materials 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 1 2 3
*
ID
Department of Mechanical Engineering, BMS College of Engineering, Bengaluru 560019, India Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway;
[email protected] Department of Management and Engineering (DTG), University of Padova, Stradella S. Nicola, 3 I-36100 Vicenza, Italy;
[email protected] (F.B.);
[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
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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 Section 2 of the present paper. The tensile properties have been reported and discussed in Section 3 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 Figure 1a, is suitable for various kinds of characterization, as it has round fatigue and stress-corrosion bars, corrosion-Erichsen test plate, fluidity and Charpy test
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appendices, besides the flat tensile bars. The dimensions of round and flat tensile specimens are shown in Figure Materials1b,c 2017, [8,9]. 10, 1011 3 of 12 Materials 2017, 10, 1011
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(a) (b) (c) (a)casting #1 for high pressure die casting; (b) (b) flat; and (c) round tensile (c) specimens Figure 1. (a) Reference
Figure 1. (a) Reference casting #1 for high pressure die casting; (b) flat; and (c) round tensile specimens (dimensions in mm) [8,9]. Figure 1. (a) casting #1 for high pressure die casting; (b) flat; and (c) round tensile specimens (dimensions inReference mm) [8,9]. (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 2reference die is description made by two AISI H11 parts, a fixed side andof ejector side, and is shown in Figure A detailed of the features and the optimization the die design, This[9]. reference die is made by two AISI H11 parts, a fixed side and ejector side, and isdeveloped shown in Figure 2 [9]. A detailed description of the features and the optimization of the die design, through numerical simulation, is given [9]. The diethe was carefully designed obtaindeveloped a developed uniform Figure 2 [9]. A detailed description of thein features and optimization of the dietodesign, through numerical simulation, is given in [9]. The die was carefully designed to obtain a uniform molten metal front and a favorable thermal evolution inside the die cavity. Figure 3 shows some through numerical simulation, is given in [9]. The die was carefully designed to obtain a uniform molten metal front and a favorable thermal evolution inside the die cavity. Figure 3 shows some results of the numerical the filling process, inside which provides evidence of the velocity molten metal front and simulation a favorableofthermal evolution the die cavity. Figure 3 melt shows some results of the numerical simulation of the filling process, which provides evidence of the melt velocity distribution the die cavity at different percentages of the die cavityoffilling [9].velocity Further results of the inside numerical simulation of three the filling process, which provides evidence the melt distribution inside the die atatthree different percentages ofthe the diecavity cavity filling[9]. [9]. Further experimental details, such as the mold filling time and the initial melt quality, can befilling accessed from [9]. distribution inside the diecavity cavity three different percentages of die Further experimental details, and the theinitial initialmelt meltquality, quality,can can accessed from experimental details,such suchasasthe themold moldfilling filling time and bebe accessed from [9].[9].
(a) (b) (a)the die for diecasting of specimens: (a) fixed side and (b) Figure 2. Layout of (b) ejector side [9]. Figure Layoutofofthe thedie diefor fordiecasting diecasting of of specimens: specimens: (a) side [9].[9]. Figure 2. 2. Layout (a)fixed fixedside sideand and(b) (b)ejector ejector side
Timelli et al. [10,11] successfully used this reference die to determine the tensile properties and the microstructural characteristics ofused diecast alloy. It was the observed the tensile Timelli et al. [10,11] successfully thisAlSi9Cu3(Fe) reference die to determine tensile that properties and Timelli etfrom al. [10,11] successfully used this reference die to determine tensileand properties andasthe specimens the characteristics reference casting #1 show significantly higher tensile strength elongation the microstructural of diecast AlSi9Cu3(Fe) alloy. It wasthe observed that the tensile microstructural ofcasting diecast alloy. higher ItThis was is observed that the tensile specimens compared from to characteristics machined specimens from the same casting. related to heterogeneity in the specimens the reference #1AlSi9Cu3(Fe) show significantly tensile strength and elongation as from the reference casting #1 show significantly higher tensile strength and elongation as compared microstructure from the periphery to the center of the cross section of the die-cast specimens. is to compared to machined specimens from the same casting. This is related to heterogeneity in Itthe machined specimens from the same casting. This is related to heterogeneity in the microstructure from reported that there a surfacetolayer of about 1.3-mm in of thethe tensile barsspecimens. that is free microstructure from exists the periphery the center of the cross thick section die-cast It of is porosity [10,11], which contributes tosection higher properties in die-cast specimens ifthere compared thereported periphery to the center ofathe cross ofabout the die-cast specimens. It is reported exists that there exists surface layer ofmechanical 1.3-mm thick in the tensile barsthat that is free of a to machined ones. porosity [10,11], which contributes to higher mechanical properties in die-cast specimens if compared to machined ones.
