The Influence of Foundry Scrap Returns on Chemical Composition and Microstructure Development of AlSi9Cu3 Alloy

Citation: Šmalc, J.; Vonˇcina, M.; Mrvar, P.; Balaško, T.; Krutiš, V.; Petriˇc, M. The Influence of Foundry Scrap Returns on Chemical Composition and Microstructure Development of AlSi9Cu3 Alloy. Crystals 2023,13, 757. https:// doi.org/10.3390/cryst13050757 Academic Editor: Hongbin Bei Received: 12 April 2023 Revised: 28 April 2023 Accepted: 30 April 2023 Published: 3 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). crystals Article The Influence of Foundry Scrap Returns on Chemical Composition and Microstructure Development of AlSi9Cu3 Alloy Jan Šmalc 1, Maja Vonˇcina 2, Primož Mrvar 2, Tilen Balaško 2, Vladimír Krutiš 3and Mitja Petriˇc 2,* 1Laboratory for Heat Treatment and Materials Testing, Faculty of Mechanical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia; [email protected] 2Department of Materials and Metallurgy, Faculty of Natural Sciences and Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia; [email protected] (M.V.); [email protected] (P.M.); [email protected] (T.B.) 3Institute of Manufacturing Technology, Faculty of Mechanical Engineering, Brno University of Technology, 616 69 Brno, Czech Republic; [email protected].cz *Correspondence: [email protected]; Tel.: +386-31-317-155 Abstract: Recycling is now, more than ever, an important part of any foundry process due to the high cost of energy. The basis of the work presented here is a study of the addition of foundry scrap returns to the melt in order to reduce material and energy costs. The most important issue in such a process is the quality of both the prepared melt and final product. In this work, scrap returns were added to the AlSi9Cu3 base alloy in different proportions. Chemical composition was monitored, the solidification path was predicted by CALPHAD calculations and monitored by thermal analysis, and the formed microstructure was studied. The mechanical properties were also determined. The results showed that as the amount of scrap returns increased, elements such as Fe, Ni, Pb, Sr, etc. were more built up and elements such as Mg, Mn, Cr, etc. were decreased due to oxidation. The different chemical composition led to a reduced Mn:Fe ratio, resulting in the formation of needle-like Fe-rich phases and a decrease in mechanical properties. Keywords: casting; foundry scrap return; recycling; solidification; Al cast alloys 1. Introduction In foundries and within the aluminium industry, the demand for aluminium alloys for the automotive industry is constantly increasing due to their good specific strength and corrosion resistance. Most of the aluminium comes from primary production, i.e., from electrolysis. However, the production of secondary aluminium is increasing rapidly [ 1 , 2 ]. Firstly, due to the high price of primary alloys, foundries often use recycled materials to increase the efficiency and productivity of the process. Secondly, the reuse of recycled and scrap parts saves up to 95% energy compared to the production of primary aluminium, resulting in less greenhouse gas emissions [3–5]. The solidification of theAlSi9Cu3(Fe) alloy has been studied by various researchers [6–8] . Solidification depends on the chemical composition and cooling rate and starts with the nucleation of primary αAl dendrites at about 610–570 ◦ C. Soon after, the formation of ironrich phases such as α -Al 15 (Mn,Fe) 3 Si 2 and/or β -Al 5 FeSi occurs. Eutectic crystallisation of ( αAl + βSi ) begins at 566 ◦ C, followed by the formation of Mg 2 Si (if the criteria are met). Subsequently, the formation of the copper-rich phase begins in the final stage of solidification, in which the first formation of the θ-Al2Cu phase takes place. Solidification is completed with the complex eutectic Q-Al 5 Mg 8 Si 6 Cu 2 phase in the temperature range of ~495–451 ◦C [9,10]. When using secondary aluminium, the efficiency of the process is lower due to the higher slag content. Secondly, many impurities are introduced into the melt with secondary Crystals 2023,13, 757. https://doi.org/10.3390/cryst13050757 https://www.mdpi.com/journal/crystals Crystals 2023,13, 757 2 of 13 aluminium. Iron is one of the main elements that form iron-rich intermetallic phases, including the less harmful α -Al 15 (Mn,Fe) 3 Si 2 phase with “Chinese script” morphology and harmful β -Al 5 FeSi needles [ 11 ]. To avoid the formation of β -Al 5 FeSi, different methods can be used: melt