Hot Gas Forming of Aluminum Alloy Tubes Using Flame Heating

Journal of Manufacturing and Materials Processing

Communication Hot Gas Forming of Aluminum Alloy Tubes Using Flame Heating

Ali Talebi-Anaraki 1,* , Mehdi Chougan 2, Mohsen Loh-Mousavi 3 and Tomoyoshi Maeno 4

1 Department of Mechanical Engineering, Materials Science, and Ocean Engineering, Graduate School of Engineering Science, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan 2 College of Engineering, Design and Physical Sciences, Brunel University London, Uxbridge, Middlesex UB8 3PH, UK; [email protected] 3 Department of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Isfahan 81857-93768, Iran; [email protected] 4 Division of Systems Research, Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan; [email protected] * Correspondence: [email protected] or [email protected]

Received: 23 May 2020; Accepted: 13 June 2020; Published: 16 June 2020

Abstract: Hot metal gas forming (HMGF) is a desirable way for the automotive industry to produce complex metallic parts with poor formability, such as aluminum alloys. A simple hot gas forming method was developed to form aluminum alloy tubes using flame heating. An aluminum alloy tube was heated by a flame torch while the tube was rotated and compressed using a lathe machine and simultaneously pressurized with a constant air pressure. The effects of the internal pressure and axial feeding on expansion and wall thickness distribution were examined. The results showed that the proposed gas forming method was effective for forming aluminum alloy tubes. It was also indicated that axial feeding is a vital parameter to prevent reductions in wall thickness by supplying the material flow during the forming process.

Keywords: gas forming; hot forming; tube forming; bulging; ﬂame heating

1. Introduction In recent years, light-weight materials such as aluminum alloys have been widely used in the automotive and aerospace industries. Tube hydroforming is a well-known forming process that is utilized to form tubular components with variable cross-sections using internal pressure media. However, it is difficult to gain a proper pressure path in hydroforming, and forming defects such as rupture or folding can occur. The poor formability of low ductility materials such as aluminum alloys at room temperature limits their applications. Therefore, warm and hot forming processes have been proposed to increase the formability of materials with low ductility at room temperature. Alzahrani et al. [1] analytically modeled a multi-nose symmetric tube in the hydroforming process based on a mechanistic approach. Afshar et al. [2] investigated the forming limit diagrams of hydroformed aluminum tubes to predict bursting, and they compared the numerical results with experimental tests. Keigler et al. [3] enhanced the formability of aluminum components via warm hydroforming, and they studied the effect of temperature controlling on wall thickness and microstructure before and after the forming process. Yi et al. [4] introduced a new combined heating system for the warm hydroforming of light-weight alloy tubes. Hwang et al. [5] examined the T-shape hydroforming of an AZ61 magnesium alloy at 150 and 300 ◦C. Chan and Kot [6] studied the formability of an AZ31B magnesium alloy made by three different loading paths for quadrilateral tubular components fabricated by the warm tube