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surface layer of about 1.3-mm thick in the tensile bars that is free of porosity [10,11], which contributes to Materials higher mechanical properties in die-cast specimens if compared to machined ones. Materials 2017, 10,10, 1011 of 12 2017, 1011 4 of412
(a) (a)
(b) (b)
(c)
(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 Figure 3. Calculated velocity at (a) 44%; (b) 66%; and (c) 92% of die filling. The color code indicates velocity in melt m s−1 [9]. velocity in m s−1 [9]. −1 indicates velocity in m s [9].
The reference casting #1 has been also adopted for the experimental validation of an analytical the experimental The reference casting #1 predicting has been also validation of an analytical method for explaining and the adopted quality offor castings, by means of the root means square method explaining and predicting the quality of castings, by means of method for explaining and predicting the quality of castings, by means of the root means square plunger acceleration and the plunger speed extracted from the plunger displacement curve [12,13]. the plunger speed extracted from [12,13]. mechanicaland behavior of Al foundry alloys can alsothe be plunger estimated by means of curve the reference plungerThe acceleration the plunger displacement displacement curve [12,13]. dieThe (Figure 4a) designed, built and tested alloys by HYDRO in be cooperation with NTNU ofdie mechanical behavior of can estimated byby means of (University the reference mechanical behavior ofAl Alfoundry foundry alloys canalso also be estimated means of the reference Science and Technology, Trondheim, Norway). The dimensions of the cylindrical tensile test (Figure 4a) designed, built and tested by HYDRO in cooperation with NTNU (University of Science die (Figure 4a) designed, built and tested by HYDRO in cooperation with NTNU (University of specimens shown in Figure 4b [14].The Some previous demonstrated that the tensiletest and Technology, Trondheim, Norway). dimensions of the[15,16] cylindrical test specimens are Science and are Technology, Trondheim, Norway). The works dimensions of thetensile cylindrical tensile specimens show porosity the grip section as compared to thethat gauge Moreover, the shown in Figure 4bhigher [14]. previous works [15,16] demonstrated the section. tensile specimens show specimens are shown in Some Figure 4bin[14]. Some previous works [15,16] demonstrated that the tensile findings indicate thatgrip the central region of the grip solidifies than both thefindings surface region higher porosity the sectionin asthe compared to section the section. Moreover, the indicate specimens showinhigher porosity grip section as gauge compared tolater the gauge section. Moreover, the and the gauge section (see Figure 3c). Similar observations to those made for the tensile specimens that the central of central the gripregion section later than both the region the region gauge findings indicateregion that the of solidifies the grip section solidifies latersurface than both the and surface from (see the reference casting #1 have been proposed Timelli al.tensile for thespecimens reference casting #2 [11]. section Figure 3c). Similar observations to thosebymade foretthe from the reference
and the gauge section (see Figure 3c). Similar observations to those made for the tensile specimens casting have been proposed by Timelli et al. for by theTimelli reference casting #2reference [11]. from the#1reference casting #1 have been proposed et al. for the 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
Figure Figure 4. 4. (a) (a) HPDC HPDC reference reference casting casting #2; #2; (b) (b) round round tensile tensile bar bar (dimensions (dimensions in in mm) mm) [14]. [14].
