance angle of portholes, introducing the baffle plate, and adjusting the bearing length.

Keywords: aluminum proﬁle; porthole die; small mandrel; ALE formulation; die structure modiﬁcations; simulation

1. Introduction Aluminum alloys are ideal lightweight material for automotive, rail transportation, communication, and aerospace applications, due to their low density, high specific strength, good corrosion resistance, and good recycling ability [1,2]. With the rapid development of extrusion technology, the demand for aluminum profiles becomes more and more important due to the use of space-frame constructions in auto body and high-speed train. The design of extrusion die is one of the key technologies for producing aluminum profiles. A well-designed extrusion die should have favorable metal flow behavior in porthole die to ensure the extruded profiles with uniform flow velocity [3,4]. Otherwise, the extruded profiles can easily be subjected to distortion, thinning, or bending along the extrusion direction. In addition, if the porthole die is not well designed, the mandrel, port bridges, and die bearing maybe generate elastic or plastic deformation under

Materials 2018, 11, 1517; doi:10.3390/ma11091517 www.mdpi.com/journal/materials Materials 2018, 11, 1517 2 of 20 non-uniform pressure and metal velocity distribution, which leads to die scrap and a large size error of products. For the aluminum profiles with high length–width ratio and small cavity, it is difficult to control the metal flow through porthole die with uniform velocity owing to the complicated affecting factors, such as the die structures and process parameters. Due to the real extrusion production being carried out in the closed container and porthole die, it is practically impossible to measure the temperature, exit velocity distribution, and welding pressure which determine the profile quality. The conventional die design for hollow profiles is generally based on the expertise and practice experience of die designers or expensive plant trials. Thus, it is not easy to guarantee the quality and performance of extruded profiles. To avoid or reduce costly extrusion trials, accurate finite element (FE) simulation can be carried out to investigate the metal flow, extrusion load, and temperature distribution during the whole extrusion cycle, and many other process-related issues [5,6]. By means of FE simulations, the extrusion defects could be forecasted in advance, thereby allowing immediate modification on the die structures and process parameters before production. Kim et al. [7] investigated the non-steady state extrusion process of Al alloy tubes through 3D-FE simulation and analyzed the influence of process parameters on welding pressure, metal flow, and surface defects. Chen et al. [3] proposed a logical and effective route for designing the multi-hole porthole die. The major design steps include finding the optimal position of die orifices, reasonable design of the welding chamber levels, and adjusting the die bearings. Lee et al. [8] investigated the influence of welding chamber shape on die deformation and material flow during condenser tubes extrusion by means of numerical simulation. With the decrease of welding chamber height, the bearing deflection, and dead zone increase. Donati et al. [9] examined the effect of feeder dimension on the weld strength and distortion of extruded profiles under different processing conditions. Bigger feeder dimension allows higher extrusion speed and wider range of process parameters. Liu et al. [10] have optimized the porthole die of hollow thin-walled aluminum profiles using arbitrary Lagrangian–Eulerian formulation (ALE) algorithm. An optimization design was proposed to obtain a uniform metal flow velocity at bearing exit. Zhang et al. [11] investigated the effects of extrusion speed on metal flow, weld strength, extrusion load, etc. for a hollow aluminum profile. It was found that an optimal extrusion speed exists for exit velocity distribution. As extrusion speed increases, seam weld quality and extrusion load increase. Sun et al. [12] proposed an optimization method for the second-step chamber for a condenser tube extrusion die based on the response surface method and genetic algorithm. The optimal shape and height of the second-step chamber were obtained. Zhao et al. [13] optimized the structure parameters of baffle plates near the die orifice for aluminum profiles extrusion to balance the metal flow. Through multiple adjustments, a uniform flow velocity at die exit is obtained. Xue et al. [14] investigated the influence of key die structures such as the billet buckle angle, the porthole bevel angle, the depth of the welding chamber, and the type of bridge on the metal flow balance during multi-output porthole extrusion process through the use of thermo-mechanical modeling combined with the Taguchi method. Gagliardi et al. [15] investigated the influences of geometric variables of porthole die on the extrusion load, the maximum pressure inside the welding chamber, and the material flow homogeneity by ANOVA technique. Then, the Grey relational analysis was introduced to determine the optimal combination of geometric parameters according to specific process needs. Chen et al. [16] designed a porthole die of hollow aluminum profile based on the theory of metal plastic forming and investigated the effect of height and shape of baffle plate on the metal flow velocity in porthole die. Koopman et al. [17] presented an accurate 2D method to simulate the filling of porthole die at the startup of extrusion process based on the pseudo concentration technique. The effect of the location and velocity of flow front on the die deformation was investigated. Lee et al. [18] developed a new porthole extrusion die for improving the welding pressure in the welding chamber by using numerical analysis. Based on the above literature data, many studies have been done to investigate the metal flow behavior in porthole dies and optimize the die structures and process parameters. However, the research objects were mainly focused on the hollow profiles with simple cross-sections or large Materials 2018, 11, x FOR PEER REVIEW 3 of 20

Based on the above literature data, many studies have been done to investigate the metal flow behavior in porthole dies and optimize the die structures and process parameters. However, the researchMaterials 2018 objects, 11, 1517 were mainly focused on the hollow profiles with simple cross-sections or large3 of 20 cavities. Little research has been reported on the extrusion process or die design for aluminum profilecavities.s with Little high research length– haswidth been ratio reported and small on thecavity extrusion; as a result, process there or are die few design related for documents aluminum toprofiles be referred with highfor practical length–width production. ratioand Therefore, small cavity; in the aspresent a result, study there, the are steady few related-state porthole documents die extrusionto be referred process for practicalof a typical production. hollow profile Therefore, with the in length the present–width study, ratio theof 42.6 steady-state and a small porthole cavity die of Φextrusion 4.04 mm process was simulated of a typical using hollow HyperXtrude profile with software the length–width based on ALE ratio formulation of 42.6 and. a Firstly, small cavity the m ofetaΦl flow4.04 mm behavior was simulated in the porthole using HyperXtrude die and die software strength based for on the ALE traditional formulation. design Firstly, scheme the metal were investigated.flow behavior Then, in the aiming porthole at die achieving and die strength a unifor form flowthe traditional velocity anddesign enough scheme die were strength, investigated. three kindsThen, of aiming die structure at achieving modifications a uniform for flow the velocity porthole and die enough were proposed. die strength, After three modifications kinds of die, structure desired simulationmodifications result fors the with porthole uniform die weremetal proposed. flow velocity After at modifications, die exit and desired small diesimulation deformation results were with obtained.uniform metalBy such flow systematic velocity at study, die exit a anddesign small route die of deformation porthole die were for obtained. the aluminum By such profile systematic with highstudy, length a design–width route ratio of portholeand small die cavity for the was aluminum proposed profile to the withdie designers high length–width. ratio and small cavity was proposed to the die designers. 2. Porthole Die Design and Simulation Procedures 2. Porthole Die Design and Simulation Procedures 2.1. Traditional Design Scheme and Geometry Modeling 2.1. Traditional Design Scheme and Geometry Modeling Figure 1 shows the shape and size of cross-section for the aluminum profile with high lengthFigure–width1 shows ratio and the shapesmall cavity and sizestudied of cross-section in this work. for It ha thed aluminuma length of profile107.57 withmm in high the horizontallength–width direction ratio and and small a minimum cavity studied wall in thickness this work. of It had2.525 a lengthmm. The of 107.57 cross mm-section in theal horizontalarea was 2 454.75direction mm and2, with a minimum the ratio of wall length thickness to width of 2.525up to mm.42.6. Thus, The cross-sectional the metal flow area behavior was454.75 in porthole mm , diewith is the quite ratio comple of lengthx and to it width is a challenging up to 42.6. task Thus, to the obtain metal a flow uniform behavior velocity in porthole distribution die ison quite the crosscomplex-section and of it isdie a exit challenging. In addition, task tothe obtain profile a uniformhad a small velocity cavity distribution with diameter on the of cross-section4.04 mm at the of outerdie exit. end In of addition, profile, which the profile further had aggravates a small cavity the withdifficulty diameter of extrusion. of 4.04 mm at the outer end of profile, which further aggravates the difficulty of extrusion.

FigureFigure 1. 1. GeometricGeometric model model of of aluminum aluminum profile proﬁle with with small small cavity: cavity: ( (aa)) 3D 3D shape shape and and (b (b) )cross cross-section.-section. (unit(unit:: mm). mm).

FigureFigure 22 showsshows thethe simplifiedsimplified 3D3D geometrygeometry modelsmodels ofof portholeporthole diedie forfor thethe traditionaltraditional designdesign scheme.scheme. Four Four potholes potholes were were arranged arranged symmetrically symmetrically on the on die the surface die to surface assign to material assign reasonably. material reasonably.The outer diameter The outer of diameter the porthole of the die porthole was 250 die mm was and 250 the mm heights and ofthe upper height ands of lower upper die and were lower 102 dieand were 64 mm, 102 respectively.and 64 mm, Arespectively. mandrel was A utilizedmandrel to was form utilized the internal to form contour the internal of small contour cavity of and small was supported by the interlaced form of port bridges. The widths of port bridges in the transverse and longitudinal directions were 26 and 18 mm, respectively. A chamfering 40◦ was designed in the bottom of port bridges to decrease the metal flow resistance. The lower die was comprised of the welding Materials 2018, 11, x FOR PEER REVIEW 4 of 20 cavity and was supported by the interlaced form of port bridges. The widths of port bridges in the Materials 2018, 11, 1517 4 of 20 transverse and longitudinal directions were 26 and 18 mm, respectively. A chamfering 40° was designed in the bottom of port bridges to decrease the metal flow resistance. The lower die was chamber,comprised die of orifice,the welding die bearing, chamber, and die run-out, orifice, asdie shown bearing, in and Figure run2b.-out, The as welding shown in chamber Figure 2b. with The a heightwelding of chamber 18 mm was with utilized a height to of reweld 18 mm the wa metals utilized streams to reweld splitting the by metal port streams bridges splitting under the by effect port ofbridges high pressureunder the and effect temperature. of high pressure The die and bearing temperature. was used The to formdie bearing the outer was contour used to and form size the of profileouter contour and adjust and thesize exit of profile flow velocity and adjust of metal. the exit The flow run-out velocity near of the metal. die exit The was run designed-out near to the avoid die scratchingexit was designed the extruded to avoid profile. scratching Besides, the the extruded die orifice profile was amplified. Besides, 1.01the timesdie orifice the theoretical was amplified size to compensate1.01 times the for theoretical shrinkage duringsize to cooling.compensate for shrinkage during cooling.

