Evaluating the Tensile Properties of Aluminum Foundry Alloys Through Reference Castings—A Review

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Review Evaluating the Tensile Properties of Aluminum Foundry Alloys through Reference Castings—A Review

A.R. Anilchandra 1,*, Lars Arnberg 2, Franco Bonollo 3, Elena Fiorese 3 and Giulio Timelli 3 ID

1 Department of Mechanical Engineering, BMS College of Engineering, Bengaluru 560019, India 2 Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway; [email protected] 3 Department of Management and Engineering (DTG), University of Padova, Stradella S. Nicola, 3 I-36100 Vicenza, Italy; [email protected] (F.B.); elena.ﬁ[email protected] (E.F.); [email protected] (G.T.) * Correspondence: [email protected]; Tel.: +91-990011686

Received: 14 March 2017; Accepted: 5 August 2017; Published: 30 August 2017

Abstract: The tensile properties of an alloy can be exploited if detrimental defects and imperfections of the casting are minimized and the microstructural characteristics are optimized through several strategies that involve die design, process management and metal treatments. This paper presents an analysis and comparison of the salient characteristics of the reference dies proposed in the literature, both in the ﬁeld of pressure and gravity die-casting. The specimens produced with these reference dies, called separately poured specimens, are effective tools for the evaluation and comparison of the tensile and physical behaviors of Al-Si casting alloys. Some of the ﬁndings of the present paper have been recently developed in the frame of the European StaCast project whose results are complemented here with some more recent outcomes and a comprehensive analysis and discussion.

Keywords: reference casting; aluminum alloy; foundry; tensile property; microstructure

1. Introduction The foundry industry has constantly tried to address the challenge of producing high quality and cost effective castings as the ﬁnal applications demand stringent conditions. There have been constant efforts to minimize defects and imperfections in the castings and to optimize the microstructure, keeping in mind the main variables related to the employed alloy, the initial melt quality and the process conditions. From the design viewpoint, the knowledge of tensile properties of cast products is a relevant topic, which is not fully covered by existing international standards, except for the newly introduced document CEN/TR 16748:2014 [1]. As a matter of fact, the European standards on foundry Al alloys (EN 1676 and EN 1706) provide interesting information, which should be integrated with more recent studies. Particularly, the EN 1706 standard [2] specifies the chemical composition limits for Al casting alloys and their tensile properties. As specified in [2], the yield strength (YS), the ultimate tensile strength (UTS) and the elongation to fracture (EL%) in as-die-cast condition are, respectively, 140 MPa, 240 MPa and less than 1% for the most high-pressure die-cast (HPDC) alloys, while these are around 90 MPa, 170 MPa and in the range 1–2.5% for the main gravity die-cast (GDC) alloys. Hence, the EN 1706 standard reports minimal and conservative values of the tensile properties, by considering that the average content of defects and imperfections in Al alloy castings could worsen their static strength. Recently, a consortium called StaCast [3], comprised of various institutions from European countries, was constituted and successfully proposed in [1], which should be used together with the existing standards for the evaluation of Al

Materials 2017, 10, 1011; doi:10.3390/ma10091011 www.mdpi.com/journal/materials Materials 2017, 10, 1011 2 of 12 alloy tensile properties. In general, the CEN/TR shows higher values of tensile properties for Al alloys based on studies that optimize and improve die parameters, by consequently minimizing defects and imperfections and by improving microstructure in the castings. The variables affecting the tensile strength of cast Al alloys are certainly well articulated and complex, involving casting and testing issues. Indeed, the static strength of foundry alloy components mainly depends on alloy composition, casting conditions, heat treatment, geometry of separately poured specimens and test conditions. Since the effects of alloying and heat treatment do not belong to the scope of the work, this review presents analysis and comparison of the tensile properties measured on the separately poured specimens obtained using dies and process parameters that were recently made part of [1] in order to explore the tensile strength of die cast Al-foundry alloys. The most popular Al foundry alloys were identified through a questionnaire: about 60 foundries, which cover a consistent percentage of the European production, have answered to the questionnaire [3]. It was found that the most common alloy category is comprised of Al-Si based alloys, both for HPDC and GDC processes. For instance, StaCast consortium results revealed that the AlSi9Cu3(Fe) alloy is used by around 59% of European foundries, while both AlSi7Mg0.3 and AlSi12Cu1(Fe) alloys are used by around 35% [3]. Moreover, 31% of foundries employ AlSi11Cu2(Fe) and a percentage lower than 30% employs other alloys, such as AlSi12(Fe) [3]. The focus of this paper is on the alloys that have large diffusion with the aim of proposing a significant contribution both for scientific and industrial fields. The alloys that have been studied in this work are usually employed for manufacturing housings, thin-wall parts and safety components [3]. Characteristics of reference dies for testing tensile behavior of foundry alloys are described in Section2 of the present paper. The tensile properties have been reported and discussed in Section3 for exploring the maximum static strength of foundry alloys with the aim of proposing additional tools for selecting material and designing components. Microstructure features and casting defects have also been mentioned for the alloys investigated. For more details about these aspects, the authors have recently published a work, in which the frequency of different kinds of defects is analyzed both for HPDC and GDC [4].

2. Reference Castings for Testing Tensile Behavior A reference die is a permanent mold, designed according to the state-of-the-art methodologies and made of steel or cast iron, suitable for the evaluation of the static strength of a cast alloy. The geometry of such a die varies in accordance with the applied kind of process, i.e., HPDC or GDC. The specimens manufactured using such a reference die are called separately poured specimens. As for process, the casting parameters affecting the quality of components were deeply reviewed in previous works for HPDC [5] and GDC [6]. It was observed that, by modifying the existing standard permanent molds, higher tensile properties could be obtained from conventional Al foundry alloys. Numerical simulation studies have been quite effective in understanding the distribution of porosity during melt flow and optimizing die parameters [7]. Regarding mechanical test conditions, capacity of machine, sensor system, cross-head speed, data elaboration, time between manufacturing and test, and testing temperature affect the behavior of castings. Tensile specimens can be round or flat, machined or not, and are characterized by a specific geometry (i.e., gauge length, gauge width and radius). In this work, the focus is on two different kinds of reference dies for testing tensile behavior of Al alloys, both in HPDC and GDC.