J. Manuf. Mater. Process. 2020, 4, 56; doi:10.3390/jmmp4020056 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 56 2 of 9 hydroforming process. However, warm tube hydroforming has a limitation on the heating temperature: it is generally kept below 300 ◦C to prevent the evaporation of fluid media such as oil and water. The limitation of the heating temperature in the tube forming can be eliminated by utilizing gas as a pressure medium. The hot metal gas forming (HMGF) of tubes is a novel hot-forming process that is utilized to form hollow steel and aluminum alloy parts. In this process, a tube is formed into a specific shape by gas or air pressure while the tube is heated before or during the forming process. Dykstra et al. [7] proposed the hot gas forming process to form hot metallic tubes. However, it is difficult to simultaneously control the heating temperature and internal pressure during gas forming due to the short deformation time of tubes in gas forming. Since internal gas pressure is compressive and the volume of the pressure medium is automatically expanding, it is difficult to control the deformation behavior. Most often, electrical resistance or electromagnetic induction methods are used to heat tubes [8–12]. However, Joule heating requires a certain electrical resistance for heating materials, and it is not suitable for heating an aluminum alloy with a high electric conductivity. Therefore, other heating methods such as flame heating can be effective for the heating of aluminum alloy tubes. Vadillo et al. [13] compared their simulation and experimental results of the hot gas forming technology for high-strength steel and stainless-steel tubes. Maeno et al. [14] developed a hot gas bulging process for an aluminum alloy tube using resistance heating. Maeno et al. [15] optimized the forming condition in the hot tube gas bulging of aluminum alloys using resistance heating set into dies. He et al. [16] investigated the mechanical properties and formability of TA2 extruded tubes in HMGF at elevated temperatures. Maeno et al. [17] formed an ultra-high-strength steel hollow part by stamping an air-filled steel tube using resistance heating. Liu et al. [18] determined the formability of titanium tubular components for high-pressure gas forming at elevated temperatures. Paul and Strano [19] studied the influence of the process variables on the gas forming and press hardening of steel tubes. They also determined the effects of internal pressure and preheating on hardening and the surface finish. Talebi Anaraki et al. [20] developed the gas forming of tubes using pulsating pressure and oscillating heating. Their results revealed that, upon applying pulsating pressure, the wall thickness gradually decreased. The oscillated heating, accompanied by pulsating pressure, increased the bulging area. Though large deformation zone is suitable for forming with dies, it is difficult to use forming dies with flame heating. The flame heating technique is suitable for the dieless forming of tubes. In hot tube forming using local heating, dieless forming has been studied. Hwang et al. [21] developed the dieless drawing of stainless steel for tube- or wire-drawing processes using a high-frequency heating source. Furushima and Manabe [22] reported the effectiveness of the dieless drawing process for microtube fabrication. The dieless gas forming of tubes is an advanced material-forming technology that can be classified as a flexible forming process with the absence of die materials. Dieless gas forming can be utilized to manufacture automobile parts, pressure vessel heads, large elbow joints, etc. In this paper, to utilize flame heating for dieless gas forming, local deformation behavior was investigated. The gas forming of an aluminum alloy tube using fixed flame heating was performed. The effects of the internal pressure and axial feeding on the expansion and wall thickness distribution of the deformed tubes were evaluated in experiments. In addition, the effect of the gas forming process on the microstructure was studied.

2. Hot Gas Forming of Aluminum Alloy Tubes Using Flame Heating

Experimental Procedure To investigate the bulging behavior in the local area, a hot gas forming apparatus was utilized to simplify the equipment for the experimental approach and the heating of tubes using ﬂame heating. It is diﬃcult to control the generation of heat using resistance heating because it is dependent on J. Manuf. Mater. Process. 2020, 4, 56 3 of 9