FigureFigure 2.2. 3D3D modelsmodelsof of the the traditional traditional designed designed die die with with the the welding welding chamber chamber designed designed in lower in lower die: (die:a) the (a) upperthe upper die and die (andb) the (b) lower the lower die. die.

2.2. Establishment of 3D 3D-FE-FE Model There are three methods can be adoptedadopted t too simulate the porthole die extrusion by by FE m methods.ethods. The transient updated Lagrangian (UL) formulation, in which the FE meshmesheses are attached to the deforming workpiece, workpiece, is is able able to to capture capture the the material material flow flow in ina very a very intuitive intuitive way way [19] [.19 However,]. However, the themesh mesheses are areprone prone to distortion to distortion in inlarge large deformation deformation regions regions and and the the local local remeshing remeshing is is inevitable, inevitable, which makesmakes it incompatibleit incompatible with with the demand the demand of the efficient of the efficien and accuracy.t and Besidesaccuracy. for Besides UL formulation, for UL theformulation, weld problem the weld between problem adjacent between metal adjacent streams metal is difficult streams to is solve. difficult The to steady solve state. The Eulerian steady state (SS) formulationEulerian (SS [)20 formulation], where the FE[20 meshes], where are the fixed FE inmesh space,es isare fast fixed but the in thermalspace, is mechanical fast but the stationarity thermal maymechanical not be well stationarity established may actually not be and well cannot establi provideshed actual any transiently and cannot information. provide The any key transient idea of ALEinformation. formulation The iskey to idea separate of ALE the formulation material and is mesh to separate displacements the material [21], and thus mesh the mesh displacements distortion and[21], frequent thus the remeshing mesh distortion in UL formulation and frequent is avoided. remeshing Also, in ULthe shortcomingformulation ofis SSavoid formulationed. Also, can the alsoshortcoming be eliminated of SS formulation since the procedure can also be is essentiallyeliminated since incremental. the procedure In recent is essentially years, the incremental. application ofIn ALE recent formulation years, the in application the simulation of ALE of portholeformulation die in extrusion the simulation process hasof porthol been usede die even extrusion more extensivelyprocess has [been10–14 used]. Therefore, even more in this extensively study, the [10 3D-FE–14]. model Therefore, for simulating in this study, the porthole the 3D-FE die model extrusion for processsimulating of aluminumthe porthole profile die with extrusion small cavity process was of established aluminum using profile HyperXtrude with small software-based cavity was ALEestablish formulation.ed using The HyperXtrude computational software domains-based of theALE metal form throughulation the. The container computational and porthole domain die wass of builtthe metal first bythrough CATIA the V5R20 container software, and then porthole input intodie thewas HyperXtrude built first by software CATIA inV5R20 STEP software, format. Before then meshinginput into the the flow HyperXtrude domains, it issoftware necessary in to STEP manually format. clean Before up the meshing tiny entities the flow of geometric domains model, it is thatnecessary influence to manually seriously clean the sizeup t andhe tiny quality entities of mesh of geometric generating. model Figure that 3influence shows the seriously 3D-FE modelsthe size forand the quality flow of domains, mesh generating. and upper Figure and lower 3 shows dies the with 3D initial-FE model elements.s for the For flow convenience domains, of and meshing upper and establishinglower dies with the boundary initial elements. conditions, For convenience the metal through of meshing extrusion and tools establishing was divided the boun intodary five differentcondition components.s, the metal The through pentahedron extrusion mesh tools type was was divided used in into the five components different of components. extruded profile The andpentahedron die bearing, mesh while type thewas tetrahedron used in the meshcomponent type wass of adoptedextruded inprofile other and components. die bearing, To while improve the thetetrahedron simulation mesh accuracy type wa ands adopted save computer in other source, components. the five T componentso improve the were simulation allocated accuracy for different and meshsave computer sizes according source, to the the five amount component of locals deformation. were allocated The for mesh different size in mesh the component sizes according near the to the die orificeamount which of local suffers deformation. a severe shear The deformationmesh size in was the setcomponent to be 0.5 mmnear to the have die at orifice least four which nodes suffers in the a profilesevere shear thickness, deformation while the wa meshs set sizeto be in 0.5 the mm welding to have chamber, at least portholes, four nodes and in billetthe profile were aboutthickness, 2, 5, andwhile 20 the mm, mesh respectively. size in the Moreover, welding in chamber, order to analyze portholes, the dieand strength, billet were the portholeabout 2, die5, and also 20 need mm, to berespectively. meshed, and Moreover, tetrahedron in element order to type analyze was used.the die The strength, mesh sizes the of portholethe upper die and also lower need dies to were be about 0.5–10 mm. The deformation analysis of the dies was carried out by using HyperXtrude solver’s Materials 2018, 11, x FOR PEER REVIEW 5 of 20

Materialsmeshed,2018 and, 11 ,tetrahedron 1517 element type was used. The mesh sizes of the upper and lower dies5 were of 20 about 0.5–10 mm. The deformation analysis of the dies was carried out by using HyperXtrude solver’s coupled flow, thermal, and stress analysis. The location at the outer diameter of porthole coupleddie and flow,die exposed thermal, region and stress were analysis. specified The with location X, Y, atZ displacement the outer diameter = 0, while of porthole in other die region and die of exposedporthole region die X, were Y, Z specified traction with= 0 is X, specified Y, Z displacement to allow =the 0, die while to infreely other move. region The of porthole total number die X, Y,of Zelements traction was = 0 isabout specified 800,000. toallow After themodeling, die tofreely the FE move. simulation The totals were number performed of elements in the DELL wasabout T7610 800,000.workstation. After modeling, the FE simulations were performed in the DELL T7610 workstation.

Figure 3. Mesh generation of (a) the ﬂow domains, (b) upper die, and (c) lower die in FE model. Figure 3. Mesh generation of (a) the flow domains, (b) upper die, and (c) lower die in FE model.

InIn FE FE simulations, simulations aluminum, aluminum alloy alloy AA6063 AA6063 and and H13 H13 steel steel were were utilized utilized to material to material for the for billet the andbillet porthole and porthole die, respectively. die, respectively. The physicalThe physical properties properties of AA6063 of AA6063 and and H13 H13 are are listed listed in Tablein Table1. The1. The H13 H13 steel steel has high has hardness,high hardness thermal, thermal stability, stability, and good and abrasion good resistance, abrasion resistance, which can withstandwhich can towithstand be repeatedly to be heated repeatedly and cooled.heated Toand simulate cooled. the To metalsimulate flow the behavior metal flow and analyzebehavior the and die analyze deflection, the thedie billetdeflection, was defined the billet as thermoplasticwas defined as material thermoplastic and the material porthole an died the was porthole defined die as elastic was defined material. as Theelastic Sellars–Tegart material. The model Sellars was–Tegart implemented model was in FE implemented simulations toin describeFE simulation the flows to stress describe behavior the flow of AA6063 aluminum alloy. For this model, the flow stress σ is given by stress behavior of AA6063 aluminum alloy. For this model, the flow stress  is given by 1 1 1 Z n n σ = sinh,1 −1 1 Z , (1)  β sinh A  , (1)   A  where β, A, n are the material parameters, which is independent of temperature and can be determined where β, A, n are the material parameters, which is independent of temperature and can be by fitting to the flow stress dates, Z is the Zener–Hollomon parameter, expressed by determined by fitting to the flow stress dates, Z is the Zener–Hollomon parameter, expressed by . Q Z = εe RT ,Q (2) RT (2) Z  e , . where R is the universal gas constant, T is the absolute temperature, ε is the effective strain rate, where R is the universal gas constant, T is the absolute temperature,  is the effective strain rate, and Q is the activation energy for deformation. Hot compression experiments were performed and Q is the activation energy for deformation. Hot compression experiments were performed using a Gleeble-1500 thermomechanical simulator to obtain stress–strain curves over a temperature using a Gleeble-1500 thermomechanical simulator to obtain stress–strain curves over a temperature range from 400 to 520 ◦C and strain rate range from 0.01 to 10 s–1 with a true strain of 1 (Figure4). range from 400 to 520 °C and strain rate range from 0.01 to 10 s–1 with a true strain of 1 (Figure 4). Those flow stress dates obtained were then corrected for flow softening due to deformation heating Those flow stress dates obtained were then corrected for flow softening due to deformation heating during hot compression tests, and then were used to determine the material parameters. For AA6063 during hot compression tests, and then were used to determine the material parameters. For aluminum alloy used in the present simulations, the parameters in Equations (1) and (2) were as AA6063 aluminum alloy used in the present simulations, the parameters in Equations (1) and (2) follows: Q = 1.71 × 105 J/mol, A = 1.18 × 109 s–1, β = 3.66 × 10–8 m2/N, n = 7.89, R = 8314 J/(mol·k). were as follows: Q = 1.71 × 105 J/mol, A = 1.18 × 109 s–1, β = 3.66 × 10–8 m2/N, n = 7.89, R = 8314 J/(mol·k). Table 1. Physical properties of AA6063 aluminum alloy and AISI H13 steel.