2.1. Reference Castings for High-Pressure Die-Cast Al-Si Alloys The static strength of HPDC Al-Si alloys can be evaluated by the reference die designed, built and tested in the frame of the NADIA Project (New Automotive components Designed for and manufactured by Intelligent processing of light Alloys, Contract No. 026563-2, 2006–2010). The reference casting #1, shown in Figure1a, is suitable for various kinds of characterization, as it has round fatigue and stress-corrosion bars, corrosion-Erichsen test plate, fluidity and Charpy test Materials 2017, 10, 1011 3 of 12 appendices, besides the flat tensile bars. The dimensions of round and flat tensile specimens are shown in FigureMaterials1 2017b,c, [ 810,9, 1011]. 3 of 12 Materials 2017, 10, 1011 3 of 12

(a) (b)(c) (a) (b)(c) FigureFigure 1. (1.a )(a Reference) Reference casting casting #1 #1 for for high high pressurepressure diedie casting; ( (bb)) flat; flat; and and (c ()c )round round tensile tensile specimens specimens (dimensionsFigure(dimensions 1. (a in) Reference in mm) mm) [8 ,[8,9].9 casting]. #1 for high pressure die casting; (b) flat; and (c) round tensile specimens (dimensions in mm) [8,9]. This reference die is made by two AISI H11 parts, a fixed side and ejector side, and is shown in This reference die is made by two AISI H11 parts, a fixed side and ejector side, and is shown in FigureThis 2 [9]. reference A detailed die is description made by two of the AISI featur H11es parts, and thea fixed optimization side and ejectorof the dieside, design, and is developed shown in FigureFigurethrough2[ 29 ]. [9].numerical A A detailed detailed simulation, description description is given ofof thethe in featuresfeatur [9]. esThe and die the was optimization optimization carefully designed of of the the die die to design, obtain design, developeda developeduniform throughthroughmolten numerical metalnumerical front simulation, simulation,and a favorable isis givengiven thermal inin [[9].9]. evolut The die dieion was wasinside carefully carefully the die designed cavity. designed Figure to to obtain obtain 3 shows a uniform a uniform some moltenmoltenresults metal ofmetal the front numericalfront and and a simulationa favorable favorable of thermalthermal the filling evolutionevolut process,ion inside which providesthe the die die cavity. cavity.evidence Figure Figure of the 33 meltshows shows velocity some some resultsresultsdistribution of of the the numerical insidenumerical the simulation diesimulation cavity ofat of thethreethe fillingfilling different process, percentages which which provides provides of the dieevidence evidence cavity of filling of the the melt [9]. melt velocityFurther velocity distributiondistributionexperimental inside inside details, the the such die die cavityas cavity the mold atat threethree filling differentdifferent time and percentages the initial melt of of the thequality, die die cavity cavitycan be filling accessed filling [9]. [9 from].Further Further [9]. experimentalexperimental details, details, such such as as the the mold mold fillingfilling time and the the initial initial melt melt quality, quality, can can be be accessed accessed from from [9]. [ 9].

(a) (b)

Figure 2. Layout of(a )the die for diecasting of specimens: (a) fixed side and ((bb) ejector side [9].

FigureFigure 2. 2.Layout Layout of of the the die die for fordiecasting diecasting ofof specimens: ( (aa)) fixed ﬁxed side side and and (b ()b ejector) ejector side side [9]. [9 ]. Timelli et al. [10,11] successfully used this reference die to determine the tensile properties and the microstructuralTimelli et al. [10,11] characteristics successfully of used diecast this AlSi9C referenceu3(Fe) die toalloy. determine It was theobserved tensile thatproperties the tensile and thespecimensTimelli microstructural et from al. [ 10the,11 characteristicsreference] successfully casting of used #1diecast show this reference AlSi9Csignificantlyu3(Fe) die tohigher alloy. determine tensileIt was the streobserved tensilength and propertiesthat elongation the tensile and as the microstructuralspecimenscompared fromto characteristicsmachined the reference specimens of casting diecast from #1 AlSi9Cu3(Fe) show the samesignif icantlyca alloy.sting. higher It This was is observedtensile related stre thattongth heterogeneity the and tensile elongation specimens in the as fromcomparedmicrostructure the reference to machined from casting the #1specimensperiphery show signiﬁcantly tofrom the the center same higher of cathesting. tensile cross This strengthsection is related of and the elongation die-castto heterogeneity specimens. as compared in Itthe is to machinedmicrostructurereported specimens that there from from existsthe theperiphery a samesurface casting. to layer the centerof This abou isof relatedt the1.3-mm cross to heterogeneitythicksection in ofthe the tensile indie-cast the bars microstructure specimens. that is free It from ofis thereportedporosity periphery [10,11],that to thethere which center exists contributes of a the surface cross to sectionlayer higher of mechanical of abou thet die-cast 1.3-mm properties specimens. thick inin die-castthe It istensile reported specimens bars that that if there comparedis free exists of a porosityto machined [10,11], ones. which contributes to higher mechanical properties in die-cast specimens if compared to machined ones.

Materials 2017, 10, 1011 4 of 12 surface layer of about 1.3-mm thick in the tensile bars that is free of porosity [10,11], which contributes

MaterialstoMaterials higher 2017 2017 mechanical, 10, ,10 1011, 1011 properties in die-cast specimens if compared to machined ones. 4 of4 12of 12