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 9 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 9 thickness. The heat generated in this case was concentrated on the heating portion due to the high makesJ. Manuf. it suitable Mater. Process. for 2020the , heating4, x FOR PEER of aluminum REVIEW alloys with a low resistivity. The sequence of the 3 of hot9 currentmakes it density. suitable However, for the heating by using of flame aluminum heating, alloys the heating with a portionlow resistivity. can be controlled, The sequence which of makes the hot it gas forming of a tube using a lathe machine by flame heating and a schematic drawing of the tube suitablegasmakes forming for it suitable the of heatinga tube for usingthe of aluminumheating a lathe of machine alloysaluminum with by alloys aflame low with resistivity. heating a low and resistivity. The a sequenceschematic Theof sequence drawing the hot gasof of the the forming hot tube and air sealing components are shown in Figures 1 and 2, respectively. Conical bushes and brass ofand agas tubeair forming sealing using of acomponents lathea tube machine using are a bylathe shown flame machine heatingin Figures by andflame 1 aand schematicheating 2, respectively. and drawing a schematic ofConical the drawing tube bushes and of airtheand sealingtube brass housings were used to seal the tube. The tube was filled with compressed air and provided with a componentshousingsand air weresealing are used shown components to inseal Figures the are tube.1 shown and The2, respectively.in tube Figures was 1filled and Conical 2,with respectively. bushes compressed and Conical brass air housingsand bushes provided and were brass with used a flow control valve and an air compressor. The air that filled the tube was heated with a fixed flame toflow sealhousings control the tube. were valve The used and tube to an wasseal air filledthe compressor. tube. with The compressed tubeThe airwas that airfilled andfilled with provided the co mpressedtube with was a heatedair flow and control withprovided a valve fixed with and flame a an torch placed a specific distance away during the experiments. To increase the uniformity of the airtorchflow compressor. placed control a valve Thespecific air and that distanceanfilled air compressor. theaway tube during was The heated airthe that experiments. with filled a fixed the tube flame To wasincrease torch heated placed the with uniformity a specific a fixed distanceflame of the temperature distribution of the tube during the forming process, the tube was rotated by employing awaytemperaturetorch during placed distribution the a experiments.specific ofdistance the To tube increaseaway during during the the uniformity theforming experiments. process, of the temperature Tothe increase tube was distributionthe rotated uniformity by ofemploying theof the tube a three-jaw chuck of the lathe machine. duringa three-jawtemperature the forming chuck distribution of process, the lathe of the the machine. tube tube was during rotated the fo byrming employing process, a three-jawthe tube was chuck rotated of the by lathe employing machine. a three-jaw chuck of the lathe machine.

Figure 1. Sequence of hot gas forming of the tube using flame heating: (a) setup and charge with Figure 1.1. Sequence of hot gas forming of thethe tubetube usingusing ﬂameflame heating:heating: (a) setupsetup andand chargecharge withwith compressedFigure 1. Sequenceair; (b) start of hotof flame gas forming heating of and the rotating tube using of tube; flame and heating: (c) start (a )of setup axial and feeding. charge with compressedcompressed air;air; ((bb)) startstart ofof ﬂameflame heatingheating andand rotatingrotating ofof tube;tube; andand ((cc)) startstart ofof axialaxial feeding.feeding. compressed air; (b) start of flame heating and rotating of tube; and (c) start of axial feeding.

Figure 2. A schematic drawing of the tube and air sealing components. FigureFigure 2. 2. A A schematic schematic drawing drawing of the tube andand air air sealing sealing components. components.