PhysicalTable Properties 1. Physical properties of AA AA60636063 aluminum Aluminum alloy Alloy and AISI H13 AISI steel H13. Steel DensityPhysical (Kg/m Properties3) AA60632700 Aluminum Alloy AISI 7870 H13 Steel Young’sDensity modulus (Kg/m (MPa)3) 68,9002700 210,0007870 Poisson’s ratio 0.3 0.33 Young’s modulus (MPa) 68,900 210,000 Thermal conductivity (W/(m·K)) 198 24.3 SpeciﬁcPoisso heat (J/(kgn’s ratio·K)) 9000.3 4600.33 ThermalThermal expansion conductivity coefﬁcient (W/(m (1/K) ·K)) 1.0 × 10–5 198 - 24.3 Specific heat (J/(kg·K)) 900 460

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Thermal expansion coefficient (1/K) 1.0 × 10–5 － Materials 2018, 11, 1517 6 of 20

90 90 (a) 0.01 s-1 (b) 520 ℃ 80 80 480 ℃ 0.1 s-1 70 440 ℃ 70 1 s-1 400 ℃ 60 10 s-1 60

50 50

40 40

30 Stress (MPa)

Stress (MPa)

20 20

10 10

0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Strain Strain

FigFigureure 4. 4. TrueTrue stress stress–strain–strain curves curves of of AA6063 AA6063 aluminum aluminum alloy alloy:: ( (aa)) at at different different strain strain rate ratess in in 4 48080 °C;◦C; –1. ((b)) at different temperature temperaturess in strain rate 1 s–1 .

The process parameters and thermal boundary conditions used in simulations were identical The process parameters and thermal boundary conditions used in simulations were identical to to those in actual extrusion production, as shown in Table 2. The diameter of container was 210 mm those in actual extrusion production, as shown in Table2. The diameter of container was 210 mm and and the extrusion ratio (area of container/area of profile) was 75.8, which belongs to the range of the extrusion ratio (area of container/area of profile) was 75.8, which belongs to the range of large large extrusion ratio. The temperature of billet was 480 °C and the temperature of porthole die was extrusion ratio. The temperature of billet was 480 ◦C and the temperature of porthole die was 50 ◦C 50 °C lower than that of billet. The speed of extrusion ram was 2 mm/s. To obtain accurate lower than that of billet. The speed of extrusion ram was 2 mm/s. To obtain accurate simulation simulation results, reasonable friction and heat transfer conditions at the billet/tools interfaces results, reasonable friction and heat transfer conditions at the billet/tools interfaces should be adopted. should be adopted. The heat transfer coefficient between the billet and porthole die was set to be The heat transfer coefficient between the billet and porthole die was set to be 3000 W/m2·◦C[22,23]. 3000 W/m2·°C [22,23]. When the extrusion temperature was greater than 400 °C, the friction type at When the extrusion temperature was greater than 400 ◦C, the friction type at the interface between the the interface between the billet and porthole dies (except die bearing) was considered to be sticking billet and porthole dies (except die bearing) was considered to be sticking friction [24]. However, at the friction [24]. However, at the interface between the billet and die bearing was defined as sliding interface between the billet and die bearing was defined as sliding friction and the friction coefficient friction and the friction coefficient was set to be 0.3 [7]. was set to be 0.3 [7]. Table 2. Process parameters used in simulation. Table 2. Process parameters used in simulation. Conditions Values BilletConditions diameter (mm) Values210 BilletBillet diameter length (mm)(mm) 210350 BilletExtrusion length (mm) ratio 75.8 350 Extrusion ratio 75.8 Extrusion speed (mm/s) 2 Extrusion speed (mm/s) 2 BilletBillet temperaturetemperature (◦ C)(°C) 480480 ContainerContainer andand die die temperature temperature (◦C) (°C) 430430 FrictionFriction coefficient coefficient atat billet/container billet/container and and die die Sticking Sticking FrictionFriction coefficient coefficient at at billet/die billet/die bearing bearing 0.30.3 Heat thermal coefficient between billet/container and die (W/(m2·◦C)) 3000 Heat thermal coefficient between billet/container and die 3000 (W/(m2.°C )) 3. FE Simulation of Traditional Die Design 3. FE TheSimulation principal of design Traditiona goal ofl Die porthole Design die is to obtain a uniform exit velocity and high die strength. At theThe condition principal of non-uniform design goal metal of porthole flow, the die billet is to is obtainextruded a uniformout from exitthe die velocity orifice and with high different die strength.velocities At throughout the condition its cross-section, of non-uniform resulting metal flow, in the the defects billet ofis profileextruded such out as from bending, the die twisting, orifice withthickness different reduction, velocities etc. Thus, throughout the exit velocity its cross uniformity-section, resulting should be in highly the defects valued by of the profile die designers. such as bending,To evaluate twisting, quantitatively thickness the reduction, velocity distribution etc. Thus, the of profile exit velocity at die exit,uniformity the standard should deviation be highly of valuedvelocity by (SDV) the die was designer introduced,s. To evaluate which is definedquantitatively by the velocity distribution of profile at die exit, the standard deviation of velocity (SDV) was introduced,v which is defined by u 2 u ∑ (vi − v) t = SDV = i 1 , (3) n

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2 (vi  v) i1 (3) SDV  , n Materials 2018, 11, 1517 7 of 20 where v is the normal velocity for node i , v is the average velocity, and n is the nodes i number. In order to obtain a more actual velocity distribution, the velocity values of all nodes on the where vi is the normal velocity for node i, v is the average velocity, and n is the nodes number. In order crossto obtain-section a more of extruded actual velocity profile distribution, were extracted. the velocity The smaller values ofthe all value nodes of on SDV, the cross-section the better the of extruded quality profile wereis. extracted. The smaller the value of SDV, the better the extruded quality is. The deformation and exitexit velocityvelocity distributiondistribution ofof extruded extruded profile profile for for the the traditional traditional die die design design is isshown shown in Figure in Figure5. It 5 is. clearly It is clear thatly the that metal the flow metal velocity flow velocity on the cross on the section cross of section die exit of is dieextremely exit is extremelynonuniform. nonuniform. From the solid From end the of solid the profileend of the (See profile region (See 1) to region the small 1) to cavity the small (See cavity region (See 2), the region flow 2),velocity the flow of material velocity gradually of material decreases. gradually The decreases. maximum The velocity maximum in Regionvelocity 1 in is Region 182.3 mm/s 1 is 182.3 and mm/sminimum and velocity minimum in velocity Region 2 in is Region only 111.0 2 is mm/s, only 111.0 which mm/s, deviates which seriously deviates fromseriously the theoretical from the theoreticalvalue of 151.8 value mm/s. of 151.8 The mm/s. calculated The calculated value of SDV value reaches of SDV 19.63 reaches mm/s, 19.63 which mm/s, indicates which aindicates very poor a vvelocityery poor distribution. velocity distribution. Owing to theOwing large to nonuniformity the large nonuniformity of exit velocity of exit distribution, velocity distribution, the region with the regionhigh velocity with high continuously velocity continuously squeezes that squeezes with low velocity,that with consequently low velocity, causing consequently the extruded causing profile the extrudeddistorted profile or bent distorted along its length.or bent along its length.

FigFigureure 5 5.. VelocityVelocity distribution in the extrud extrudeded profile proﬁle for the traditional die design.

Figure6 6shows shows the the velocity velocity distributions distributions of four of sections four sections extracted extracted from the from entrance the of entrance portholes of portholesto the bottom to theof welding bottom chamber of welding along chamber the extrusion along direction, the extrusion which reflects direction, the material which reflects flow status the materialin porthole flow die status during in the porthole whole die extrusion during process. the whole Figure extrusion6a shows process. the metal Figure flow 6 statea shows at the the inlet metal of flowfour portholes.state at the Theinlet contours of four portholes. drawn in theThe figures contours show drawn the in velocities the figure scalars show values the onvelocities the specified scalar valuessurfaces on perpendicular the specified tosurfaces the extrusion perpendi direction.cular to Bluethe extrusion and red represent direction. the Blue minimum and red and represent maximum the minimumvelocity values, and maximum respectively. velocity The same values, as below.respectively. It is clear The from same Figure as below.6a that It theis clear metal from flow Figure faster in6a thatPortholes the metal 3 and flow 4 than faster in in Portholes Portholes 1 and3 and 2, 4 which than in causes Portholes a large 1 and velocity 2, which difference causes ina large the Regions velocity 1 differenceand 2 (See in Figure the Regions5). In addition, 1 and 2 (See the metalFigure in 5 the). In center addition, of every the metal porthole in the moves center faster of every than porthole that at movesthe periphery. faster than This that is mainly at the periphery. because of This an intense is mainly sticking because friction of an on intense the portholes sticking wall.friction Figure on the6b portholesshows the wall. material Figure flow 6b stateshows in the four mater portholes.ial flow By state comparison in four portholes. with Figure By comparison6a, a reducing with velocity Figure 6gradienta, a reducing in each velocity porthole gradient is observed, in each which porthole is also is due observed, to the increasing which is also friction due resistance to the increasing between frictionthe billet resistance and portholes. between The intense the billet friction and on portholes. the porthole The wall intense prevents friction the billet on the skin porthole from entering wall ptherevents extruded the billet profiles. skin Figurefrom entering6c shows the the extrud materialed profiles flow state. Figure at the 6c entrance shows the of material welding flow chamber. state atThere the areentrance four dead of welding metal zones chamber. under There the port are bridgesfour dead where metal the zones velocities under are the nearly port zero.bridges Figure where6d theshows velocities the velocity are nearly distribution zero. Figure in the 6 mid-heightd shows the of veloci weldingty distribution chamber.The in the welding mid-height chamber of welding is filled chamber.by the barreling The welding legs, and chamber the two is adjacent filled by metal the streamsbarreling rejoin legs, and and begin the two welded adjacent each other.metal Figure streams6e rejoinshows and the begin metal welded flow state each at theother. inlet Figure of the 6 diee shows orifice. the The metal metal flow of state welding at the chamber inlet of flowsthe die through orifice. Thethe diemeta bearingl of welding to form chamber the final flows shape through of the hollow the die profile. bearing Once to form the metal the final extrudes shape through of the hollow the die profile.bearing, Once the flow the metal velocity extrudes reaches through maximum the during die bearing, the whole the flow extrusion velocity cycle. reaches The smallmaximum cavity during region thewith whole relatively extrusion lower cycle. velocity The compared small cavity to theregion other with regions relatively is the lower result velocity of the additional compared friction to the otherresistance regions between is thethe result small of mandrelthe additional and metal, friction which resistance leads to between a nonuniform the small velocity mandrel distribution and metal, on the cross-section of die exit and a warped profile, as shown above in Figure5.