Gravity casting, being the simplest and conventional route of casting, has few disadvantages 2.2.such Reference Castings forbubbles Gravityand Die-Cast Alloys as entrained air, oxide Al-Si 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 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 critical in minimizing defects and imperfections. It can be achieved by critical calculations, like those designed for GDC [18], which is also called a Stahl mold. Efforts made by researchers and foundries
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
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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 designed for2017, GDC [18], which is also called a Stahl mold. Efforts made by researchers and 5foundries Materials 10, 1011 of 12 to adopt the Stahl mold for obtaining thethe best mechanical alloys are arereviewed reviewedinin [6]. to adopt the Stahl mold for obtaining best mechanicalproperties properties of of cast alloys to the mold for obtaining the best mechanical properties of cast alloys are reviewed in and [6].adopt The limitation of themold Stahl moldthat was itwell-defined has well-defined dimensions termsof ofgauge gauge length The limitation of Stahl the Stahl was itthat has dimensions ininterms length [6]. The limitation of Stahl mold was that it has well-defined dimensions in terms of gauge length and thickness, with the consequence that it was not allowed to study different solidification rates. thickness, with the consequence that it was not allowed to study different solidification rates. To overcome and thickness, with the consequence that itdeveloped was not allowed toconfiguration study different rates. To overcome limitation, the Aluminum Association developed a step configuration the die this limitation, thethis Aluminum Association a step ofsolidification the die of with variable To overcome limitation, the Aluminum Association developed stepshown configuration of 5the die6 with variable this thickness [19]. The design proposed by Grosselle et al.a[20], in Figures and thickness [19]. The design proposed by Grosselle et al. [20], shown in Figures 5 and 6 as reference casting with variablecasting thickness Theof design proposed by Grosselle [20], shown in Figures 5 and 6 as reference #3, [19]. consists four steps varying from 5 to et 20al. mm, from which flat tensile bars #3, consists of four steps #3, varying from 5 to 20 mm, from which flat tensile bars can be machined as per the as casting of four varying from 5 to 20 mm,and from which flat tensile bars3 canreference be machined as perconsists the ASTM B557steps [21] with gauge length, width thickness of 30, 10 and ASTM B557 [21] with as gauge length, width thickness 30, 10width and 3and mm,thickness respectively. can berespectively. machined per the ASTM B557 and [21] with gaugeoflength, of 30, 10 and 3 mm, 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 Figure (a) Reference casting #3 for gravity die casting; (b) sectioning scheme for mechanical property testing 5. (dimensions in mm) [20,22]. 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
The stepastep casting was fromthe the bottom offor theobtaining thinnest the the riser over The casting was gated gated bottom of the thinnest step, while thewhile riser over casting ensures good feeding. This from configuration allows a step, range of cooling rates and the ensures a good feeding. ThisThis configuration for obtaining a range of cooling and and consequently microstructural scales inallows theallows casting [20].obtaining As shown Figure therates two-part casting ensures a different good feeding. configuration for ainrange of6,cooling rates consequently different microstructural scales in in the the casting [20]. shown Figure 6,assembly the die is splitdifferent along a vertical joint line passing through the pouring basin. Toinfacilitate and consequently microstructural scales casting [20].AsAs shown in Figure 6, two-part the two-part 3 die is split along a vertical joint line passing through the pouring basin. To facilitate assembly and mutual location, the die halves are hinged. The dimension of the whole die is 310 × 250 × 115 mm die is split along a vertical joint line passing through the pouring basin. To facilitate assembly and 3 mutual arehalves hinged. The dimension the whole [20,22]. 310 × 250 × 115 mm andlocation, the location, thicknesses thehalves twoare die are 45 dimension and 75 mm,of respectively mutual the the dieofdie halves hinged. The of the wholedie dieis is 310 × 250 × 115 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].
Figure ofthe thedie die[20]. [20]. Figure6.6. Layout Layout of
Variations of the step design have been recently studied to improve the quality of test specimens Variations of the step design have beenrecently recently totoimprove thethe quality test obtained. Itof is worth noticing the optimization of thestudied step mold design proposed byofTimelli et specimens al. for Variations the step design have been studied improve quality of specimens test obtained. is worth noticing the optimization of of the stepstep mold design proposed by Timelli al. for et al. Mg alloys, could be probably used for other foundry alloys [23]. obtained. It isItwhich worth noticing the optimization the mold design proposed by et Timelli Mgalloys, alloys, which could be used forfor other foundry alloys [23].by The mechanical behavior of GDC Al-Si alloys canfoundry be evaluated another step reference die for Mg which could beprobably probably used other alloys [23]. The mechanical GDC Al-Si alloys can be evaluated by another step reference7491, die designed, built and behavior tested byofNTNU (University of Science and Technology, Trondheim, designed, built and tested by SINTEF NTNU (Trondheim, (University of Science As andshown Technology, Trondheim, 7491, Norway) in cooperation with Norway). in Figure 7, the reference Norway) cooperation (Trondheim, Norway). shown ranging in Figurefrom 7, the reference casting #4inhas five steps,with 250 SINTEF mm length, 120 mm width and As thickness 5 to 30 mm casting has five steps, length, 120from mmsteps width and30, thickness ranging 5 to 30while mm [24,25]. #4 Round tensile bars250 canmm be machined with 20, 15 and 10 mmfrom thickness, [24,25]. Round tensile bars can be machined from steps with 30, 20, 15 and 10 mm thickness, while
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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 Figure 7, 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 mm step.from The round bars have 36round mm gauge length and gauge 10 mmlength total diameter, while flat bars flat bars the 5 mm step. The bars have 36 mm and 10 mm total the diameter, have 32 mm gauge length, 10 mm total width and 5 mm thickness. while the flat bars have 32 mm gauge length, 10 mm total width and 5 mm thickness.