The tailstock ofof thethe lathelathe machinemachine provided provided the the required required axial axial feeding feeding to to decrease decrease the the thinning thinning of TheThe tailstock tailstock of of the the lathe lathe machine machine providedprovided the requiredquired axialaxial feeding feeding to to decrease decrease the the thinning thinning theof the tube. tube. Due Due to the to formingthe forming conditions conditions in the in present the present forming forming method, method, the lathe the machine’s lathe machine’s tailstock of ofthe the tube. tube. Due Due to to the the forming forming conditionsconditions in the present formingforming method, method, the the lathe lathe machine’s machine’s pushedtailstocktailstock the pushed pushed tube during the the tube tube the during experimentsduring the the experiments experiments and fed the and tube. fed The thethe tailstock tube.tube. The The was tailstock tailstock pushed was andwas pushed controlledpushed and and by tailstock pushed the tube during the experiments and fed the tube. The tailstock was pushed and thecontrolledcontrolled automation by by the the feeding automation automation of the feeding lathefeeding carriage of of the the lathelathe to simplify carriage the toto formingsimplifysimplify the setup.the forming forming This setup. simple setup. This mechanismThis simple simple controlled by the automation feeding of the lathe carriage to simplify the forming setup. This simple eliminatedmechanismmechanism theeliminated eliminated extra components the the extra extra components components to form a rotating toto formform tubea rotating using tubetube a lathe using using machine a alathe lathe machine with machine the with proposed with the the mechanism eliminated the extra components to form a rotating tube using a lathe machine with the heatingproposedproposed mechanism. heating heating mechanism. mechanism. The experimental The The experimental experimental apparatus appa of theratus hot ofof gas thethe forming hothot gas gas offorming forming an aluminum of of an an aluminum aluminum alloy tube proposed heating mechanism. The experimental apparatus of the hot gas forming of an aluminum usingalloyalloy tube ﬂame tube using using heating flame flame is heating shown heating is in is shown Figureshown 3in in. FigureFigure An aluminum 3. An aluminum 6063-T5 6063-T5 6063-T5 tube that tube tube was that that 25 was was mm 25 25 inmm mm outer in in alloy tube using flame heating is shown in Figure 3. An aluminum 6063-T5 tube that was 25 mm in diameterouterouter diameter diameter and 1.5 and mm and in1.5 1.5 wall mm mm thickness inin wallwall was thicknessthickness utilized was in the utilized experiment. inin thethe The experiment.experiment. chemical compositionsThe The chemical chemical of outer diameter and 1.5 mm in wall thickness was utilized in the experiment. The chemical thecompositionscompositions aluminum of alloy of the the are aluminum aluminum presented allo allo iny Tabley areare 1 presented.presented The velocity in Table of the 1.1. pushing TheThe velocityvelocity of the of tailstockof the the pushing pushing was constantof ofthe the compositions of the aluminum alloy are presented in Table 1. The velocity of the pushing of the attailstock 0.07tailstock mm was was/rev, constant constant and the at at tube0.07 0.07 (130mm/rev, mm/rev, mm and inand length) thethe tubetube was (130 rotated mmmm inin with length)length) a constant was was rotated rotated rotational with with a velocityconstanta constant of tailstockrotational was velocity constant of at22.4 0.07 RPM mm/rev, by the andlathe the mach tubeine. (130 The mm dimensions in length) and was conditions rotated ofwith the a hot constant gas 22.4rotational RPM byvelocity the lathe of 22.4 machine. RPM Theby the dimensions lathe mach andine. conditions The dimensions of the hot and gas co formingnditions of of the the tubes hot gas are rotationalforming velocityof the tubes of 22.4 are shownRPM by in theTable lathe 2. machine. The dimensions and conditions of the hot gas shownforming in of Table the tubes2. are shown in Table 2. forming of the tubes are shown in Table 2.

Figure 3. Set-up and tools for hot gas forming of aluminum alloy tubes using flame heating. Figure 3. Set-upSet-up and tools for hot gas forming of al aluminumuminum alloy tubes using ﬂameflame heating. Figure 3. Set-up and tools for hot gas forming of aluminum alloy tubes using flame heating.

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Table 1. Chemical composition of the A6063 tube (wt %).

Al Mg Si Pb Fe 98.3 0.396 0.388 0.439 0.342

Table 2. Dimensions and conditions of hot gas forming process.

Parameters Value Outer diameter (mm) 25 Thickness (mm) 1.5 Length (mm) 130 Rotational velocity (RPM) 22.4 Velocity of pushing of tailstock (mm/rev) 0.07

Internal pressure and axial feeding (stroke) are the most critical parameters in the gas forming process. The internal pressure (p), which was measured by the pressure gauge at the plug, was kept constant during the experiment. The stroke of the axial feeding (s) term was defined as the axial compression in the axial direction of the tube. According to the designed equipment, the stroke was defined as the total amount of moving of the tailstock, which was equal to the difference in the tube length before and after forming. The bulging was the difference between the tube’s diameter before and after deformation. In this study, the tube freely bulged until bursting occurred to achieve the maximum diameter. It should be noted that the desired final geometry in dieless gas forming can be tuned by controlling the heating position or using contact tools. The internal pressure (p) and the total amount of stroke (s) were varied in the experiment, as shown in Table3. The free bulging (i.e., forming without dies) was carried out using three di fferent values of constant internal pressure. The tubes were subjected to pressures of 0.4, 0.55, and 0.7 MPa to investigate the effect of internal pressure on the expansion and thickness distribution of the deformed parts. Then, the tubes were bulged by a maximum internal pressure of 0.7 MPa with and without axial feeding to investigate the effect of the axial feeding during the forming process of the tubes at a constant air pressure. In addition, due to the instability of the fracture phenomenon, all of the experiments were done three times until the coefficient of variation of the bulging diameter results was less than 3%, and then the average values were calculated.