Materials 2018, 11, x FOR PEER REVIEW 8 of 20

Materialswhich leads2018, 11 to, 1517 a nonuniform velocity distribution on the cross-section of die exit and a warped8 of 20 profile, as shown above in Figure 5.

(a) (b) (c)

(d) (e)

FigureFigure 6.6. MetalMetal flowflow atat differentdifferent stages stages during during the the whole whole extrusion extrusion process: process: (a )(a velocity) velocity distribution distribution at theat the entrance entrance of the of portholes; the portholes; (b) velocity (b) velocity distribution distribution in the portholes,in the portholes (c) metal, (c flowing) metal the flowing welding the chamber,welding ( cdhamber,) metal flowing (d) metal in the flowing mid-height in the welding mid-height chamber, welding and (e ) chamber, velocity distributionand (e) velocity at the entrancedistribution of the at the die entrance orifice. of the die orifice.

In billet-on-billet continuous extrusion, the porthole die usually works under high temperature, In billet-on-billet continuous extrusion, the porthole die usually works under high temperature, pressure and friction conditions, which easily causes porthole die elastic deformation or cracking. pressure and friction conditions, which easily causes porthole die elastic deformation or cracking. The deformation of mandrel and orifice seriously influences the accuracy and dimension of The deformation of mandrel and orifice seriously influences the accuracy and dimension of extruded extruded profiles and the die life. By analyzing the die strength, the unreasonable design of porthole profiles and the die life. By analyzing the die strength, the unreasonable design of porthole die could die could be found in advance and unnecessary waste of die is avoidable. In aspect of the aluminum be found in advance and unnecessary waste of die is avoidable. In aspect of the aluminum profile profile with high length–width ratio and small cavity, for the upper die, strength checking mainly with high length–width ratio and small cavity, for the upper die, strength checking mainly focuses on focuses on the small mandrel and port bridge, which are usually considered as simply supported the small mandrel and port bridge, which are usually considered as simply supported girder beams. girder beams. The mandrel with a diameter of 4.04 mm is especially liable to deflection and break The mandrel with a diameter of 4.04 mm is especially liable to deflection and break during extrusion. during extrusion. Therefore, mandrel strength is one of the important factors to produce high Therefore, mandrel strength is one of the important factors to produce high accuracy profiles. For the accuracy profiles. For the lower die, there exists long cantilever beams in which deformation also lower die, there exists long cantilever beams in which deformation also occurs easily. The displacement occurs easily. The displacement distributions of the upper and lower dies for the initial design distributions of the upper and lower dies for the initial design scheme are shown in Figure7. It is scheme are shown in Figure 7. It is clear that maximum displacement in the upper die is 0.107 mm, clear that maximum displacement in the upper die is 0.107 mm, which lies at the intersection of port which lies at the intersection of port bridges. During extrusion process, the port bridges directly bridges. During extrusion process, the port bridges directly suffer higher compressive stress, which suffer higher compressive stress, which may lead to the port bridges collapse along the extrusion may lead to the port bridges collapse along the extrusion direction. The displacement in the end of the direction. The displacement in the end of the small mandrel is up to 0.0925 mm. It can be explained small mandrel is up to 0.0925 mm. It can be explained as follows: first, the small mandrel itself has bad as follows: first, the small mandrel itself has bad rigidity and is easy to distortion. Second, there is rigidity and is easy to distortion. Second, there is lager pressure difference in radial and axial direction lager pressure difference in radial and axial direction of small mandrel due to the nonuniform metal of small mandrel due to the nonuniform metal flow (See Figure6). Deflection of the mandrel seriously flow (See Figure 6). Deflection of the mandrel seriously affected the size accuracy of small cavity. affected the size accuracy of small cavity. Besides, the small mandrel is exposed to the outside of Besides, the small mandrel is exposed to the outside of upper die and 26 mm higher than the die upper die and 26 mm higher than the die surface. During splitting or transporting, the small mandrel surface. During splitting or transporting, the small mandrel can easily deflect and break. As a result, can easily deflect and break. As a result, we did not use it for the extrusion die. For the lower die, we did not use it for the extrusion die. For the lower die, the maximum displacement is 0.0245 mm, the maximum displacement is 0.0245 mm, which is located at the pier. which is locatedMaterials 2018 at, 11the, x FORpier. PEER REVIEW 9 of 20 In order to improve the quality of profile and mandrel strength, the conventional die structure should be optimized. So it is crucial to find an appropriate way of porthole die optimization for balancing the metal flow and reducing mandrel deflection.

(a) (b)

Figure 7. Displacement of the traditional designed die: (a) upper die and (b) lower die. Figure 7. Displacement of the traditional designed die: (a) upper die and (b) lower die. 4. Redesign of Porthole Die In the extrusion die design, there are many means to balance the metal flow in porthole die, such as by rationally designing the shape and position of portholes. Metal supplies can be balanced by adjusting the welding chamber height and welding angle; the metal flow resistance in porthole die can be controlled by adjusting the bearing length; the metal flow uniformity at the die exit can be improved greatly. The mandrel strength is mainly affected by the size of mandrel, the area of support surface, pressure in the welding chamber, pressure difference around mandrel, etc. The entrance angle of portholes, and the length and width of port bridges directly influence the strength of the bridges. In this study, the modification schemes of porthole die were suggested as follows: Firstly, increasing the guiding angle in the entrance of portholes and adjusting the welding chamber from the lower die to the upper die. Then, the baffle plates are adopted in the welding chamber to encourage more metal to flow into the small cavity region. Finally, the bearing lengths are adjusted reasonably to obtain a more uniform exit velocity of profile.

4.1. First Step: Rearranging the Welding Chamber in Upper Die In the initial die design, the welding chamber was designed to in the lower die and the height of the welding chamber was 18 mm. With the rising of welding chamber height, the pressure around the mandrel increases, which leads to that the small mandrel is more easily deflected [7]. To improve the strength of upper die and prevent the mandrel from breaking in transporting or splitting, the welding chamber is redesigned to in the upper die. The port bridges were sunk with a value of 24 mm. There, the welding chamber height was decreased to 15 mm. On the other hand, the entrance angle of portholes was designed to 30° for guiding the material to easily flow into the portholes, resulting in decreasing the supporting force of port bridges and improving the die strength. Figure 8 shows the 3D geometric model of modified porthole die. Keeping the same process parameters to the initial extrusion process, the FE simulation was carried out with the modified porthole die. Figure 9 shows the displacement distributions of the upper and lower dies. In contrast to the simulation results of the initial porthole die, the die deflection is reduced significantly. The maximum displacement in the port bridges is decreased from 0.107 to 0.0655 mm, and the displacement of small mandrel is decreased from 0.0925 to 0.0518 mm. The maximum displacement of the lower die is located in the die orifice, the value is 0.00768 mm. Figure 10 shows the metal flow velocity distribution at the die exit obtained by the first modification scheme. In comparison with the initial die design, the maximum velocity is decreased from 182.3 to 172.8 mm/s, and the minimum velocity is increased from 111.0 to 114.4 mm/s. The SDV is decreased from 19.63 to 16.148 mm/s. However, there is still a large difference in metal flow velocity between the small cavity and other regions, which leads to the profile being bent and warped. Thus, it is necessary to take further measures to modify the die structure and balance the exit velocity of the profile.

Materials 2018, 11, 1517 9 of 20

In order to improve the quality of profile and mandrel strength, the conventional die structure should be optimized. So it is crucial to find an appropriate way of porthole die optimization for balancing the metal flow and reducing mandrel deflection.

4. Redesign of Porthole Die In the extrusion die design, there are many means to balance the metal flow in porthole die, such as by rationally designing the shape and position of portholes. Metal supplies can be balanced by adjusting the welding chamber height and welding angle; the metal flow resistance in porthole die can be controlled by adjusting the bearing length; the metal flow uniformity at the die exit can be improved greatly. The mandrel strength is mainly affected by the size of mandrel, the area of support surface, pressure in the welding chamber, pressure difference around mandrel, etc. The entrance angle of portholes, and the length and width of port bridges directly influence the strength of the bridges. In this study, the modification schemes of porthole die were suggested as follows: Firstly, increasing the guiding angle in the entrance of portholes and adjusting the welding chamber from the lower die to the upper die. Then, the baffle plates are adopted in the welding chamber to encourage more metal to flow into the small cavity region. Finally, the bearing lengths are adjusted reasonably to obtain a more uniform exit velocity of profile.

4.1. First Step: Rearranging the Welding Chamber in Upper Die In the initial die design, the welding chamber was designed to in the lower die and the height of the welding chamber was 18 mm. With the rising of welding chamber height, the pressure around the mandrel increases, which leads to that the small mandrel is more easily deflected [7]. To improve the strength of upper die and prevent the mandrel from breaking in transporting or splitting, the welding chamber is redesigned to in the upper die. The port bridges were sunk with a value of 24 mm. There, the welding chamber height was decreased to 15 mm. On the other hand, the entrance angle of portholes was designed to 30◦ for guiding the material to easily flow into the portholes, resulting in decreasing the supporting force of port bridges and improving the die strength. Figure8 shows the 3D geometric model of modified porthole die. Keeping the same process parameters to the initial extrusion process, the FE simulation was carried out with the modified porthole die. Figure9 shows the displacement distributions of the upper and lower dies. In contrast to the simulation results of the initial porthole die, the die deflection is reduced significantly. The maximum displacement in the port bridges is decreased from 0.107 to 0.0655 mm, and the displacement of small mandrel is decreased from 0.0925 to 0.0518 mm. The maximum displacement of the lower die is located in the die orifice, the value is 0.00768 mm. Figure 10 shows the metal flow velocity distribution at the die exit obtained by the first modification scheme. In comparison with the initial die design, the maximum velocity is decreased from 182.3 to 172.8 mm/s, and the minimum velocity is increased from 111.0 to 114.4 mm/s. The SDV is decreased from 19.63 to 16.148 mm/s. However, there is still a large difference in metal flow velocity between the small cavity and other regions, which leads to the profile being bent and warped. Thus, it is necessary to take further measures to modify the die structure and balance the exit velocity of the profile. Materials 2018, 11, 1517 10 of 20 MaterialsMaterials 20182018,,, 1111,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 1010 ofof 2020

FigFigureFigureure 88.8... GeometricGeometric model model of of porthole porthole diedie forfor thethe firstﬁrstfirst modiﬁcationmodificationmodification scheme.scheme.