(a) (b) (c) (a) (b) (c) Figure 3. Calculated melt velocity at (a) 44%; (b) 66%; and (c) 92% of die filling. The color code Figure 3. Calculated melt velocity at (a) 44%; (b) 66%; and (c) 92% of die filling. The color code indicates Figureindicates 3. Calculated velocity in meltm s−1 [9].velocity at (a) 44%; (b) 66%; and (c) 92% of die filling. The color code velocity in m s−1 [9]. indicates velocity in m s−1 [9]. The reference casting #1 has been also adopted for the experimental validation of an analytical methodThe reference for explaining casting and #1 predictinghas been also the adoptedquality of fo for castings,r the experimental by means of validation the root meansof an analyticalsquare methodplunger for acceleration explaining and and the predicting plunger speed the quality extracte of d fromcastings, the plunger by by means displa of cement the root curve means [12,13]. square plungerThe acceleration mechanical and behavior the plunger of Al foundry speed extractedextracte alloys cand from also the be plungerestimatedplunger displacementdispla by meanscement of curvethecurve reference [[12,13].12,13]. dieThe (Figure mechanical 4a) designed, behaviorbehavior built ofof Al andAl foundry foundry tested alloysby alloys HYDRO can can also alsoin becooperation be estimated estimated with by by means NTNU means of (University theof the reference reference of die die(FigureScience (Figure4a) and designed, 4a) Technology,designed, built andbuilt Trondheim, tested and tested by HYDRONorway). by HYDRO inThe cooperation dimensionsin cooperation with of NTNUthe with cylindrical (UniversityNTNU (Universitytensile of Science test of Scienceandspecimens Technology, and areTechnology, shown Trondheim, in FigureTrondheim, Norway). 4b [14]. TheNorway).Some dimensions prev iousThe worksdimensions of the [15,16] cylindrical ofdemonstrated the tensile cylindrical test that specimens thetensile tensile test are specimensshownspecimens in Figure are show shown4b higher [ 14 in]. SomeFigureporosity previous 4b in [14]. the grip worksSome sectio prev [15n,16 iousas] compared demonstrated works [15,16] to the that gaugedemonstrated the section. tensile specimensMoreover, that the tensile the show specimenshigherfindings porosity indicateshow in higher thethat grip the porosity central section inregion as the compared grip of the sectio grip ton section the as gaugecompared solidifies section. to later the Moreover, thangauge both section. the thefindings surface Moreover, region indicate the findingsthatand the the centralindicate gauge region sectionthat the of (see central the Figure grip region section 3c). Similarof solidifiesthe grip observations section later than solidifies to boththose thelatermade surface than for theboth region tensile the andsurface specimens the region gauge andsectionfrom the (seethegauge reference Figure section3c). casting Similar(see Figure#1 observationshave 3c). been Similar proposed to those observations by made Timelli for toet the al.those tensile for themade specimens reference for the casting fromtensile the #2 specimens reference[11]. fromcasting the #1 reference have been casting proposed #1 have by Timellibeen proposed et al. for by the Timelli reference et al. casting for the #2 reference [11]. casting #2 [11].

(a) (b)

Figure 4. (a) HPDC reference casting #2; (b) round tensile bar (dimensions in mm) [14]. (a) (b) 2.2. Reference Castings for Gravity Die-Cast Al-Si Alloys FigureFigure 4. ((aa)) HPDC HPDC reference reference casting casting #2; #2; ( (b)) round round tensile bar (dimensions in mm) [14]. [14]. Gravity casting, being the simplest and conventional route of casting, has few disadvantages 2.2.such Reference as entrained Castings air, for bubbles Gravity and Die-Cast oxide Al-Sibi-films. Alloys Th e design of sprue, runner and gating system is critical in minimizing defects and imperfections. It can be achieved by critical calculations, like those Gravity casting, being the simplest and conventional route of casting, has few disadvantages proposed by Campbell [17]. such as entrained air, bubbles and oxide bi-films. The design of sprue, runner and gating system is The ASTM B108 gives dimensions and details of a standard round tensile specimen specifically criticaldesigned in minimizing for GDC [18], defects which and is alsoimperfections. called a Stah Itl canmold. be Efforts achieved made by bycritical researchers calculations, and foundries like those proposed by Campbell [17].

The ASTM B108 gives dimensions and details of a standard round tensile specimen specifically designed for GDC [18], which is also called a Stahl mold. Efforts made by researchers and foundries

Materials 2017, 10, 1011 5 of 12

2.2. Reference Castings for Gravity Die-Cast Al-Si Alloys Gravity casting, being the simplest and conventional route of casting, has few disadvantages such as entrained air, bubbles and oxide bi-ﬁlms. The design of sprue, runner and gating system is critical in minimizing defects and imperfections. It can be achieved by critical calculations, like those proposed by Campbell [17]. The ASTM B108 gives dimensions and details of a standard round tensile specimen specifically Materials 2017, 10, 1011 5 of 12 designedMaterials for 2017 GDC, 10, 1011 [18 ], which is also called a Stahl mold. Efforts made by researchers and5 foundries of 12 to adoptto adopt the Stahl the Stahl mold mold for obtainingfor obtaining the the best best mechanical mechanical properties properties ofof castcast alloys are are reviewed reviewed in in [6]. The limitationto[6]. adopt The limitation the of Stahl the Stahl ofmold the mold Stahlfor obtaining wasmold that was theit that hasbest itwell-defined hasmechan well-definedical properties dimensions dimensions of cast in in termsalloys terms ofare ofgauge gaugereviewed length in and thickness,[6].and The thickness, with limitation the consequencewith of the Stahlconsequence that mold it waswas that notthat it allowed itwas has not well-defined toallowed study differentto dimensions study different solidification in terms solidification of rates. gauge To length rates. overcome this limitation,andTo overcome thickness, the this with Aluminum limitation, the consequence Associationthe Aluminum that it developed was Association not allowed a stepdeveloped to configuration study a stepdifferent configuration of solidification the die of with the rates. variabledie Towith overcome variable thisthickness limitation, [19]. Thethe Aluminumdesign proposed Association by Grosselle developed et al. a[20], step shown configuration in Figures of 5the and die 6 thickness [19]. The design proposed by Grosselle et al. [20], shown in Figures5 and6 as reference casting withas reference variable casting thickness #3, [19].consists The of design four stepsproposed varying by Grossellefrom 5 to et20 al. mm, [20], from shown which in Figuresflat tensile 5 and bars 6 #3, consists of four steps varying from 5 to 20 mm, from which flat tensile bars can be machined as per the ascan reference be machined casting as #3,per consists the ASTM of fourB557 steps [21] withvarying gauge from length, 5 to 20width mm, and from thickness which flat of 30,tensile 10 and bars 3 ASTMcanmm, B557 be respectively. machined [21] with as gauge per the length, ASTM width B557 and[21] with thickness gauge of length, 30, 10 width and 3 and mm, thickness respectively. of 30, 10 and 3 mm, respectively.