superheating, higher cooling rates, and chemical modification. In practise, the Mn:Fe ratio = 0.5 is most often maintained [ 12 ]. With the addition of recycled and scrap parts, the chemical composition changes and it is known that the concentrations of Mg and Mn decrease due to oxidation. Both elements play an important role, firstly in the formation of α -Al 15 (Mn,Fe) 3 Si 2 , and secondly, the presence of Cu and Mg allows additional hardening of the AlSi9Cu3(Fe) alloy by heat treatment or natural ageing. During ageing, θ -Al 2 Cu and/or Mg 2 Si precipitates are formed, which lead to an increase in tensile strength (Rm) and hardness of the AlSi9Cu3(Fe) alloy [ 13 ]. Furthermore, grain refinement of primary αAl with Al-Ti-B master alloy is often used to improve Rm and hardness of aluminium alloys [ 14 , 15 ]. Modification of eutectic βSi by the addition of Sr is also a well-known practise [ 16 ]. When Sr-modified scrap is added to the melt, the chemical composition of Ti, B, and Sr increases, and enrichment of these elements can lead to the formation of new Sr-rich intermetallic Al2SrSi2[17,18]. The aim of the present study was to observe the effects of different amounts of scrap returns on the chemical composition of the castings. The main focus of the study was on the cooling curves and changes in mechanical properties, such as tensile strength, yield strength, and hardness, which change due to natural ageing. 2. Materials and Methods The work was based on the production of specimens from a base alloy with different amounts of foundry scrap returns. Foundry scrap returns in this context meant the rejected castings from gating and feeder systems that are recycled during the production of castings at every foundry and are a necessity in casting production. The amount of castings rejected from gating and feeder systems during brut casting is usually from 30% to 50% or more, which is why foundry scrap returns are an important part of the charge material. Usually, the foundry scrap returns used in the production of new castings are from the same alloy as the castings produced. The alloy used for the experiment was a standard AlSi9Cu3 base alloy (chemical composition given in Table 1) to which scrap returns described above (chemical composition given in Table 2) were added in quantities ranging from 0% to 100%. Table 2shows the designations of the samples and the additions of return scrap. The samples of about 2000 g were melted in an induction furnace with a steel crucible. After melting, the temperature was raised to 700 ◦ C, and grain refiners (0.02 wt% Ti in the AlTi5B1 master alloy) and modifiers (0.01 wt% Sr in the AlSr10 master alloy) were added. The melt was degassed with a laboratory impeller at 600 rpm and an argon gas flow of 3 L/min for 120 s. The melts prepared in this way were poured into a Croning measuring cell for simple thermal analysis, where the temperature was recorded over time. The remainder of the melt was poured into a square steel mould that was preheated to 450 ◦ C. The castings from the Croning measuring cell were cut for chemical analysis, differential scanning calorimetry (DSC), and metallographic observations. The castings from the steel mould were used to prepare tensile and hardness test specimens. Figure 1shows the melting procedure, the cast samples, and the sections for the above analyses. Chemical analysis was carried out by inductively coupled plasma–optical emission spectrometry (ICP–OES) using an Agilent 5800 VDV instrument and metallographic observations were made by optical metallography and scanning electron microscopy (SEM) using an Olympus BX 61 microscope and Jeol JSM-6500F SEM with an EDS detector, respectively. Round tensile test specimens (DIN 50125) were tested 35 days after casting the samples, with four replicates using the INSTRON 8802 machine according to SIST EN ISO 6892-1 A224. Brinell hardness (HBW 2.5/62.5) measurements were taken for 35 days after casting to follow the natural ageing process, with two repetitions for each sample using a NEXUS 7500 tester. Based on the chemical composition, CALPHAD simulations of the phase diagrams were calculated Crystals 2023,13, 757 3 of 13 for all alloys using ThermoCalc 2020a software and the TCAL6 database. In addition, the non-equilibrium solidification course for each alloy was predicted using the Scheil model. Table 1. Chemical composition of AlSi9Cu3 base alloy and foundry scrap returns in wt%. Element Si Fe Cu Mn Mg Cr Ni Zn Base 