FigureFigFigureure 9.99... DisplacementDisplacement distribution distribution of of the the first ﬁrstfirst modification modiﬁcationmodification scheme: scheme: ( (aa))) thethe the upper upperupper die diedie and andand (((bb))) thethethe lowerlowerlower die.die.die.

FigFigureFigureure 10 10.10... VelocityVelocity distributiondistribution inin thethe eextrudedextrudxtrudeded profileprofileprofile forforfor thethethe first firstfirstfirst modification modificationmodificationmodification scheme. scheme.scheme.scheme.

4.2.4.2. Second Second Step: Step: Introducing Introducing the the Baffle BaffleBaffle Plates Plates BasedBased on on thethe FEFE resultsresults ofof firstfirstfirst modificationmodificationmodification scheme,scheme, the the metal metal flow flowflow velocity velocity in in the the part part with with smallsmall cavitycavitycavity isis stillstillstill relativelyrelativelyrelatively low,low,low, farfarfar lesslessless thanthanthan thethethe meanmeanmean velocityvvelocityelocity at atat a aa value valuevalue of ofof 151.8 151.8151.8 mm/s. mm/s.mm/s. Thus, additionaladditional baffle bafflebaffle plates plates onon thethe the edgeedge edge ofof of thethe the diedie die orificeorifice orifice areare are introducedintroduced introduced toto to increaseincrease increase thethe the flowflow flow resistanceresistance resistance inin inthethethe the regionsregionsregions regions ininin in whichwhichwhich which mmm metaletaletal flowsflows flows fasterfaster faster thanthan than thethe the smallsmall small cavitycavity cavity region.region. region. FigureFigure Figure 111111 showsshows thethe locationlocationlocation andand shape shape shape of o off introducing introducingintroducingintroducing the the the the baffle bafflebaffle plates. plates. plates. Baffle Baffle Baffle plates plates plates play play play an important an an important important role for role role balancing forfor balancing balancing the metal the the flowmetalmetal in flowflow the weldinginin thethe weldingwelding chamber. chamber.chamber. The main TheThe principle mainmain principleprinciple of baffle ofof plates bafflebaffle design platesplates is: designdesign where isis the::: wherewherewhere metal thethethe flow metalmetalmetal out theflowflow die outout orifice the the die die is fast, orifice orifice a high isis fast, fast, baffle a a high high plate baffle baffle should plate plate be applied; should should be inbe contrast, applie applied;d; whereinin contrast, contrast, the flow where where resistance the the flow flow is large,resistanceresistance no baffle is is large, large, plate no no or a baffle baffle low one plate plate is used or or a a [ low13 low]. The one one location is is used used of[[1313 baffle]]... TheThe plates location location is an of of important baffle baffle plates plates factor is is that an an affectsimportantimportantimportant the velocity factor factor factor that that that distribution affects affects affects the the the of extruded velocity velocity velocity profile distribution distribution distribution at die ofexit, of of extruded extruded extruded which is profile profile profileparticularly at at at die die die discussed exit, exit, exit, which which which in this is is is study.particularlyparticularly Figure discusseddiscussed 12 shows in thein tthishis detailed study.study. dimensions FigureFigure 1122 showsshows of baffle thethe plates detaileddetailed in the dimensionsdimensions welding chamber. ofof bafflebaffle The platesplates height inin thethe of baffleweldingwelding plates chamber.chamber. was 4 TheThe mm heightheight and the ofof distancebafflebaffle platesplates from waswas the 44 side mmmm edge andand thethe of baffle distancedistance plates fromfrom to thethe die sideside orifice edgeedge was ofof 1bafflebaffle mm. Inplatesplates the parttoto diedie with orificeorifice small waswas cavity, 11 mm.mm. the lengthInIn thethe ofpartpart baffle withwith plates smallsmall to cavity, thecavity, die centerthethe lengthlength was 35.71ofof baffbaff mm.lelele platesplatesplates In the tototo solid thethethe end, diediedie centercenter waswas 35.7135.71 mm.mm. InIn thethe solidsolid end,end, thethe distancedistance betweenbetween thethe bafflebaffle platesplates andand thethe diedie centercenter isis describeddescribed as as the the variable variable dd... The The The distance distance distance was was was adjusted adjusted adjusted from from from 55 55 55 to to to 65 65 65 mm, mm, mm, and and and the the the relevant relevant relevant FE FE FE

Materials 2018, 11, 1517 11 of 20

the distance between the bafﬂe plates and the die center is described as the variable d. The distance Materials 2018, 11, x FOR PEER REVIEW 11 of 20 wasMaterials adjusted 2018, 11 from, x FOR 55 PEER to 65 REVIEW mm, and the relevant FE analyses were carried out. The meshing principle,11 of 20 analysesboundary were conditions carried out. and The bearings meshing lengths principle, in FE simulationsboundary conditions kept consistency and bearings in each lengths case to in make FE analyses were carried out. The meshing principle, boundary conditions and bearings lengths in FE simulationseffective comparisons. kept consistency in each case to make effective comparisons. simulations kept consistency in each case to make effective comparisons.

FigFigureure 11 11.. LocationLocation and and shape shape of of introducing introducing the the baffle bafﬂe plates. plates. Figure 11. Location and shape of introducing the baffle plates.

Figure 12. Detailed dimensions of baffle plates in the welding chamber (mm). FigureFigure 12.12. DetailedDetailed dimensionsdimensions ofof bafflebaffle platesplates inin thethe weldingwelding chamberchamber (mm).(mm). The distortion and velocity distribution of extruded profiles under different distances d is TheThe distortion distortion and and velocity velocity distribution distribution of extruded of extruded profiles profiles under under different different distances distanced is showns d is shown in Figure 13. With the varying position of baffle plates, a considerable velocity difference is shown in Figure 13. With the varying position of baffle plates, a considerable velocity difference is observedin Figure at 13 die. With exit. the When varying d is position55 mm, ofthe baffle metal plates, flow avelocity considerable in the velocitysolid end difference is obviously is observed faster atobserved die exit. at When die exit.d is 55When mm, d the is metal55 mm, flow the velocity metal flow in the velocity solid end in isthe obviously solid end faster is obviously than part faster with than part with small cavity. A severe bending deformation occurs in the solid end. When d increases than part with small cavity. A severe bending deformation occurs in the solidd end. When d increases tosmall 57.5 cavity. mm, a A uniform severe exitbending velocity deformation is obtained occurs in the in extrudatethe solid end. and When little distortionincreases is to appeared. 57.5 mm, to 57.5 mm, a uniform exit velocity is obtained in the extrudate and little distortion is appeared. However,a uniform the exit small velocity cavity is obtained has a relatively in the extrudate larger velocity and little comparing distortion iswith appeared. the solid However, end in thefurther small However, the small cavity has a relatively larger velocity comparing with the solid end in further increasingcavity has the a relatively value of d larger. Table velocity 3 shows comparing the comparison with the of maximum solid end in and further minimum increasing velocities, the value SDVs, of dincreasing the value of d. Table 3 shows the comparison of maximum and minimum velocities, SDVs, and. Table displacement3 shows theof extrudate comparison with of maximumdifferent positions and minimum of baffle velocities, plates. It SDVs,is clear and that displacement the maximum of and displacement of extrudate with different positions of baffle plates. It is clear that the maximum andextrudate minimum with velocitiesdifferent positions for the ofCase baffle 2 areplates. 157.7 It is and clear 149.9 that the mm/s, maximum respectively. and minimum The maximum velocities and minimum velocities for the Case 2 are 157.7 and 149.9 mm/s, respectively. The maximum velocityfor the Casedifference 2 are 157.7on the and cross 149.9 section mm/s, of respectively. profile is 7.8 The mm/s. maximum The calculated velocity difference SDV is 3.656 on the mm/s, cross velocity difference on the cross section of profile is 7.8 mm/s. The calculated SDV is 3.656 mm/s, indicatingsection of thatprofile the is mater 7.8 mm/s.ial flow The in calculated the welding SDV chamber is 3.656 mm/s, could indicating be well regulated that the material by effectively flow in indicating that the material flow in the welding chamber could be well regulated by effectively designingthe welding the chamberlocations could and shapes be well of regulated the baffle by plates. effectively For Case designing 2, the thedisplacement locations and of extrudate shapes of in the designing the locations and shapes of the baffle plates. For Case 2, the displacement of extrudate in thebaffle front plates. end is For 0.927 Case mm, 2, thewhich displacement is much less of than extrudate the other in the cases. front end is 0.927 mm, which is much lessthe front than theend other is 0.927 cases. mm, which is much less than the other cases.

Materials 2018, 11, 1517 12 of 20 Materials 2018, 11, x FOR PEER REVIEW 12 of 20

(a)

(b)

(c)

(d)

Figure 13. Velocity distribution in the extruded profiles at different positions of baffle plates: (a) d = Figure 13. Velocity distribution in the extruded proﬁles at different positions of bafﬂe plates: 55 mm; (b) d = 57.5 mm; (c) d = 60 mm; (d) d = 65 mm. (a) d = 55 mm; (b) d = 57.5 mm; (c) d = 60 mm; (d) d = 65 mm.