(a) (b) (a) (b) Figure 5. (a) Reference casting #3 for gravity die casting; (b) sectioning scheme for mechanical property Figure 5. (a) Reference casting #3 for gravity die casting; (b) sectioning scheme for mechanical property Figuretesting 5.(dimensions (a) Reference in mm) casting [20,22]. #3 for gravity die casting; (b) sectioning scheme for mechanical property testing (dimensions in mm) [20,22]. testing (dimensions in mm) [20,22]. The step casting was gated from the bottom of the thinnest step, while the riser over the casting TheensuresThe step astep castinggood casting feeding. was was gated gatedThis fromconfiguration from the the bottom bottom allows of the of forthinnest the obtaining thinnest step, whilea step,range the while ofriser cooling over the the riserrates casting overand the castingensuresconsequently ensures a good a different good feeding. feeding. microstructural This This configuration conﬁguration scales inallows the allowscasting for obtaining for[20]. obtaining As showna range a in range Figureof cooling of 6, cooling the rates two-part ratesand and consequentlyconsequentlydie is split different along different a vertical microstructural microstructural joint line passing scales scales inthroughin the casting casting the pouring [20]. [20 ].As Asbasin. shown shown To in facilitate Figure in Figure 6, assembly the6, thetwo-part two-part and die isdiemutual split is split along location, along a vertical thea vertical die jointhalves joint line areline passinghinged. passing The throughthrough dimension the the pouring pouring of the wholebasin. basin. dieTo To facilitateis 310 facilitate × 250assembly × assembly 115 mmand3 and 3 mutualmutualand location, the location,thicknesses the the die of die halves the halves two are die are hinged. halves hinged. ar The eThe 45 dimensionanddimension 75 mm, of ofrespectively the the whole whole [20,22].die die is is310 310 × 250× 250 × 115× mm115 mm3 and the thicknesses of the two die halves are 45 and 75 mm, respectively [20,22]. and the thicknesses of the two die halves are 45 and 75 mm, respectively [20,22].

Figure 6. Layout of the die [20]. FigureFigure 6. 6. LayoutLayout of of the the die die [20]. [20 ]. Variations of the step design have been recently studied to improve the quality of test specimens Variationsobtained.Variations It ofis worth the of the step noticing step design design the have haveoptimization been been recentlyrecently of the studied studiedstep mold to to improve design improve proposed the the quality quality by of Timelli test of specimens test et specimensal. for obtained.obtained.Mg alloys, It is It which worthis worth could noticing noticing be probably thethe optimization used for other of ofth foundrye the step step mold alloys mold design [23]. design proposed proposed by Timelli by et Timelli al. for et al. for MgMg alloys, alloys,The mechanical whichwhich could could behavior be be probably probably of GDC used used Al-Si for for otheralloys other foundry can foundry be evaluatedalloys alloys [23]. by [23 another]. step reference die designed,The mechanical built and behaviortested by of NTNU GDC Al-Si(University alloys canof Science be evaluated and Technology, by another stepTrondheim, reference 7491, die designed,Norway) inbuilt cooperation and tested with by SINTEFNTNU (Trondheim,(University ofNorway). Science Asand shown Technology, in Figure Trondheim, 7, the reference 7491, Norway)casting #4 in has cooperation five steps, with 250 SINTEFmm length, (Trondheim, 120 mm widthNorway). and Asthickness shown rainnging Figure from 7, the 5 toreference 30 mm casting[24,25]. #4Round has fivetensile steps, bars 250 can mm be machinedlength, 120 from mm steps width with and 30, thickness 20, 15 and ranging 10 mm from thickness, 5 to 30 while mm [24,25]. Round tensile bars can be machined from steps with 30, 20, 15 and 10 mm thickness, while

Materials 2017, 10, 1011 6 of 12

The mechanical behavior of GDC Al-Si alloys can be evaluated by another step reference die designed, built and tested by NTNU (University of Science and Technology, Trondheim, 7491, Norway) in cooperation with SINTEF (Trondheim, Norway). As shown in Figure7, the reference casting #4 has five steps, 250 mm length, 120 mm width and thickness ranging from 5 to 30 mm [24,25]. Round Materials 2017, 10, 1011 6 of 12 tensile bars can be machined from steps with 30, 20, 15 and 10 mm thickness, while flat bars from the 5 mmflat bars step. from The roundthe 5 mm bars step. have The 36 round mm gauge bars ha lengthve 36 mm and gauge 10 mm length total diameter,and 10 mm while total thediameter, flat bars havewhile 32 mmthe flat gauge bars length, have 32 10 mm mm gauge total length, width 10 and mm 5 mm total thickness. width and 5 mm thickness.

(a) (b)

FigureFigure 7. 7.(a )(a Reference) Reference casting casting #4 #4 for for gravitygravity diedie casting;casting; ( (b)) sectioning sectioning scheme scheme fo forr mechanical mechanical property property testing (dimensions in mm) [24,25]. testing (dimensions in mm) [24,25].

3. Results on the Expected Tensile Strength of Al-Si Alloys Cast in Permanent Mold 3. Results on the Expected Tensile Strength of Al-Si Alloys Cast in Permanent Mold The expected tensile strength is the mechanical behavior that can be achieved by Al-Si alloys, castThe in reference expected dies tensile with strength state-of-the-art is the mechanical knowledge behavioron die design, that canprocess be achievedmanagement by Al-Siand alloy alloys, casttreatments in reference properly dies with applied state-of-the-art to minimize knowledge casting defects on die and design, imperfections processmanagement and to improve and the alloy treatmentsmicrostructure. properly The appliedexpected totensile minimize strength casting of Al-S defectsi alloys was and estimated imperfections by means and of totensile improve testing the microstructure.performed on specimens The expected obtained tensile through strength the above-described of Al-Si alloys dies. was estimated by means of tensile testing performed on specimens obtained through the above-described dies. 3.1. Expected Tensile Strength of High-Pressure Die-Cast Alloys 3.1. Expected Tensile Strength of High-Pressure Die-Cast Alloys The tensile strength of HPDC alloys was been obtained through reference casting #1 [1,8,9]. TableThe 1 tensilecollects strengththe chemical of HPDC composition alloys of was the allo beenys obtainedtested, which through have been reference selected casting based #1 on [ 1the,8, 9]. Tableconsiderations1 collects the made chemical in compositionthe Introduction. of the The alloys addition tested, whichof iron have permits been selectedavoiding based soldering on the considerationsphenomenon made due to in high the Introduction.velocity and pre Thessure addition typical of of iron the permits HPDC process avoiding [26]. soldering phenomenon due to high velocity and pressure typical of the HPDC process [26]. Table 1. Chemical composition of the investigated high pressure die cast Al-Si alloys (wt.%).