(a)
(b)
Figure Referencecasting casting#4 #4for forgravity gravity die die casting; casting; (b) property Figure 7. 7. (a)(a) Reference (b) sectioning sectioningscheme schemefor formechanical mechanical 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, The expected dies tensile strength is the mechanical that can be achieved by Al-Si alloys, cast in reference with state-of-the-art knowledge behavior on die design, process management and alloy cast in reference dies with state-of-the-art knowledge on die design, process management and alloy treatments properly applied to minimize casting defects and imperfections and to improve the treatments properly applied to minimize casting defects and imperfections and to improve microstructure. The expected tensile strength of Al-Si alloys was estimated by means of tensile testingthe microstructure. The expected tensile strength of Al-Si alloys dies. was estimated by means of tensile performed on specimens obtained through the above-described 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]. The1 tensile of HPDC alloysofwas been obtained through reference casting Table collectsstrength the chemical composition the alloys tested, which have been selected based#1 on[1,8,9]. the Table 1 collects themade chemical composition of theThe alloys tested, of which been avoiding selected based on the considerations in the Introduction. addition ironhave permits soldering phenomenonmade due to velocity and pressure typical thepermits HPDC process [26]. considerations inhigh the Introduction. The addition ofof iron avoiding 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.%).
Table 1. Chemical investigated high die Alloy Si composition Fe Cu of the Mn Mg Cr pressure Ni Zncast 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. Alloy 10.895 Si0.889Fe 1.746 Cu 0.219 Mn 0.224 Mg 0.082 Cr 0.084 Ni Zn Pb AlSi11Cu2(Fe) 1.274 0.089 Sn 0.029 Ti0.047 Albal. AlSi12Cu1(Fe) 0.7210.799 0.941 0.354 0.083 0.055 0.026 0.025 0.041 0.038bal.bal. AlSi9Cu3(Fe)10.5108.227 2.8250.232 0.2610.242 0.252 0.045 0.083 0.080 0.081 0.895 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. AlSi12Cu1(Fe) 10.510 0.721 0.941 ingots, 0.232 were 0.242melted 0.045 in 0.080 0.055 in 0.025 0.038 furnace bal. The alloys, supplied as commercial a 3000.354 kg crucible a gas-fired
set up at (800 ± 10) °C and maintained at this temperature for at least 3 h. The temperature of the melt was then gradually decreased by following thewere furnace inertia tokg (690 ± 5) °C.inThe molten metal The alloys, supplied as commercial ingots, melted in aup 300 crucible a gas-fired 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 campaigns was estimated by Foseco H-Alspek to measure hydrogen level and reduced pressure mm. A bi-film index is generally used to determine the molten metal quality: theby higher the bi-film testindex (RPT) to measure bi-film index. The hydrogen content was lower than 0.15 mL/100 g Al during is, the lower the tensile test values and the higher the scatter are [25,29]. Periodically, the molten themetal entirewas experimental campaigns while the bi-film indexes were in the range between 10 and 18 mm. manually skimmed with a coated paddle. Cast-to-shape specimens were produced using HPDC reference casting #1 in a cold chamber diecasting 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,
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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. Table 2 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 (%)
AlSi9Cu3(Fe)
Flat Round
309 ± 6 342 ± 8
163 ± 1 168 ± 6
3.6 ± 0.3 5.1 ± 0.4
AlSi11Cu2(Fe)
Flat Round
312 ± 2 342 ± 7
153 ± 1 158 ± 3
3.5 ± 0.1 5.5 ± 0.7
AlSi12Cu1(Fe)
Flat Round
283 ± 2 315 ± 7
137 ± 1 131 ± 2
3.5 ± 0.1 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 Table 2 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 Figure 8a, and some sample defects detected in the specimen at higher magnification are
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shown in Figure 8b,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 properties be found in compendium [4]. Materials 2017,could 10, 1011 8 of 12
(a)
(b)
(c)
Figure 8. 8. SEM Figure SEM micrographs micrographs of of the the fracture fracture surface surface of of the the round round specimen specimen obtained obtained through through reference reference die #1 #1 and and AlSi12Cu1(Fe) AlSi12Cu1(Fe) alloy: die alloy: (a) (a) low low magnification magnification image; image; (b) (b) cold cold shot shot and and (c) (c) micro-porosity. micro-porosity. These defects defects are are small small thanks thanks to to the the optimized optimized geometry geometry of of the the die. die. These