Table 3. Comparison of maximum and minimum velocities, SDVs, and displacements of extruded Table 3. Comparison of maximum and minimum velocities, SDVs, and displacements of extruded profiles at different position of baffle plates. profiles at different position of baffle plates. Design Schemes of Baffle Plates Case 1 Case 2 Case 3 Case 4 Design SchemesLength of of d Baffle (mm) Plates Case 155 Case 257.5 Case60 3 Case65 4 Max.Length velocity of d (mm) (mm/s) 55217.8 57.5 157.7 60162.8 65168.8 Max.Min. velocity velocity (mm/s) (mm/s) 217.8141.2 157.7 149.9 162.8149.8 168.8140.8 Min. velocity (mm/s) 141.2 149.9 149.8 140.8 SDVSDV 16.26616.266 1.892 1.892 3.6563.656 8.8258.825 DisplacementDisplacement of of profiles profiles (mm) (mm) 6.6576.657 0.927 0.927 1.4991.499 3.3183.318

4.3.4.3. Third Third Step: Step: Adjusting Adjusting the the Die Die Bearings Bearings DieDie bearing bearing plays plays a critical a critical role role in the in the produc productiontion of high of high-performance-performance extruded extruded profiles. profiles. It is Ita is commona common method method in practice in practice to balance to balance the the material material flow flow and and eliminate eliminate the the extrusion extrusion defects defects by by adjustingadjusting the the lengths lengths of ofdie die bearing bearings.s. The The design design rule rule of ofdie die bearing bearing is that is that a long a long bearing bearing is isused used to to increaseincrease additional additional frictional frictional resistance resistance in in the the zone zone with with higher higher velocity, velocity, thereby thereby reducing reducing the velocity the velocityby which by which metal flows metal out flows the out die the orifice. die orifice. On the On contrary, the contrary, a short a bearing short bearing is utilized is utilized to decrease to decreasethe fictional the fictional resistance. resistance. According According to the informationto the information of exit of velocity exit velocity from Figurefrom Fig 13b,ure the 13b, bearing the bearinglengths length of fives of regions five regions in the profilein the profile were artificially were artificially adjusted adjusted reasonably, reasonably, as is illustrated as is illustrated in Figure in 14 . FigTableure 144 shows. Table the4 shows bearing the lengths bearing for len thegth initials for the and initial optimal and dies.optimal The d effecties. The of effect optimal of designoptimal of design of die bearings on the velocity distribution of extruded profile at the die exit was examined. Figure 15 shows the deformation and velocity distribution of extruded profile with adjusting

Materials 2018, 11, 1517 13 of 20

Materialsdie bearings 2018, 11 on, x FOR the velocityPEER REVIEW distribution of extruded profile at the die exit was examined. Figure13 of 1520 showsMaterials the 2018 deformation, 11, x FOR PEER and REVIEW velocity distribution of extruded profile with adjusting bearing length.13 of 20 bearingBy comparing length. Figures By comparing 13b and Fig 15,ure thes 1velocity3b and difference 15, the velocity on the difference cross section on the of profile cross section is further of profiledecreasedbearing is length.further to from Bydecreased 7.71 comparing to 2 to mm/s, from Fig andure7.71sthe to 13 b2 SDV mm/s, and reduces 15 and, the the from velocity SDV the re differenceinitialduces 0.927from on mm/sthe the initial cross to 0.4480.927 section mm/smm/s of toatprofile present.0.448 is mm/s further Even at present.decreased more important, Even to from more extrusion 7.71 important, to 2 mm/s, defects—such extrusion and the defects asSDV distortion re—ducessuch andfromas distortion warp the initial on and the 0.927 extrudedwarp mm/s on theprofile—areto 0.448 extruded mm/s basically profile at present.— eliminated.are basicallyEven more eliminated. important, extrusion defects—such as distortion and warp on the extruded profile—are basically eliminated. l l5 l1 l2 3 l4 l l5 l1 l2 3 l4

FigFigureure 1 14.4. UnevenUneven bearing lengths in the profile proﬁle (mm). Figure 14. Uneven bearing lengths in the profile (mm). Table 4. Bearing lengths for the initial and optimal dies. Table 4. Bearing lengths for the initial and optimal dies. Table 4. Bearing lengths for the initial and optimal dies. Position l1 l2 l3 l4 l5 InitialPosition designPosition (mm) l1 2ll21 4l2 l3 6.4l3 l44.3l4 l5 2.8l5 OptimalInitialInitial design design design (mm) (mm)(mm) 22.75 42 4.064 6.4 6.356.4 4.34.184.3 2.8 3.322.8 OptimalOptimal design design (mm) 2.752.75 4.06 4.06 6.35 6.35 4.184.18 3.32 3.32

Figure 15. Velocity distribution of extruded profile with adjusting bearing lengths. FigureFigure 15.15. Velocity distribution ofof extrudedextruded profileprofile withwith adjustingadjusting bearingbearing lengths.lengths. 5. Comparison between the Initial and Optimal Porthole Dies 5.5. Comparison betweenbetween thethe InitialInitial andand OptimalOptimal PortholePorthole DiesDies 5.1. Metal Flow Pattern 5.1.5.1. Metal Flow Pattern The metal flow pattern can be plotted by the streamlines panel of HyperView software using The metal flow pattern can be plotted by the streamlines panel of HyperView software using node Theor element metal flow vector pattern fields. can Fig beure plotted 16 shows by the the streamlines metal flow panelpattern ofs HyperViewin the porthole software die for using the node or element vector fields. Figure 16 shows the metal flow patterns in the porthole die for the initial initianodel orand element optimal vector dies, whichfields. representFigure 16 theshows flow the paths metal of nodeflow patternparticless infrom the the porthole billet todie the for final the and optimal dies, which represent the flow paths of node particles from the billet to the final profile. profileinitial .and It can optimal be seen dies, that, which for the represent two die the design flows, pathsthere ofalso node exist particles multiple from dead the metal billet zones to the in final the It can be seen that, for the two die designs, there also exist multiple dead metal zones in the corner of cornerprofile of. It the can container be seen that, and forporthole the two die die and design betweens, there the alsosidewall exist andmultiple bottom dead of weldingmetal zones chamber. in the the container and porthole die and between the sidewall and bottom of welding chamber. This may be Thiscorner may of thebe explainedcontainer andby the porthole strong diesticking and between friction thebetween sidewall the andbillet bottom and extrusion of welding tools chamber. under explained by the strong sticking friction between the billet and extrusion tools under the non-lubricated theThis non may-lubricated be explained conditions by the during strong portholesticking frictiondie extrusion. between For the the billet initial and die extrusion design, the tools material under conditions during porthole die extrusion. For the initial die design, the material flow through porthole flowthe non throug-lubricatedh porthole conditions die is severely during not porthole uniform. die Atextrusion. the region For with the smallinitial cavity,die design, the inlet the edgematerials of die is severely not uniform. At the region with small cavity, the inlet edges of portholes and the bottom portholesflow throug andh porthole the bottom die is of severely port bridge not uniform.s, a few At metal the region streamlines with small existed. cavity, At the inlet die exit,edge thes of of port bridges, a few metal streamlines existed. At the die exit, the particles in the solid end have particlesportholes in andthe solid the bottom end have of larger port bridge velocity,s, a and few they metal squeeze streamlines the particles existed. in Atthe thepart die with exit, small the larger velocity, and they squeeze the particles in the part with small cavity where with low velocity. cavityparticles where in the with solid low end velocity. have larger This squeezing velocity, andaction they leads squeeze to a large the particles distortion in of the extrud part edwith profile small. This squeezing action leads to a large distortion of extruded profile. Figure 16b shows the metal flow Figcavityure where16b shows with low the velocity. metal flow This pattern squeezing for action the optimal leads to die a large design. distortion By the of three extrud stepsed profile of die. pattern for the optimal die design. By the three steps of die structure modifications, the particle lines structureFigure 1 6modificationsb shows the , metalthe particle flow patternlines at forthe thedie exit optimal are almost die design. parallel By and the all three particles steps almost of die at the die exit are almost parallel and all particles almost have the same velocity. This phenomenon havestructure the modifications same velocity., the This particle phenomenon lines at the indicates die exit thatare almost the allocation parallel and of materialall particles and almost flow indicates that the allocation of material and flow behavior in porthole die are almost reasonable. behaviorhave the in same porthole velocity. die are This almost phenomenon reasonable. indicates that the allocation of material and flow behavior in porthole die are almost reasonable.

MaterialsMaterials2018 2018, 11, ,11 1517, x FOR PEER REVIEW 1414 of of 20 20 Materials 2018, 11, x FOR PEER REVIEW 14 of 20