AlloyTable 1. ChemicalSi compositionFe Cu of theMnMg investigated high Cr pressure Ni die Zn cast Al-Si Pb alloys Sn (wt.Ti %). Al AlSi9Cu3(Fe) 8.227 0.799 2.825 0.261 0.252 0.083 0.081 0.895 0.083 0.026 0.041 bal. AlSi11Cu2(Fe)Alloy 10.895 Si0.889 Fe 1.746 Cu 0.219 Mn 0.224 Mg 0.082 Cr 0.084 Ni 1.274 Zn Pb0.089 Sn0.029 Ti0.047 Albal. AlSi12Cu1(Fe)AlSi9Cu3(Fe) 10.5108.227 0.721 0.799 0.941 2.825 0.232 0.261 0.242 0.252 0.045 0.083 0.080 0.081 0.8950.354 0.0830.055 0.0260.025 0.0410.038bal. bal. AlSi11Cu2(Fe) 10.895 0.889 1.746 0.219 0.224 0.082 0.084 1.274 0.089 0.029 0.047 bal. TheAlSi12Cu1(Fe) alloys, supplied10.510 as commercial 0.721 0.941 ingots, 0.232 were 0.242 me 0.045lted in 0.080 a 300 0.354 kg crucible 0.055 in 0.025 a gas-fired 0.038 furnacebal. set up at (800 ± 10) °C and maintained at this temperature for at least 3 h. The temperature of the melt wasThe then alloys, gradually supplied decreased as commercial by following ingots, the were furnac meltede inertia in aup 300 to kg(690 crucible ± 5) °C. in The a gas-fired molten metal furnace was degassed with Ar for 15 min. Since the initial metal quality can deeply affect the castability of an set up at (800 ± 10) ◦C and maintained at this temperature for at least 3 h. The temperature of the melt alloy [27,28] and the final properties of castings, the quality of the materials used in the experimental was then gradually decreased by following the furnace inertia up to (690 ± 5) ◦C. The molten metal campaigns was estimated by Foseco H-Alspek to measure hydrogen level and by reduced pressure was degassed with Ar for 15 min. Since the initial metal quality can deeply affect the castability of an test (RPT) to measure bi-film index. The hydrogen content was lower than 0.15 mL/100 g Al during alloy [27,28] and the final properties of castings, the quality of the materials used in the experimental the entire experimental campaigns while the bi-film indexes were in the range between 10 and 18 campaignsmm. A bi-film was estimatedindex is generally by Foseco used H-Alspek to determine to measure the molten hydrogen metal quality: level and the by higher reduced the bi-film pressure testindex (RPT) is, tothe measure lower the bi-film tensile index. test values The and hydrogen the higher content the scatter was lower are [25,29]. than 0.15Periodically, mL/100 the g Al molten during themetal entire was experimental manually skimmed campaigns with while a coated the bi-filmpaddle. indexes were in the range between 10 and 18 mm. Cast-to-shape specimens were produced using HPDC reference casting #1 in a cold chamber die- casting machine with a locking force of 2.9 MN. The nominal plunger velocity was 0.2 m/s for the first phase and 2.7 m/s for the filling phase; a pressure of 40 MPa was applied once the die cavity was full. The optimal experimental conditions at steady-state, which guarantee a high quality of castings,

Materials 2017, 10, 1011 7 of 12

A bi-film index is generally used to determine the molten metal quality: the higher the bi-film index is, the lower the tensile test values and the higher the scatter are [25,29]. Periodically, the molten metal was manually skimmed with a coated paddle. Cast-to-shape specimens were produced using HPDC reference casting #1 in a cold chamber die-casting machine with a locking force of 2.9 MN. The nominal plunger velocity was 0.2 m/s for the first phase and 2.7 m/s for the filling phase; a pressure of 40 MPa was applied once the die cavity was full. The optimal experimental conditions at steady-state, which guarantee a high quality of castings, were found to be: pouring temperature 690 ◦C, melt velocity at in-gates 51 m/s and filling time 9.7 ms. Indeed, the process parameters influence defect content and microstructure of castings, as it has been deeply studied in [13]. The cycle time was approximately 45 s. The weight of the Al alloy die-casting was 0.9 kg, including the runners, gating and overflow system. About 15 castings were scrapped after the start-up, in order to reach a quasi-steady-state temperature in the shot chamber and the die. Oil circulation channels in the die served to stabilize the temperature (at ~230 ◦C). The melt was transferred in 18 s from the holding furnace and poured into the shot sleeve by means of a coated ladle. The fill fraction of the shot chamber, with 70 mm inner diameter, was 0.28. The surface finish of samples was adequately accurate to avoid machining, and only some excess flash along the parting line of the die was manually removed. The tensile tests have been done on a tensile testing machine. The crosshead speed used was 2 mm/min and the strain was measured using a 25-mm extensometer. Experimental data have been collected and processed to provide yield stress (YS or 0.2% proof stress), ultimate tensile strength (UTS) and elongation to fracture (%EL). At least 10 tensile tests were conducted for each condition. The specimens were maintained at room temperature for five months before testing. Table2 summarizes the tensile strength of the alloys tested.

Table 2. Tensile strength properties of the investigated Al-Si HPDC alloys obtained through reference die #1. Flat specimens with 3 mm thickness and round specimens with 6 mm diameter.