3.2. Expected Strength of of Gravity Gravity Die-Cast Die-Cast Alloys Alloys 3.2. Expected Tensile Tensile Strength The tensile tensile strength strength of GDC alloys alloys was was evaluated evaluated through castings #3 #3 and The of GDC through reference reference castings and #4. #4. Table Table 33 collects the composition of the alloys tested. These alloys were selected based on the popular Al collects the composition of the alloys tested. These alloys were selected based on the popular Al foundry alloys foundry alloys identified identified through through aa questionnaire questionnaire [3] [3] (as (as described described in in the the Introduction Introduction of of this this work) work) and are frequently used for manufacturing automotive components, such as cylinder heads, wheels and are frequently used for manufacturing automotive components, such as cylinder heads, wheels and carter. carter. The final and The chemical chemical composition composition of of the the alloys alloys was was properly properly chosen chosen for for improving improving the the final properties of castings. For instance, the addition of Cu permits enhancing strength and workability properties of castings. For instance, the addition of Cu permits enhancing strength and workability of of an alloy, while presence of improves Fe improves wear resistance [22,26]. an alloy, while thethe presence of Fe wear resistance [22,26]. Table 3. Chemical Chemical composition composition of of the the investigated investigated gravity gravity die die cast cast Al-Si Al-Si alloys alloys (wt. (wt. %). %). Table 3. Alloy Si Alloy Si AlSi7Mg0.3 6.5 AlSi6Cu4 6.56.0 AlSi7Mg0.3 AlSi6Cu4 6.0
Fe Fe 0.1 1.0 0.1 1.0
Cu Mn Cu Mn 0.002 0.007 4.0 0.5 0.002 0.007 4.0 0.5
Mg Ni Zn Ti Al Mg Ni Zn Ti 0.3 0.003 0.006 0.1 bal. Al 0.1 0.3 1.0 0.2 0.3 0.003 0.006 0.1 bal.bal. 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 (720alloys, ± 10) °C and maintained at this ingots, temperature. The molten was then degassed with set Ar supplied as commercial were melted in 70 metal kg electric resistance furnace for at 15(720 min.±The content of in the holding furnace waswas analyzed by hydrogen up 10) ◦hydrogen C and maintained at the thismelt temperature. The molten metal then degassed with analyzer, and it showed values lower than 0.1 mL/100 g Al during the entire experimental Ar for 15 min. The hydrogen content of the melt in the holding furnace was analyzed bycampaign. hydrogen Periodically, moltenvalues metal lower was manually skimmed with a coated paddle. Sr modification was analyzer, andthe it showed than 0.1 mL/100 g Al during the entire experimental campaign. carried out as well as Ti grain refining; lower mechanical properties can be expected without these Periodically, the molten metal was manually skimmed with a coated paddle. Sr modification was metal treatments. were produced using reference properties castings #3 can andbe #4.expected The temperature the carried out as wellCastings as Ti grain refining; lower mechanical without of these die was maintained at around °C by means oil circulation and about three castings metal treatments. Castings were300 produced using of reference castingschannels, #3 and #4. The temperature of the ◦ were scrapped after the start-up to reach a quasi-steady-state temperature in the die. A ceramic filter die was maintained at around 300 C by means of oil circulation channels, and about three castings with a pore size of 10 ppi was used. The optimal filling time was 5–6 s. were scrapped after the start-up to reach a quasi-steady-state temperature in the die. A ceramic filter tensile test withused. rectangular cross filling sectiontime were drawn with Flat a pore size of 10bars ppi was The optimal was 5–6 s.from each step, in the middle zonesFlat of tensile the castings and the dimensions were maintained as per theeach ASTM-B557 standard. test bars with rectangular cross section were drawn from step, in the middle The tensile haveand been on a tensile machine crosshead speed of 1.5The mm/min. zones of thetests castings thedone dimensions were testing maintained as perwith the ASTM-B557 standard. tensile The strain was measured using a 25-mm extensometer. tests have been done on a tensile testing machine with crosshead speed of 1.5 mm/min. The strain Table 4 summarizes the tensile strength of the investigated alloys. Several specimens (around 10) was measured using a 25-mm extensometer. fromTable each step were tested. 4 summarizes the tensile strength of the investigated alloys. Several specimens (around 10) from each step were tested.