Figure 16. Comparison of metal flow patterns in porthole die: (a) the initial die and (b) the optimal Figure 16. Comparison of metal flow patterns in porthole die: (a) the initial die and (b) the optimal Figuredie. 16. Comparison of metal flow patterns in porthole die: (a) the initial die and (b) the optimal die. die. 5.2.5.2. Welding Welding Pressure Pressure 5.2. Welding Pressure InIn porthole porthole die die extrusion, extrusion, the the metal metal flowing flowing through through the the portholes portholes has has to to split split up up around around the the In porthole die extrusion, the metal flowing through the portholes has to split up around the webswebs and and then then rejoin rejoin creating creating longitudinal longitudinal welds welds that that extend extend along along the the whole whole profile. profile. The The quality quality of of webs and then rejoin creating longitudinal welds that extend along the whole profile. The quality of weldweld seams seams is an is important an important quality quality index indexfor evaluating for evaluating hollow aluminum hollow aluminum profiles. Adhesive profiles. bonding Adhesive weld seams is an important quality index for evaluating hollow aluminum profiles. Adhesive underbonding pressure under plays pressure a dominant plays a role dominant in the solid-state role in the welding solid-state of the welding seams [6of,25 the,26 seams]. In recent [6,25,26 years,]. In bonding under pressure plays a dominant role in the solid-state welding of the seams [6,25,26]. In therecent issue years, of longitudinal the issue welding of longi hastudinal attracted weld lotsing of has interest attracted from lots researchers, of interest and from several researchers, criteria were and recent years, the issue of longitudinal welding has attracted lots of interest from researchers, and proposedseveral criteri for evaluatinga were proposed its quality, for such evaluating as the maximum its quality, pressure such as criterion the maximum [27], pressure-time-flow pressure criterion several criteria were proposed for evaluating its quality, such as the maximum pressure criterion criterion[27], pressure [28], and-time pressure-time-temperature-strain-flow criterion [28], and pressure rate-time criterion-temperature [29]. It- hasstrain been rate widely criterion accepted [29]. It [27], pressure-time-flow criterion [28], and pressure-time-temperature-strain rate criterion [29]. It thathas higher been widely welding accepted pressure that is beneficial higher welding for improving pressure welding is beneficial quality. for Akeret improving and Lee welding et al. [quality.27,30] has been widely accepted that higher welding pressure is beneficial for improving welding quality. confirmedAkeret and through Lee et experimentsal. [27,30] confirmed that the through extruded experiments profiles can that be wellthe extruded welded whenprofiles the can welding be well Akeret and Lee et al. [27,30] confirmed through experiments that the extruded profiles can be well pressurewelded inwhen the weldingthe welding chamber pressure exceeds in the the weldi effectiveng chamber stress. exceeds Li et al. [the31] effective concluded stress. that Li when et al. the [31] welded when the welding pressure in the welding chamber exceeds the effective stress. Li et al. [31] peakconcluded welding that pressure when on the the peak welding welding plane pressure is three on times the beyond welding the plane yield is stress three of times material, beyond then the concluded that when the peak welding pressure on the welding plane is three times beyond the canyield be recognized stress of material, that the hollow then can profiles be recognized could weld. that The the peak hollow welding profiles pressure could is almost weld. located The peak in yield stress of material, then can be recognized that the hollow profiles could weld. The peak thewelding middle pres heightsure of is welding almost chamber located in [6 ,26 the]. middle The welding height pressures of welding in the chamber middle [6,26]. height The of welding welding welding pressure is almost located in the middle height of welding chamber [6,26]. The welding chamberpressures for in the the initial middle and height optimal of designwelding schemes chamber are for shown the initial in Figure and 17 optimal. It can design be seen schemes that there are pressures in the middle height of welding chamber for the initial and optimal design schemes are existshown four in weld Figure zones, 17. It i.e., can four be seen weld that seams there form exist in four the surfaceweld zones, of extruded i.e., four profile. weld seams The pressure form in inthe shown in Figure 17. It can be seen that there exist four weld zones, i.e., four weld seams form in the thesurface weld zones of extruded is obviously profile lower. The thanpressure that in in the the non-welded weld zones zones. is obviously For the initiallower design than that scheme, in the surface of extruded profile. The pressure in the weld zones is obviously lower than that in the thenon mean-welded pressure zones. in For four the weld initial zones design is 45.7scheme, MPa, the which mean is pressure nearly equal in four to weld the effective zones is stress45.7 MPa, of non-welded zones. For the initial design scheme, the mean pressure in four weld zones is 45.7 MPa, material.which is A nearly lower weldingequal to pressurethe effec maybetive stress lead of to material. a undesirable A lower welding welding strength. pressure The maybe mean welding lead to a which is nearly equal to the effective stress of material. A lower welding pressure maybe lead to a pressureundesirable in four welding weld zonesstrength. for theThe optimal mean welding die is 197.3 pressure MPa, in being four 331.72%weld zones higher for the than optimal that of die the is undesirable welding strength. The mean welding pressure in four weld zones for the optimal die is initial197.3 die. MPa, The being ratio 331.72% of mean higher welding than pressure that of tothe the initial effective die. stressThe ratio of the of materialmean welding is 4.5. Althoughpressure to 197.3 MPa, being 331.72% higher than that of the initial die. The ratio of mean welding pressure to thethe welding effective chamber stress of height the material of the optimalis 4.5. Although die is decreased the welding from chamber 18 to 15 height mm, theof the introduction optimal die of is the effective stress of the material is 4.5. Although the welding chamber height of the optimal die is baffledecreased plates from increases 18 to greatly15 mm, the the flow introduction resistance of of baffle material plates near increases the die greatly orifice. the Thus, flow the resistance welding of decreased from 18 to 15 mm, the introduction of baffle plates increases greatly the flow resistance of qualitymaterial of extrudednear the die profile orifice. will Thus, be greatly the w improved.elding quality of extruded profile will be greatly improved. material near the die orifice. Thus, the welding quality of extruded profile will be greatly improved.

FigureFigure 17. 1Comparison7. Comparison of the of welding the welding pressure pressure distribution distribution in middle in height middle of height the welding of the chamber welding Figure 17. Comparison of the welding pressure distribution in middle height of the welding (a)chamber the initial (a) die the and initial (b) die the and optimal (b) the die. optimal die. chamber (a) the initial die and (b) the optimal die. 5.3. Temperature Distribution 5.3. Temperature Distribution

Materials 2018, 11, x FOR PEER REVIEW 15 of 20

The evolution of temperature in the billet during extrusion process is also an important indicator of attention to extrusion engineer. The temperature rise of extruded profile from the initial billet temperature may be beyond 100 °C [6]. Such strong thermal effects unavoidably affect the deformation behavior of material, extrusion load, and the microstructure of profile. In addition, the maximum temperature of extruded profile cannot exceed the critical temperature of AA6063-560 °C, since too high of a temperature may lead to hot tears outside the profile and low mechanical properties. Figure 18 shows the temperature evolution of billet during steady extrusion process for Materialsthe initial2018 , and11, 1517 optimal design schemes. It is clear that the temperature of billet firstly 15 drops of 20 gradually and then increases along extrusion direction. The temperature of billet in container 5.3.decreases Temperature from Distribution the initial temperature of 480 °C to 438–480 °C due to the heat exchange with extrusion tools. With the proceeding of extrusion process, the billet temperature in the portholes increasesThe evolution to 458–469 of temperature°C as a result in ofthe heat billet generation during extrusion in plastic process deformation is also an and important friction. indicator As the ofmetal attention streams to extrusion of portholes engineer. flow into The temperature the welding rise chamber of extruded and begins profile welded, from the more initial plastic billet ◦ temperaturedeformation mayenergy be beyondis converted 100 C[ into6]. heat Such and strong increases thermal the effects billet unavoidably temperature. affect At the deformationdie exit, the behaviorbillet temperature of material, reaches extrusion its maximum, load, and which the microstructure is caused by oflarge profile. shear In deformation addition, the in maximumdie orifice ◦ temperaturethat generates of considerable extruded profile amounts cannot of heat. exceed the critical temperature of AA6063-560 C, since too high ofBy a analysistemperature and may comparison, lead to hot atears more outside uniform the profile temperature and low mechanical distribution properties. at the die Figure exit 18 is showsobserved the in temperature the optimal evolution die. This ofis billetbeneficial during to steadyreduce extrusionthe thermal process deformation for the initialof profile and optimaland the designdifficulty schemes. of the subsequent It is clear that heat the treatment. temperature This ofphenomenon billet firstly may drops be gradually attributed and to the then reasonable increases alongallocation extrusion of material direction. and The uni temperatureform material of billetflow inin containerporthole decreasesdie for the from optimal the initial die design. temperature As a ◦ ◦ ofresult, 480 theC to optimal 438–480 dieC duemakes to the the heat distribution exchange of with the extrusion plastic deformation tools. With the heat proceeding more even. of extrusionThe exit ◦ process,temperatures the billet for the temperature initial diein and the optimal portholes die increases are both toless 458–469 than 560C °C, as a thus result surface of heat defects generation such inas plastic hot cracks deformation or overburning and friction. can As be the effectively metal streams avoided of portholes in the extruded flow into profile. the welding The maximum chamber andtemperature begins welded, for the more optimal plastic die deformation is 539.3 °C, beingenergy 8.6 is converted °C higher into thanheat that and of the increases initial the die. billet The temperature.introduction of At baffle the die plates exit, increases the billet temperaturethe heat generation reaches form its maximum, plastic deformation which is caused and frictional by large shearcontact, deformation and results in in die a higher orifice exit that temperature. generates considerable amounts of heat.

FigureFigure 18.18. Comparison ofof thethe temperature temperature distribution distribution of of billet billet (a )( thea) the initial initial die die and and (b) the(b) optimalthe optimal die. die. By analysis and comparison, a more uniform temperature distribution at the die exit is observed in5.4. the Extrusion optimal L die.oad This is beneficial to reduce the thermal deformation of profile and the difficulty of theThe subsequent other key heat aspect treatment. is the Thisinfluence phenomenon of die structure may be attributed modifications to the on reasonable the extrusion allocation load, ofbecause material the and value uniform determines material whether flow in the porthole porthole die for die theextrusion optimal process die design. can As be acarried result, theout optimalsuccessfully. die makes Besides, the too distribution high of an of extrusion the plastic load deformation will lead heat to an more intolerable even. The amount exit temperatures of wear for ◦ forthe theporthole initial diedie, and which optimal is the die major are both cause less of than die 560failure.C, thus Thus, surface the required defects suchextrusion as hot load cracks is an or overburningimportant parameter can be effectively for evaluating avoided the inporthole the extruded die design. profile. By The a seri maximumes of numerical temperature simulations, for the ◦ ◦ optimalthe required die is extrusion 539.3 C, beingloads for 8.6 theChigher different than design that of sch theemes initial could die. be The obtained introduction as listed of baffle in Table plates 5. increasesIt can be theseen heat that generation the required form extrusion plastic deformation load for initial and die frictional design contact,is relevantly and results low, the in avalue higher is exit10,617.3 temperature. KN. Through three kinds of die modifications, especially after introducing the baffle plates in the lower die, the steady extrusion load increases significantly due to the great increasing of the 5.4. Extrusion Load metal flow resistance in the welding chamber. However, the extrusion load for the optimal porthole die isThe 13,903.2 other KN, key aspectwhich isis thestill influence less than ofthe die maximum structure cap modificationsacity of the on 20 the00 ton extrusion extrusion load, press because. the value determines whether the porthole die extrusion process can be carried out successfully. Besides, too high of an extrusion load will lead to an intolerable amount of wear for the porthole die, which is the major cause of die failure. Thus, the required extrusion load is an important parameter for evaluating the porthole die design. By a series of numerical simulations, the required extrusion loads for the different design schemes could be obtained as listed in Table5. It can be seen that the required extrusion load for initial die design is relevantly low, the value is 10,617.3 KN. Through three kinds of die modifications, especially after introducing the baffle plates in the lower die, the steady Materials 2018, 11, 1517 16 of 20