Alloy Type of Specimen UTS (MPa) YS (MPa) EL (%) Flat 309 ± 6 163 ± 1 3.6 ± 0.3 AlSi9Cu3(Fe) Round 342 ± 8 168 ± 6 5.1 ± 0.4 Flat 312 ± 2 153 ± 1 3.5 ± 0.1 AlSi11Cu2(Fe) Round 342 ± 7 158 ± 3 5.5 ± 0.7 Flat 283 ± 2 137 ± 1 3.5 ± 0.1 AlSi12Cu1(Fe) Round 315 ± 7 131 ± 2 7.1 ± 0.5

Tensile properties of the round specimens obtained through AlSi9Cu3(Fe) alloy can be compared with those achieved in a previous study [10] using the same die. The values of mechanical properties are in good agreement, by highlighting effectiveness of the proposed die in evaluating the properties of castings. Properties reported in Table2 can be also compared with those obtained in another study [ 30], which used a different geometry called “one bar casting” with a single large feeding channel on one extremity of the specimen, thus to have a coaxial inﬂow with the specimen. Moreover, a large feeder was added on the other extremity of the specimen. The resultant as-cast tensile bar was cylindrical with gauge length 30 mm, total length 96 mm and gauge diameter 8 mm. For the AlSi10Cu3(Fe) alloy, the following properties were achieved: 275 MPa UTS, 150 MPa YS and 2.1% EL. Besides its effectiveness, another advantage of the proposed die is that multiple specimens for tensile, fatigue, impact, corrosion and stress-corrosion testing can be prepared from a single casting. This reference die was carefully designed and optimized to maximize the quality of castings, by reducing the scrap percentage and the presence of defects and imperfections. With the aim of demonstrating this statement, a typical fracture surface under the scanning electron microscope (SEM) is reported in Figure8a, and some sample defects detected in the specimen at higher magniﬁcation are Materials 2017, 10, 1011 8 of 12 shown in Figure8b,c. These defects are detrimental for tensile properties and could cause premature failure of castings. Nevertheless, they are small thanks to the optimized geometry of reference die and the optimal experimental conditions adopted. More on casting defects and their effect on mechanical propertiesMaterials 2017 could, 10, 1011 be found in compendium [4]. 8 of 12

(a) (b)(c)

Figure 8. SEM micrographs of the fracture surface of the round specimen obtained through reference die #1 and AlSi12Cu1(Fe) alloy: ( (aa)) low low magnification magniﬁcation image; image; ( (bb)) cold cold shot shot and and ( (cc)) micro-porosity. micro-porosity. These defects are small thanks toto thethe optimizedoptimized geometrygeometry ofof thethe die.die.

3.2. Expected Tensile Strength ofof GravityGravity Die-CastDie-Cast AlloysAlloys The tensile strength of GDC alloys was evaluatedevaluated through reference castings #3 and #4. Table 33 collects the composition of the alloys tested.tested. These These alloys alloys were were selected based on the popular Al foundry alloys identified identiﬁed through a questionnaire [3] [3] (as described in the Introduction of of this work) and are frequently used for manufacturing automotiveautomotive components, such as cylinder heads, wheels and carter. The The chemical chemical composition composition of of the the allo alloysys was properly chosen for improving the ﬁnalfinal properties ofof castings.castings. ForFor instance,instance, the the addition addition of of Cu Cu permits permits enhancing enhancing strength strength and and workability workability of anof an alloy, alloy, while while the the presence presence of Feof improvesFe improves wear wear resistance resistance [22 ,[22,26].26].

Table 3. Chemical composition of the investigated gravity diedie castcast Al-SiAl-Si alloysalloys (wt.(wt. %). Alloy Si Fe Cu Mn Mg Ni Zn Ti Al AlloyAlSi7Mg0.3 Si6.5 Fe0.1 0.002 Cu 0.007 Mn 0.3Mg 0.003 Ni 0.006 Zn 0.1 Ti bal. Al AlSi7Mg0.3AlSi6Cu4 6.56.0 0.11.0 0.0024.0 0.007 0.5 0.10.3 0.30.003 0.0061.0 0.20.1 bal. bal. AlSi6Cu4 6.0 1.0 4.0 0.5 0.1 0.3 1.0 0.2 bal. The alloys, supplied as commercial ingots, were melted in 70 kg electric resistance furnace set up atThe (720 alloys, ± 10) °C supplied and maintained as commercial at this ingots,temperat wereure. meltedThe molten in 70 metal kg electric was then resistance degassed furnace with setAr upfor at15(720 min.± The10) ◦hydrogenC and maintained content of at the this melt temperature. in the holding The molten furnace metal was was analyzed then degassed by hydrogen with Aranalyzer, for 15 min.and it The showed hydrogen values content lower than of the 0.1 melt mL/1 in00 the g Al holding during furnace the entire was experimental analyzed by campaign. hydrogen analyzer,Periodically, and the it showed molten valuesmetal lowerwas manually than 0.1 mL/100skimmed g Alwith during a coated the entire paddle. experimental Sr modification campaign. was Periodically,carried out as the well molten as Ti metalgrain refining; was manually lower skimmedmechanical with properties a coated can paddle. be expected Sr modification without these was carriedmetal treatments. out as well Castings as Ti grain were refining; produced lower using mechanical reference propertiescastings #3 canand be#4. expected The temperature without of these the metaldie was treatments. maintained Castings at around were 300 produced °C by means using of reference oil circulation castings channels, #3 and #4. and The about temperature three castings of the diewere was scrapped maintained after the at around start-up 300 to ◦reachC by a means quasi-stea of oildy-state circulation temperature channels, in and the aboutdie. A threeceramic castings filter werewith a scrapped pore size after of 10 the ppi start-up was used. to reach The optimal a quasi-steady-state filling time was temperature 5–6 s. in the die. A ceramic filter withFlat a pore tensile size oftest 10 bars ppi waswith used. rectangular The optimal cross fillingsection time were was drawn 5–6 s. from each step, in the middle zonesFlat of tensilethe castings test bars and with the rectangular dimensions cross were section maintained were drawn as per from the each ASTM-B557 step, in the standard. middle zonesThe tensile of the tests castings have and been the done dimensions on a tensile were te maintainedsting machine as per with the ASTM-B557crosshead speed standard. of 1.5 The mm/min. tensile testsThe strain have beenwas measured done on ausing tensile a 25-mm testing extensometer. machine with crosshead speed of 1.5 mm/min. The strain was measuredTable 4 summarizes using a 25-mm the tensile extensometer. strength of the investigated alloys. Several specimens (around 10) fromTable each 4step summarizes were tested. the tensile strength of the investigated alloys. Several specimens (around 10) from each step were tested.