Table 3. Experimental condition of gas forming process.

Parameters Value Internal pressure p (MPa) 0.4, 0.55, or 0.7 Total amount of stroke s (mm) 0–5

The temperature distribution along the heated tube was measured by an infrared thermometer, as shown in Figure4. The emissivity is calculated by a thermocouple. The tube was heated for 90 s before bulging to increase the temperature and to achieve a steady-state condition. The temperature distribution was measured by the moving sensor. The scanning time of the temperature distribution was about 6 s along the tube. It should be noted that the surface between the tube and the sealing components was assumed to be at a constant temperature (about 200 ◦C). The temperature distribution of the tube was not constant on the surface of the tube, which indicated that the temperature was varied along the tube. The varied temperature distribution is a positive factor in dieless forming to control the heating portion during and after the bulging of aluminum alloys. The temperature of the middle of the tube, which was heated directly by the fixed flame, was higher than in the other sections. Propane gas was used as the fuel of the flame heating, and the flame was set within 200 mm of the center J. Manuf. Mater. Process. 2020, 4, 56 5 of 9

of the tube. The temperature proﬁle was constant during the experiments, and the temperature of J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 9 theJ. Manuf. extruded Mater. Process. tube in 2020 the, 4 middle, x FOR PEER section REVIEW was about 560 ◦C. 5 of 9

FigureFigure 4. 4. Temperature Temperature distribution distribution graph graph along along the the heated heated tube. tube.

3.3. Results Results of of Bulging Bulging Behavior Behavior 3.1. Effect of Internal Pressure on Bulging Behavior 3.1.3.1. Effect Effect of of Internal Internal Pressure Pressure on on Bulging Bulging Behavior Behavior The tubes formed by three different constant pressures with a stroke of 5 mm and the duration TheThe tubestubes formedformed byby threethree differentdifferent constantconstant prespressuressures withwith aa strokestroke ofof 55 mmmm andand thethe durationduration times are shown in Figure5. The deformed tubes with lower pressures ( p = 0.55 MPa and p = 0.4 MPa timestimes are are shown shown in in Figure Figure 5. 5. The The deformed deformed tubes tubes with with lower lower pressures pressures ( (pp = = 0.55 0.55 MPa MPa and and p p = = 0.4 0.4 MPa MPa with s = 5 mm) were melted without sufficient bulging due to the longer forming process time, whereas withwith ss == 55 mm)mm) werewere meltedmelted withoutwithout sufficientsufficient bulgingbulging duedue toto thethe longerlonger formingforming processprocess time,time, for the pressure of p = 0.7 MPa with s = 5 mm, bursting occurred due to greater tensile stress in the hoop whereaswhereas for for the the pressure pressure of of p p = = 0.7 0.7 MPa MPa with with s s = = 5 5 mm, mm, bursting bursting occurred occurred du duee to to greater greater tensile tensile stress stress and axial directions; thus, the tube was bulged before melting after a lower heating time. Figure6 inin the the hoop hoop and and axial axial directions; directions; thus, thus, the the tube tube was was bulged bulged before before melting melting after after a a lower lower heating heating time. time. shows the expansion of the deformed tubes for three applied internal pressures. It can be concluded FigureFigure 66 showsshows thethe expansionexpansion ofof thethe deformeddeformed tubestubes forfor threethree appliedapplied internalinternal pressures.pressures. ItIt cancan bebe that, as the pressure increased, the expansion of the bulged tube increased. concludedconcluded that, that, as as the the pressure pressure increased, increased, the the expansion expansion of of the the bulged bulged tube tube increased. increased.