Materials 2018, 11, x FOR PEER REVIEW 16 of 20 extrusion load increases significantly due to the great increasing of the metal flow resistance in the welding chamber. However,Table 5. Required the extrusion extrusion load load for for the the optimaldifferent portholedesign schemes die is. 13,903.2 KN, which is still less than the maximum capacity of the 2000 ton extrusion press. Initial Modification Scheme Modification Scheme Modification Scheme Design Schemes TableDesign 5. Required extrusion1 load for the different2 design schemes. 3 Extrusion load 10,617.3 10,140.6 13,718.4 13,903.2 Design(KN) Schemes Initial Design Modification Scheme 1 Modification Scheme 2 Modification Scheme 3 Extrusion load (KN) 10,617.3 10,140.6 13,718.4 13,903.2 5.5. Die Displacement 5.5. Die Displacement The displacement distribution of upper die for the initial and optimal design schemes is shown in FigureThe 1 displacement9. It can be found distribution that the ofmaximum upper die displacement for the initial in and the optimal upper die design for the schemes initialis design shown schemein Figure is 19 0.107. It can mm, be which found thatis locat theed maximum in the port displacement bridges. After in the multi upper-step die formodifications the initial design, the maximumscheme is displacement 0.107 mm, which in the is located upper indie the is portreduced bridges. to 0.05945 After multi-stepmm. The displacement modifications, of the mandrel maximum is alsodisplacement decreased in from the upper 0.0925 die to is 0.04648 reduced mm. to 0.05945 In contrast mm. The to displacement the initial design of mandrel scheme, is also the decreased elastic deforfrommation 0.0925 of to the 0.04648 optimal mm. porthole In contrast die tois quitethe initial small, design thus scheme,the life of the extrusion elastic deformation die is long. of The the dimensionaloptimal porthole tolerance die is caused quite small, by the thus elastic the life deformation of extrusion of diesmall is long. mandrel The dimensional does not affect tolerance the dimensionalcaused by the accuracy elastic deformationof extruded of profiles small. mandrel The optimal does not porthole affect the die dimensionalmeets the production accuracy of requirementsextruded profiles. of large The aluminum optimal profiles porthole with die meetshigh length the production–width ratio requirements and small cavity. of large aluminum profiles with high length–width ratio and small cavity.

FigFigureure 19 19.. TheThe displacement displacement distribution distribution in inthe the upper upper die die (a ()a the) the initial initial design design scheme scheme and and (b ()b )the the optimaloptimal design design scheme. scheme.

6.6. Experimental Experimental Verifi Verificationcation ToTo validate validate the the rationality rationality of thethe optimizationoptimization scheme,scheme, a a real real porthole porthole die die was was manufactured manufactured and andextrusion extrusion experiments experiment weres were performed performed on on a 2000 a 2000 ton ton extruder. extruder. The The process process parameters—such parameters—such as as the thetemperature temperature of initial of initial billet billet and die, and billet die, diameter,billet diameter, extrusion extrusion speed, etc.—adoptedspeed, etc.—adopted in the extrusion in the experiments were exactly consistent with those in simulations, as shown in Table2. The extruded extrusion experiments were exactly consistent with those in simulations, as shown in Table 2. The profile with electrophoretic coating is given in Figure 20. It can be seen that, by subsequent drawing, extruded profile with electrophoretic coating is given in Figure 20. It can be seen that, by subsequent straightening, and correcting processes, no distinct extrusion defects such as bent or twist deformation drawing, straightening, and correcting processes, no distinct extrusion defects such as bent or twist along the length direction of profile is found. The measured and required thickness difference in the deformation along the length direction of profile is found. The measured and required thickness region of small cavity is only 0.1 mm, which meets the engineering requirements. Moreover, three difference in the region of small cavity is only 0.1 mm, which meets the engineering requirements. typical positions on the cross section of profile were chosen to observe and analyze the microstructure Moreover, three typical positions on the cross section of profile were chosen to observe and analyze (see Figure 21). For practical application, attention must be taken again to avoid overheating, as this the microstructure (see Figure 21). For practical application, attention must be taken again to avoid is detrimental to the aluminum profile. The mechanical properties of extruded profiles are mainly overheating, as this is detrimental to the aluminum profile. The mechanical properties of extruded determined by the grain structure. Figure 22 shows the optical micrographs of three different zones. profiles are mainly determined by the grain structure. Figure 22 shows the optical micrographs of It is clear that there are no remelting eutectics , local remelting that widens the grain boundaries, three different zones. It is clear that there are no remelting eutectics , local remelting that widens the and remelting triangles in the intersection of three grains, which are the criteria to judge whether grain boundaries, and remelting triangles in the intersection of three grains, which are the criteria to structure overheating occurs. It can be also seen that the deformation organization has basically judge whether structure overheating occurs. It can be also seen that the deformation organization disappeared and dynamic recrystallization occurs obviously during extrusion. The grain sizes in has basically disappeared and dynamic recrystallization occurs obviously during extrusion. The different positions of cross-section are basically equal. The main reason is that the optimized die grain sizes in different positions of cross-section are basically equal. The main reason is that the optimized die allocates the material reasonably in porthole die and improves considerably the uniformity of metal flow velocity distribution at the exit of die, leading to a more uniform

Materials 2018, 11, 1517 17 of 20 MaterialsMaterials 20182018,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 1717 ofof 2020 Materials 2018, 11, x FOR PEER REVIEW 17 of 20 distribution of temperature on the cross section of the profile (see Figure 18b). The experimental allocatesdistribution the materialof temperature reasonably on the in porthole cross section die and of improvesthe profile considerably (see Figure the 18b) uniformity. The experimental of metal results are basically consistent with the simulated ones, which indicates that the ALE formulation is distributionflowresults velocity are basically of distribution temperature consistent at the on with exitthe crossofthe die, simulated section leading of ones, to the a more whichprofile uniform indicates (see Figure distribution that 18b) the .ALE ofThe temperature formulation experimental on is effective and efficient in simulating the extrusion process with large deformation. This work could resultstheeffective cross are sectionand basically efficient of the consistent in profile simulating (seewith Figure the the simulated extrusion 18b). The ones,process experimental which with indicates large results deformation. arethat basically the ALE Thisconsistent formulation work could with is provide theoretical guidance in designing porthole dies for the kinds of large aluminum profile with effectivetheprovide simulated theoreticaland efficient ones, whichguidance in simulating indicates in designing that the theextrusion porthole ALE formulation process dies for with the is effective kindslarge ofdeformation. and large efficient aluminum This in simulating workprofile could with the high length–width ratio and small cavity. provideextrusionhigh length theoretical process–width with guidanceratio large and deformation.insmall designing cavity. Thisporthole work dies could for providethe kinds theoretical of large aluminum guidance inprofile designing with highporthole length dies–widt for theh ratio kinds and of small large cavity aluminum. profile with high length–width ratio and small cavity.

FigFigureure 2020.. QualifiedQualified extrudateextrudate withwith electrophoreticelectrophoretic coatingcoating throughthrough the the optimaloptimal die.die. Figure 20. Qualiﬁed extrudate with electrophoretic coating through the optimal die. Figure 20. Qualified extrudate with electrophoretic coating through the optimal die.

Figure 21. Distribution of the observation points on the cross section of the proﬁle. FigFigureure 2211.. DistributionDistribution ofof thethe observationobservation pointspoints onon thethe crosscross sectionsection ofof thethe profile.profile. Figure 21. Distribution of the observation points on the cross section of the profile. (a)(a) (b)(b) (a) (b)

(c)(c) (c)

Figure 22. Optical micrographs in different observation points of (a) 1#, (b) 2#, and (c) 3# on the FigureFigure 22. 22.Optical Optical micrographs micrographs in indifferent different observation observation points points of (a ) of 1#, (a ()b 1#,) 2#, (b and) 2#, (c ) and 3# on (c) the 3# proﬁle. on the profile. Figprofileure .2 2. Optical micrographs in different observation points of (a) 1#, (b) 2#, and (c) 3# on the profile. 7.7. ConclusionsConclusions 7. Conclusions

Materials 2018, 11, 1517 18 of 20

7. Conclusions In this study, the porthole die for producing an aluminum profile with small cavity was designed and optimized through numerical simulation. The steady state extrusion process was simulated using HyperXtrude software based ALE algorithm. The metal flow behavior in the porthole die for the initial die was revealed and the die strength was analyzed. Aiming at achieving a uniform flow velocity and enough die strength, three kinds of structure modifications for the porthole die were proposed. The metal flow pattern, welding pressure, temperature evolution, and die strength for the initial and optimal dies were analyzed and compared synthetically. The major findings of this study can be summarized as follows:

• The metal flow behavior in the porthole die at different stages of extrusion process for the initial die scheme is severely not uniform. The SDV at the die exit is 19.63 mm/s. The maximum displacement in the upper die and mandrel are 0.107 and 0.0925 mm, respectively. • By three steps of die structure modifications, the SDV at the die exit is reduced to 0.448 mm/s. The distortion of the profile is avoided effectively. The mean welding pressure in the welding zones for the optimal die is 197.3 MPa, being 331.72% higher than that of the initial die. The maximum temperature of extrudate at the die exit for the optimal die is 539.3 ◦C, being 8.6 ◦C higher than that of the initial die. The maximum displacement in the upper die is decreased from 0.107 to 0.0656 mm, and the mandrel deflection is decreased from 0.0925 to 0.04648 mm. • The good agreement between the simulation and experimental results shows the modification strategy of porthole die based ALE formulation is practicable and it can provide theoretical guidance for porthole die design of any other similar profiles. • A design route of porthole die for aluminum profile with a small mandrel is proposed, including sunken port bridges to design the welding chamber in the upper die, increasing the inlet angle of portholes, adding the baffle plates, and adjusting the die bearings.

Author Contributions: Z.L. and L.L. conceived and designed the porthole extrusion dies; S.L. performed the extrusion experiments; Z.L. and J.Y. analyzed the data; Z.L. and G.W. contributed reagents/materials/analysis tools; and Z.L. wrote the paper. Funding: This research was funded by the National Natural Science Foundation of China (no. U1664252, 51605234), the Open Fund of State Key Laboratory of Advanced Design and Manufacture for Vehicle Body (no. 31715011) and the Science and Technology Program of Hengyang (no. 2017KJ292). Acknowledgments: The authors wish to thank Nonfermet international XiLin Industry LTD. for the assistance of porthole die extrusion experiments. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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