Materials 2017, 10, 1011 9 of 12

Materials 2017, 10, 1011 9 of 12 Table 4. Average tensile strength of Al-Si GDC alloys evaluated by means of two different reference die. Table 4. Average tensile strength of Al-Si GDC alloys evaluated by means of two different reference die. Reference Die #3 Reference Die #4 Reference Die #3 Reference Die #4 AlloyAlloy Step Step Thickness Thickness (mm) (mm) UTSUTS (MPa) (MPa) EL EL (%) (%) UTS UTS (MPa) (MPa) EL (%) EL (%) 55 181 181 2.3 2.3 194 194 ±± 22 9.5 9.5± 1 ± 1 1010 172 172 1.7 1.7 182 182 ±± 33 7.1 7.1± 0.5± 0.5 AlSi7Mg0.3 ± ± AlSi7Mg0.3 1515 182 182 2.3 2.3 174 174 ± 11 5.6 5.6± 0.3 0.3 20 167 1.8 166 ± 2 4.4 ± 0.2 3020 - 167 1.8- 166 161 ±± 23 4.4 3.2± 0.2± 0.6 30 - - 161 ± 3 3.2 ± 0.6 5 203 0.9 - - 5 203 0.9 - - 10 207 1.0 - - AlSi6Cu4 10 207 1.0 - - AlSi6Cu4 15 194 0.8 - - 2015 188194 0.70.8 - -- - 20 188 0.7 - -

The AlSi7Mg0.3 alloy is equivalent to the popular A356 cast alloy and the minimum standard The AlSi7Mg0.3 alloy is equivalent to the popular A356 cast alloy and the minimum standard tensiletensile values values of of this this alloy alloy reportedreported in the the literature literature are are 145 145 MPa MPa (UTS) (UTS) and and 3% 3%EL. EL.The TheAlSi6Cu4 AlSi6Cu4 is is equivalentequivalent to A319 alloy alloy whose whose minimum minimum strength strength values values reported reported in inthe the literature literature are are 186 186 MPa MPa (UTS)(UTS) and and 2.5% 2.5% EL, EL, measured measured using using the the popularpopular ASTMASTM B-108 mold mold [18]. [18]. Considering Considering the the values values reportedreported in in [1 ,[1,18],18], the the values values of of Table Table4 4for for reference reference diedie #3#3 areare disappointing and and offer offer scope scope for for the the improvementimprovement in in the the mold mold design. design. Generally,Generally, lowlow elongationelongation to to fracture fracture is is related related to to the the presence presence of of diffuseddiffused microscopic microscopic casting casting defects defects such such as oxides,as oxid porosity,es, porosity, etc. Foretc. similarFor similar alloy alloy composition, composition, Akhtar et al.Akhtar [25] observedet al. [25] observed higher ductility higher ductility using reference using reference die #4. Thedie #4. comparison The comparison between between reference reference dies #3 anddies #4 #3 indicates and #4 thatindicates the die that configuration the die configuration is an important is an important parameter parameter in assessing in assessing the tensile the potentialtensile ofpotential an alloy asof an much alloy as as the much melt as quality the melt and quality the pouringand the pouring conditions. conditions. Timelli Timelli et al. [ 23et ]al. noted [23] noted that by modifyingthat by modifying the runner the and runner gating and systems gating insystems reference in reference die #3, the dieamount #3, the amount of casting of casting defects defects could be minimized.could be minimized. This was shown This was in both shown experimental in both experimental and numerical and simulationnumerical studies.simulation The studies. modified The die designmodified is represented die design in is Figurerepresented9. However, in Figure the 9. study However, was limited the study to microstructuralwas limited to microstructural characterization of magnesiumcharacterization alloys of andmagnesium needs to alloys be extended and needs to aluminum to be extended alloys. to By aluminum means of alloys. numerical By means simulation of techniques,numerical Wang simulation et al. [techniques,31] found thatWang the et gauge al. [31] section found that of the the standard gauge section Stahl moldof the (standardstandard Stahl ASTM mold (standard ASTM B-108) showed higher porosity (about 3%) compared to the AA Step mold B-108) showed higher porosity (about 3%) compared to the AA Step mold (less than 1%) in an AlSi7Mg (less than 1%) in an AlSi7Mg alloy. This comparison confirmed the statement of Singworth and Kuhn alloy. This comparison confirmed the statement of Singworth and Kuhn [32] that the Stahl mold could [32] that the Stahl mold could not produce better mechanical properties than the Step mold, due to not produce better mechanical properties than the Step mold, due to higher micro-porosity in the higher micro-porosity in the gauge section on account of micro-shrinkage. However, the AA Step gauge section on account of micro-shrinkage. However, the AA Step mold is different from reference mold is different from reference die #3 in terms of in-gate and runner design. To the best of the dieauthors’ #3 in terms knowledge, of in-gate there and is runner no research design. available To the best about of the comparative authors’ knowledge, studies between there is nothe research Stahl availablemold and about the themodified comparative Step mold. studies between the Stahl mold and the modified Step mold.

FigureFigure 9. 9.(a ()a step) step casting casting produced produced withwith referencereference die die #3; #3; ( (bb) )defects defects observed observed under under X-ray X-ray scan; scan; (c)( distributionc) distribution of of micro-porosity micro-porosity due due to to shrinkageshrinkage predicted by by magma-simulation; magma-simulation; (d ()d modified) modiﬁed step step casting;casting; (e )( improvede) improved quality quality of castingof casting as observed as observed under under X-ray X-ray scan; scan; (f) distribution (f) distribution of micro-porosity of micro- percentageporosity percentage (%) as indicated (%) as byindicated the color by codethe color [23]. code [23].