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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 StepStep Thickness (mm)(mm) Thickness UTS (MPa) UTS(MPa) (MPa) EL (%) EL (%) UTS (MPa) EL EL(%) (%) UTS
AlSi7Mg0.3
5 10 15 20 30
AlSi6Cu4
5 10 15 20
AlSi7Mg0.3
AlSi6Cu4
5 10 15 20 30 5 10 15 20
181181 172172 182 182 167 167 -
2.3 2.3 1.7 1.7 2.3 2.3 1.8 1.8 -
203 203 207 194207 188194
0.9 0.9 1.0 1.0 0.8 0.8 0.7
188
0.7
-
-
194 ±±22 194 182 ±±33 182 174 ± 1 174 ±1 166 ± 2 166 ±2 161 ± 3 161 ± 3 ---
9.5 9.5 ± 1± 1 ± 0.5 7.1 7.1 ± 0.5 5.6 ± 0.3 5.6 ± 0.3 4.4 ± 0.2 4.4 3.2 ± 0.2 ± 0.6 3.2 ± 0.6 - - -
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 tensile values in the the literature literatureare are145 145MPa MPa (UTS) AlSi6Cu4 tensile valuesofofthis thisalloy alloyreported reported in (UTS) andand 3% 3% EL. EL. TheThe AlSi6Cu4 is is equivalent to A319 alloy whose minimum strength values reported in the literature are 186 MPa equivalent to A319 alloy whose minimum strength values reported in the literature are 186 MPa (UTS) and 2.5% EL, mold [18]. [18]. Considering Considering the values (UTS) and 2.5% EL,measured measuredusing usingthe thepopular popular ASTM ASTM B-108 B-108 mold the values reported in [1,18], the values of Table 4 for reference die #3 are disappointing and offer scope for reported in [1,18], the values of Table 4 for reference die #3 are disappointing and offer scope for thethe improvement ininthe tofracture fractureisisrelated relatedtotothe the presence improvement themold molddesign. design.Generally, Generally, low low elongation elongation to presence of of diffused microscopic casting defects such as oxides, porosity, etc. For alloy alloy composition, Akhtar diffused microscopic casting defects such as oxides, porosity, etc. similar For similar composition, et al. [25] observed higher ductility using reference die #4. The comparison between reference dies #3 Akhtar et al. [25] observed higher ductility using reference die #4. The comparison between reference dies and #4 that indicates that the die configuration is an important parameter in assessing the potential tensile and #4 #3 indicates the die configuration is an important parameter in assessing the tensile of an alloy much the meltand quality and the pouring conditions. et al.noted [23] noted of potential an alloy as much asasthe meltasquality the pouring conditions. TimelliTimelli et al. [23] that by that by modifying the runner and gating systems in reference die #3, the amount of casting defects modifying the runner and gating systems in reference die #3, the amount of casting defects could be could be minimized. This was shown in both experimental andsimulation numerical studies. simulation Thedie minimized. This was shown in both experimental and numerical Thestudies. modified modified die designinisFigure represented in Figure However, the study was limited to microstructural design is represented 9. However, the9.study was limited to microstructural characterization characterization of magnesium alloys and needs to be extended to aluminum alloys. By means of of magnesium alloys and needs to be extended to aluminum alloys. By means of numerical simulation numerical simulation techniques, Wang et al. [31] found that the gauge section of the standard Stahl techniques, Wang et al. [31] found that the gauge section of the standard Stahl mold (standard 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 of in-gate andisrunner design. To the best of the the comparative authors’ knowledge, there is no knowledge, there no research available about studies between theresearch Stahl available about the comparative studies between the Stahl mold and the modified Step mold. mold and the modified Step mold.