Figure 5. Formed tubes in diﬀerent pressures with s = 5 mm for (a) 0.7 MPa, (b) 0.55 MPa, and (c) 0.4 MPa. FigureFigure 5. 5. Formed Formed tubes tubes in in different different pressures pressures with with s s = = 5 5 mm mm for for ( (aa)) 0.7 0.7 MPa, MPa, ( (bb)) 0.55 0.55 MPa, MPa, and and ( (cc)) 0.4 0.4 MPa. TheMPa. bulged specimen was cut in the hoop direction by means of a wire cutting machine, and the amounts of the wall thickness were measured by an accurate micrometer. The wall thickness distribution at the center of protrusion of the bulged tube, which was formed by the pressure of 0.7 MPa and the stroke of 5 mm, is shown in Figure7. The wall thickness reduction was measured in the hoop direction, and the results showed that the thickness decreased near the bursting portion of the bulged tube.

FigureFigure 6. 6. Diameter Diameter of of the the deformed deformed tube tubess at at different different pressures pressures with with s s = = 5 5 mm. mm.

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Figure 4. Temperature distribution graph along the heated tube.

3. Results of Bulging Behavior

3.1. Effect of Internal Pressure on Bulging Behavior The tubes formed by three different constant pressures with a stroke of 5 mm and the duration times are shown in Figure 5. The deformed tubes with lower pressures (p = 0.55 MPa and p = 0.4 MPa with s = 5 mm) were melted without sufficient bulging due to the longer forming process time, whereas for the pressure of p = 0.7 MPa with s = 5 mm, bursting occurred due to greater tensile stress in the hoop and axial directions; thus, the tube was bulged before melting after a lower heating time. Figure 6 shows the expansion of the deformed tubes for three applied internal pressures. It can be concluded that, as the pressure increased, the expansion of the bulged tube increased.

J. Manuf.Figure Mater. 5. Process. Formed2020 tubes, 4, 56 in different pressures with s = 5 mm for (a) 0.7 MPa, (b) 0.55 MPa, and (c) 0.4 6 of 9 MPa.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 9 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 9 The bulged specimen was cut in the hoop direction by means of a wire cutting machine, and the amountsThe ofbulged the wallspecimen thickness was cut were in the measured hoop direction by an by accurate means of micrometer.a wire cutting The machine, wall andthickness the distributionamounts of at the the wallcenter thickness of protrusion were measuredof the bulged by antube, accurate which wasmicrometer. formed byThe the wall pressure thickness of 0.7 MPadistribution and the strokeat the centerof 5 mm, of protrusion is shown in of Figure the bu lged7. The tube, wall which thickness was formedreduction by wasthe pressuremeasured of in0.7 the MPa and the stroke of 5 mm, is shown in Figure 7. The wall thickness reduction was measured in the hoop direction,Figure and the 6. Diameter results showed of the deformed that the tube thicskness at different decreased pressures near with the sbursting = 5 mm. portion of the hoop direction,Figure and 6. theDiameter results showed of the deformed that the tubesthickness at di ﬀdecreasederent pressures near the with burstings = 5 mm. portion of the bulged tube. bulged tube.