Figure 10 shows the microstructure of the AlSi7Mg0.3 alloy as a function of step thickness. It is Figure 10 shows the microstructure of the AlSi7Mg0.3 alloy as a function of step thickness. worth noticing that the geometry of the proposed reference die permits accurately evaluating the It is worth noticing that the geometry of the proposed reference die permits accurately evaluating mechanical properties of the alloy with thickness variations. As the step thickness reduces, the the mechanical properties of the alloy with thickness variations. As the step thickness reduces, cooling rates are higher and secondary dendritic arm spacing (SDAS) is lower (Figure 10d), resulting

Materials 2017, 10, 1011 10 of 12

Materials 2017, 10, 1011 10 of 12 the cooling rates are higher and secondary dendritic arm spacing (SDAS) is lower (Figure 10d), inresulting improved in improved mechanical mechanical properties. properties. Average SDAS Average of each SDAS step of eachhas been step hasmeasured been measured and was 45.4 and µm, was 32.445.4 µm,µm, 30.1 32.4 µmµm, and 30.1 25.0µm µm and by 25.0 reducingµm by the reducing thickness the (from thickness Figure (from 10a–d). Figure Percentage 10a–d). Percentageof porosity hasof porosity also been has estimated also been and estimated was 0.65, and 0.42, was 0.65,0.22 0.42,and 0.05 0.22 by and reducing 0.05 by reducingthe step thethickness. step thickness. Similar observationsSimilar observations were made were in made the inwork the workof Grosselle of Grosselle et al. et [20,22] al. [20,22 with] with reference reference die die #3, #3, which which is concomitant with observations made using referencereference diedie #4#4 for similarsimilar alloyalloy compositioncomposition [[24,25].24,25].

(a) 20 mm (b) 15 mm (c) 10 mm (d) 5 mm

Figure 10. 10. MicrostructureMicrostructure of ofthe theAlSi7Mg0.3 AlSi7Mg0.3 alloy alloy with withdecreasing decreasing step thickness step thickness from (a from–d). These (a–d). Thesemicrographs micrographs are related are related to interior to interior of the ofcasting the casting (see Figure (see Figure 5b). 5b).

4. Conclusions 4. Conclusions This review work indicates that the tensile properties of high-pressure and gravity die-cast Al- This review work indicates that the tensile properties of high-pressure and gravity die-cast Al-Si Si alloys are better than what has been previously estimated by the existing standards. The improved alloys are better than what has been previously estimated by the existing standards. The improved mechanical properties are due to the minimized casting defects and imperfections, and optimized mechanical properties are due to the minimized casting defects and imperfections, and optimized microstructure of specimens obtained through the reference castings proposed. microstructure of specimens obtained through the reference castings proposed. With the aim of precisely estimating the mechanical behavior of aluminum alloys, reference dies With the aim of precisely estimating the mechanical behavior of aluminum alloys, reference for HPDC and GDC should be chosen and tensile testing conditions should be standardized. For the dies for HPDC and GDC should be chosen and tensile testing conditions should be standardized. HPDC process, the reference die #1 developed in the frame of NADIA Project can be used, since it For the HPDC process, the reference die #1 developed in the frame of NADIA Project can be used, simultaneously provides specimens for tensile, fatigue, impact, corrosion and stress-corrosion since it simultaneously provides specimens for tensile, fatigue, impact, corrosion and stress-corrosion testing. The step casting obtained using reference die #4 for GDC permits evaluating the mechanical testing. The step casting obtained using reference die #4 for GDC permits evaluating the mechanical properties as a function of thickness variations, which strongly affect cooling rate and hence the properties as a function of thickness variations, which strongly affect cooling rate and hence the resulting microstructure. However, reference die #3 needs modification and could be used after resulting microstructure. However, reference die #3 needs modiﬁcation and could be used after optimizing as proposed for Mg alloys. optimizing as proposed for Mg alloys. The knowledge of the tensile strength of foundry alloys, which can be obtained through The knowledge of the tensile strength of foundry alloys, which can be obtained through reference reference castings, will give a relevant contribution to material selection and design approach, by castings, will give a relevant contribution to material selection and design approach, by allowing to allowing to choose the best solution for the envisaged application. This knowledge will significantly choose the best solution for the envisaged application. This knowledge will signiﬁcantly help the help the foundries to reduce production costs by minimizing scrap with a consequent improvement foundries to reduce production costs by minimizing scrap with a consequent improvement in the in the competitive edge. competitive edge.

Acknowledgments: TheThe authors authors would would like to like acknowledge to acknowledge the financing the financingof the European of the Project European StaCast Project (New QualityStaCast and (New Design Quality Standards and Design for Aluminum Standards Alloys for Aluminum Cast Products—FP7-NMP-2012-CSA-6, Alloys Cast Products—FP7-NMP-2012-CSA-6, Grant No. 319188). TheGrant StaCast No. 319188). project Therefers StaCast to the project University refers of to thePadu Universitya (Italy), ofthe Padua Norwegian (Italy), University the Norwegian of Science University and Technologyof Science and (Norway), Technology Aalen (Norway), University Aalen (Germany), University the (Germany), Italian Association the Italian of Association Metallurgy of(Italy), Metallurgy the Assomet (Italy), the Assomet Services (Italy), and Federation of Aluminium Consumers in Europe (Brussels, Belgium). This paper Services (Italy), and Federation of Aluminium Consumers in Europe (Brussels, Belgium). This paper presents presents the main key issues of the CEN Technical Report, which has elaborated on the expected static strength of Aluminumthe main key alloys issues and of was theapproved CEN Technical by CEN Report, Technical which Committee has elaborated 132 (Aluminum on the expected and its alloys). static strength One of theof Aluminumauthors (ARA) alloys would and likewas to approved thank the by management CEN Technical of BMS Committee College 132 of Engineering (Aluminum for and the its support alloys). rendered. One of the authors (ARA) would like to thank the management of BMS College of Engineering for the support rendered. Author Contributions: L.A. and F.B. conceived and designed the experiments; G.T., L.A. and F.B. performed the Authorexperiments; Contributions: A.R.A. and L.A. E.F. and analyzed F.B. conceived the data; A.R.A.,and designed G.T. and the E.F. experiments; wrote the paper. G.T., L.A. and F.B. performed theConflicts experiments; of Interest: A.R.A.The and authors E.F. analyzed declare no the conflict data; A.R.A., of interest. G.T. and E.F. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. References

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