Figure 9. 9.(a)(a)step die #3; #3; (b) (b)defects defectsobserved observedunder under X-ray scan; Figure stepcasting castingproduced produced with with reference reference die X-ray scan; distribution micro-porositydue dueto toshrinkage shrinkage predicted modified step (c) (c) distribution ofof micro-porosity predicted by bymagma-simulation; magma-simulation;(d)(d) modified step casting; improved quality of casting as observed under (f) distribution of microcasting; (e) (e) improved quality of casting as observed under X-rayX-ray scan;scan; (f) distribution of micro-porosity porosity percentage (%) as by indicated by code the color percentage (%) as indicated the 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
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the cooling rates are higher and secondary dendritic arm spacing (SDAS) is lower (Figure 10d), resulting in improved mechanical properties. Average SDASstep of each step has been measured and µm, was in improved mechanical properties. Average SDAS of each has been measured and was 45.4 45.4 µm, µm, 30.1 32.4 µm µm,and 30.125.0 µmµm andby 25.0 µm bythe reducing the (from thickness (from Figure 10a–d). Percentage 32.4 reducing thickness Figure 10a–d). Percentage of porosity of porosity has also been estimated and was 0.65, 0.42, 0.22 and 0.05 by reducing the step thickness. has also been estimated and was 0.65, 0.42, 0.22 and 0.05 by reducing the step thickness. Similar Similar observations were made in the work of Grosselle et al. [20,22] with reference die #3, which is is observations were made in the work of Grosselle et al. [20,22] with reference die #3, which concomitant with concomitant with observations observations made made using using reference reference die die #4 #4 for for similar similar alloy alloy composition composition [24,25]. [24,25].
(a) 20 mm
(b) 15 mm
(c) 10 mm
(d) 5 mm
Figure of of thethe AlSi7Mg0.3 alloyalloy withwith decreasing step thickness from (a–d). Figure 10. 10. Microstructure Microstructure AlSi7Mg0.3 decreasing step thickness from These (a–d). micrographs are related to interior of the casting (see Figure 5b). These micrographs are related to interior of the casting (see Figure 5b).
4. Conclusions 4. Conclusions This review work indicates that the tensile properties of high-pressure and gravity die-cast AlThis 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: The authors would like tolike acknowledge the financing of the European Project StaCastProject (New Acknowledgments: The authors would to acknowledge the financing of the European Quality and Design Standards for Aluminum Alloys Cast Products—FP7-NMP-2012-CSA-6, Grant No. 319188). StaCast (New Quality and Design Standards for Aluminum Alloys Cast Products—FP7-NMP-2012-CSA-6, GrantStaCast No. 319188). StaCast refers of to the University (Italy), University the Norwegian University The project The refers to theproject University Padua (Italy), of thePadua Norwegian of Science and of Science and Technology (Norway), Aalen University the ItalianofAssociation Metallurgy (Italy), Technology (Norway), Aalen University (Germany), the(Germany), Italian Association Metallurgyof (Italy), the Assomet 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 the main key issues theapproved CEN Technical which has elaborated on the expected static strength of Aluminum alloys andofwas by CENReport, Technical Committee 132 (Aluminum and its alloys). One of the Aluminum alloys and was approved by CEN Technical Committee 132 (Aluminum and its alloys). One of the authors (ARA) would like to thank the management of BMS College of Engineering for the support rendered. 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 experiments; A.R.A. andL.A. E.F. and analyzed the data; A.R.A., G.T. andthe E.F. wrote the paper. Author Contributions: F.B. conceived and designed experiments; G.T., L.A. and F.B. performed the experiments; A.R.A. E.F. analyzed theconflict data; A.R.A., G.T. and E.F. wrote the paper. Conflicts of Interest: Theand authors declare no of interest. Conflicts of Interest: The authors declare no conflict of interest.
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