FigureFigureFigure 7. 7. The7. The The distribution distribution distribution of ofof wall wallwall thickness thickness reduction reduction in inin hoop hoop hoop direction direction direction at at centerat center center ofof tubeoftube tube forfor forp p= =p 0.7 0.7= 0.7 MPa andMPaMPas = and 5and mm. s s= =5 5 mm. mm. 3.2. Influence of Axial Feeding on Expansion 3.2.3.2. Influence Influence of of Axial Axial Feeding Feeding onon ExpansionExpansion The effect of axial feeding on deformed parts for p = 0.7 MPa is shown in Figure8. This figure TheThe effect effect of of axial axial feeding feeding onon deformed parts forfor pp = = 0.7 0.7 MPa MPa is is shown shown in in Figure Figure 8. 8.This This figure figure showsshowsshows that, that, that, after after after eliminating eliminating eliminating the thethe axial axialaxial feeding,feeding, thethe forming forming time time time in in increasedcreasedcreased and and and the the thebulging bulging bulging area area areawas was was melted.melted.melted. Therefore, Therefore, Therefore, bursting bursting bursting occurred occurredoccurred andand the amountamount ofof of bulge bulge bulge height height height was was was significantly significantly significantly smaller smaller smaller than than than thatthatthat of of theof the the other other other tube. tube. Figure Figure Figure 999 coco comparesmparesmpares the the cross-sectional cross-sectional shape shape shape of of the the of tubes tubes the with tubes with and and with without without and axial without axial axialfeeding;feeding; feeding; the the theresults results results showedshowed showed thatthat that thethethe axial axial feedingfeeding feeding increasedincreased increased the the theexpansion expansion expansion of of the ofthe bulged the bulged bulged tube tube tubeby by by providingprovidingproviding better better better material material material flow flowflow during duringduring thethe formingforming process,process, and and and it it itpostpon postpon postponededed bursting bursting bursting by by supplying by supplying supplying materialmaterialmaterial in in thein the the deformation deformation deformation area. area.area. TheTheThe results alsoalso indicatedindicated indicated that, that, that, as as asthe the the amount amount amount of of the of the theaxial axial axial feeding feeding feeding increased,increased,increased, the the the expansion expansion expansion of of of the thethe bulged bulgedbulged tubetube alsoalso increased. increased.

Figure 8. Bulged tubes (a) without and (b) with axial feeding for p = 0.7 MPa. Figure 8. Bulged tubes (a) without and (b) with axial feeding for p = 0.7 MPa. Figure 8. Bulged tubes (a) without and (b) with axial feeding for p = 0.7 MPa.

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Figure 9. Cross-sectional shape of the formed tubes for p = 0.7 MPa with and without axial feeding. Figure 9. Cross-sectional shape of the formed tubes for p = 0.7 MPa with and without axial feeding. 3.3. Effect of Gas Forming on Microstructure 3.3. Effect of Gas Forming on Microstructure 3.3. EffectSamples of Gas from Forming the fractured on Microstructure part of the deformed tube were taken, and the average grain size of Samples from the fractured part of the deformed tube were taken, and the average grain size of the sampleSamples was from compared the fractured with the part undeformed of the deformed tube. tuThebe sampleswere taken, were and prepared the average by polishing grain size with of the sample was compared with the undeformed tube. The samples were prepared by polishing with theabrasive sample silicone was compared paper and with etched the undeformedby Keller’s etch tube. solution The samples for 10 s.were The prepared grain morphologies by polishing before with abrasive silicone paper and etched by Keller’s etch solution for 10 s. The grain morphologies before abrasiveand after silicone the forming paper process and etched for the by center Keller’s of the etch bulged solution tube for are 10 compared s. The grain in Figuremorphologies 10. This beforefigure and after the forming process for the center of the bulged tube are compared in Figure 10. This figure andshows after that the grain forming growth process appeared for the after center the of forming the bulged process. tube areThe compared average grainin Figure size 10.of theThis sample figure shows that grain growth appeared after the forming process. The average grain size of the sample after showsafter the that bulging grain growthprocess appeared increased after by almostthe forming 73%, process.from 55.36 The to average 95.88 µm.grain It size can of be the seen sample that the bulging process increased by almost 73%, from 55.36 to 95.88 µm. It can be seen that increasing afterincreasing the bulging the temperature process increased obviously by almoststretched 73%, the from grains 55.36 in tothe 95.88 deformation µm. It can direction. be seen thatThe the temperature obviously stretched the grains in the deformation direction. The deformation process increasingdeformation the process temperature also made obviously the grain stretched structur esthe amorphous, grains in thea phenomenon deformation caused direction. by largeThe also made the grain structures amorphous, a phenomenon caused by large amounts of elongation at deformationamounts of elongation process also at high made temperatures. the grain structur The otheres amorphous, reason that thea phenomenon grain size was caused larger bywas large that high temperatures. The other reason that the grain size was larger was that there was a reduction in amountsthere was of a elongationreduction in at the high cooling temperatures. time due The to th othere open reason die characteristic that the grain of size the was free larger bulge wasforming that the cooling time due to the open die characteristic of the free bulge forming operation. This means that thereoperation. was a This reduction means in that the thecooling tube time was duequenched to the openby the die air characteristic and the grain of thesize free was bulge larger. forming These the tube was quenched by the air and the grain size was larger. These results agree well with those operation.results agree This well means with thatthose the obtained tube was by Hequenched et al. [23], by whothe air showed and the the grain effect size of hot was gas larger. forming These on obtained by He et al. [23], who showed the effect of hot gas forming on AA6061 microstructures before resultsAA6061 agree microstructures well with those before obtained and after by theHe gaset al. form [23],ing who process showed at different the effect temperatures. of hot gas forming Both sets on and after the gas forming process at different temperatures. Both sets of results showed that the grains AA6061of results microstructures showed that the before grains and wereafter thestretch gas edform at inga higher process temperature at different temperatures.along the deformation Both sets were stretched at a higher temperature along the deformation direction. ofdirection. results showed that the grains were stretched at a higher temperature along the deformation direction.