7.51 0.5973 2.955 0.2738 0.3366 0.0407 0.0244 0.6846 Scrap 7.504 0.6095 2.995 0.1838 0.2547 0.0305 0.0432 0.7343 Element Ti Ag B Be Bi Ca Cd Ce Base 0.0951 <0.00001 0.0015 0.00001 0.002 0.0022 0.00019 <0.00010 Scrap 0.108 <0.00001 0.0021 0.00001 0.0031 0.00083 0.00032 <0.00010 Element Co Ga Hg Li Na P Pb Sb Base 0.0014 0.012 <0.0001 0.00003 <0.00002 0.0009 0.0367 0.0022 Scrap 0.00083 0.0125 <0.0001 0.00003 <0.00006 0.00083 0.0582 0.0006 Table 2. Sample designations and additions of scrap returns to the base alloy. Sample Designation Base Alloy/wt% Base Alloy/g Scrap Return/wt% Scrap Return/g 226-100 100 2030.8 0 0 226-80 80 1607.0 20 414.5 226-60 60 1200.8 40 784.1 226-40 40 795.2 60 1204.7 226-20 20 395.1 80 1620.6 226-00 0 0 100 2030.7 Figure 1. ( a ) Melting in a steel crucible; ( b ) Casting from steel mould for mechanical testing; ( c ) Casting from Croning cell, sectioned for different tests. Crystals 2023,13, 757 4 of 13 3. Results and Discussion 3.1. Chemical Composition The chemical composition was analysed for all six samples listed in Table 2. Figure 2a shows the change in chemical composition as a function of the proportion of scrap returns. The main alloying elements, such as Si, Cu, and Fe, varied randomly from sample to sample. In the case of Si, the variation was between 7.4 and 8 wt%, which was still within the range of standard values. Similar fluctuations were also observed for Cu. The reason for these differences in the values for Si and Cu was the addition of residues to the base alloy. The scrap had different shapes and sizes, e.g., the feeder systems were relatively large, which meant that solidification in the feeder systems during production of the castings took place at the lowest cooling rates, resulting in a coarse and inhomogeneous microstructure. When cutting samples from such feeder systems, the areas with more or less microstructural constituents such as eutectic βSi and Cu-rich Al 2 Cu and Q-AlCuMgSi phases were randomly selected, resulting in differences in the Si and Cu values. Figure 2. ( a ) The chemical composition of all samples; ( b ) The amounts of phases in all samples predicted by CALPHAD calculations. For other alloying elements, the situation was a little different. The contents of some elements that were susceptible to oxidation were reduced at higher additions of scrap returns. This meant that the liquid metal used for the casting process was kept in a furnace for a while before casting, where elements such as Ca, Mg, and Zr were oxidised. On the Crystals 2023,13, 757 5 of 13 other hand, elements such as Cr and Mn could also oxidise or form phases that settled on the bottom of the holding furnace due to the higher density or float in the oxide layer that was removed before casting. When the return scrap was remelted in the production of samples, oxidation took place again, resulting in a further decrease in the composition of the elements mentioned. In contrast, the contents of some elements were increased. These elements were B, Bi, Sn, Sr, Ni, Pb, Ti, and Zn. These elements were more stable and did not oxidise in the melt. Ti, B, and Sr are added to the melt with the intention of refining the grain and modifying the melt. From the first sample (226-100) to the last, which had 100% scrap return content (226-00), the contents of Ti, B, and Sr were increased by 14%, 50%, and as much as 66%, respectively. 3.2. CALPHAD Simulations The result of the CALPHAD calculations was an isopleth phase diagram, as shown for sample 226-100 in Figure 3a. It showed the solidification path of an alloy in which solidification started with the αAl phase, followed by Al15Si2M4, the iron-rich α -Al 15 (Fe,Mn) 3 Si 2 phase. Another iron-rich phase, Al 9 Fe 2 Si 2 , solidified next, also referred to as Al 5 FeSi and marked as β -AlFeSi [ 19 ]. Solidification proceeded with the main eutectic ( αAl + βSi ) phase in which solidification should end. According to the calculations, precipitation occurred in the Q-Al 5 Cu 2 Mg 8 Si 6 phase, which is also referred to as Q-AlCuMgSi. The next phase to precipitate was Al 2 Cu and the low-temperature Si 2 Sr phase. Figure 3b–e shows the plots of phase fractions versus temperature for samples 226-100 and 226-00. The phase fractions were slightly different in the two alloys, but differences were also observed in the temperature ranges of solidification and precipitation of the phases and in the order of solidification of the iron-rich phases. In the first sample, the