Figure 10. Grain structures of the bulged tube (a) before andand ((b)) afterafter thethe formingforming process.process. Figure 10. Grain structures of the bulged tube (a) before and (b) after the forming process. 4. Conclusions 4. ConclusionsInIn thisthis paper,paper, thethe hothot metalmetal gasgas formingforming ofof aluminumaluminum tubestubes atat anan elevatedelevated temperaturetemperature waswas investigated, and the following conclusions were drawn: investigated,In this paper, and the the following hot metal conclusions gas forming were of drawn:aluminum tubes at an elevated temperature was 1.investigated,1. The flame flame and heating heating the following of of the the rotating rotatingconclusions tube tube usingwere using drawn:the the lathe lathe machine machine was was effective effective at simplifying at simplifying the the heating process during the hot gas forming of aluminum alloys. 1. Theheating flame process heating during of the the rotating hot gas tube forming using of the aluminum lathe machine alloys. was effective at simplifying the 2. heatingA high high constant constantprocess duringpressure the increased hot gas forming the expansion of aluminum until bursting; alloys. low pressure led to an increase 2. Ainin highthe the forming forming constant time time pressure and melted increased the thesample expansion without until susufficientffi bursting;cient bulging. low pressure led to an increase in the forming time and melted the sample without sufficient bulging.

J. Manuf. Mater. Process. 2020, 4, 56 8 of 9

3. The maximum expansion occurred in the ﬂame focusing zone due to the elevated temperature distribution along the heated tube. 4. The experimental results showed that the axial feeding increased the bulge height and improved the formability of the tubes.

To reduce the weight of automobile parts, demand is increasing for hollow aluminum alloy products. It is difficult to form aluminum alloy tubes with a low ductility by conventional tube forming processes. The present process developed using flame heating is one of the simplest for producing hollow aluminum alloy parts. Flame heating can control the heating region, i.e., the bulging area can be controlled. Thus, gas bulging using flame heating has the possibility of dieless forming. Therefore, the potential of the present process is large; however, the development of the dieless forming technology by controlling the heating position and condition should be still studied.

Author Contributions: Investigation, A.T.-A. and M.C.; writing—original draft preparation, A.T.-A.; supervision, M.L.-M. and T.M.; writing—review and editing, M.L.-M. and T.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interests.

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