iron-rich α -Al 15 (Fe,Mn) 3 Si 2 phase solidified first, followed by β -AlFeSi, but in sample 226-00, it was the other way around due to the different chemical composition. Figure 2b shows the calculated phase fractions for all samples and it can be seen that fractions of α -Al 15 (Fe,Mn) 3 Si 2 and Q-AlCuMgSi decreased as the amount of scrap returns in the alloy increased due to the lower content of Mn and Mg in the samples. The Scheil model of solidification for all samples is shown in Figure 4. The predicted solidification path was similar to that described above, with the difference being that the Cu-containing phases solidified at the end of the solidification range and did not precipitate out of the solid. The difference in the solidification of the Cu-rich Q-AlCuMgSi and Al 2 Cu phases was evident in the second sample (226-80), which contained 20% scrap returns. In the sample without scrap returns (266-100), Q-AlCuMgSi solidified first and then Al 2 Cu, but the order was reversed in sample 226-80, so that Al 2 Cu solidified before Q-AlCuMgSi. The reason for this must have been the lower Mg content and different Cu content, as mentioned above. Similarly, there was another change in the order of solidification of Fe-bearing phases, as described in the equilibrium isopleth phase diagram. In samples 226-100 to 226-40, the first of the iron phases to solidify was α -Al 15 (Fe, Mn) 3 Si 2 and the second was β -AlFeSi, but the order of solidification changed in the samples containing more than 80% scrap returns (226-20 and 226-00). It can also be seen that after the main eutectic ( αAl + βSi ) solidification ended, the β -AlFeSi phase disappeared. Figure 4shows the presence of the AlB 2 , Al 3 Ti, and Si 2 Sr phases. The reason for this was that B, Ti, and Sr were also included in the calculations, resulting in the aforementioned phases. The rest of the solidification was unaffected and the phases were not the object of the study. 3.3. Thermal Analysis Thermal analysis was carried out for all six samples. Figure 5shows the cooling curves and derivatives of samples 266-100 and 266-00, respectively. All cooling curves showed similar behaviours. The differences in all characteristic temperatures were relatively small, e.g., the liquid temperature ranged between 594.1 ◦ C and 595.3 ◦ C. Higher differences were found in the main eutectic region and the second eutectic region, reaching up to 5.5 ◦ C Crystals 2023,13, 757 6 of 13 difference. The derivative curves of all samples were also similar, except that the last peak changed with higher additions of scrap returns and split into two peaks. It was assumed that the reason for this was the changed order of solidification of the Cu-rich phases and the lower amount of the Q-AlCuMgSi phase. Figure 3. ( a ) Isopleth phase diagram for sample 226-100; ( b ) Phase fraction versus temperature for sample 226-100; ( c ) Magnification of ( b ); ( d ) Phase fraction versus temperature for sample 226-00; (e) Magnification of (d). Crystals 2023,13, 757 7 of 13 Crystals 2023, 13, x FOR PEER REVIEW 7 of 13 Figure 4. Scheil model of solidification of: (a) Sample 226-100; (b) Sample 226-80; (c) Sample 226-60; (d) Sample 226-440; (e) Sample 226-20; (f) Sample 226-00. 3.3. Thermal Analysis Thermal analysis was carried out for all six samples. Figure 5 shows the cooling curves and derivatives of samples 266-100 and 266-00, respectively. All cooling curves showed similar behaviours. The differences in all characteristic temperatures were relatively small, e.g., the liquid temperature ranged between 594.1 °C and 595.3 °C. Higher differences were found in the main eutectic region and the second eutectic region, reaching up to 5.5 °C difference. The derivative curves of all samples were also similar, except that the last peak changed with higher additions of scrap returns and split into two peaks. It was assumed that the reason for this was the changed order of solidification of the Curich phases and the lower amount of the Q-AlCuMgSi phase. Figure 4. Scheil model of solidification of: ( a ) Sample 226-100; ( b ) Sample 226-80; ( c ) Sample 226-60; (d) Sample 226-440; (e) Sample 226-20; (f) Sample 226-00. 3.4. Metallography •SEM SEM micrographs are shown in Figure 6with the corresponding EDS analyses. The same microstructural constituents were determined in all six samples, only the fractions were changed, as shown below. Figure 6a shows phases such as αAl -matrix, βSi , α -Al 15 (Fe, Mn) 3 Si 2 , β -AlFeSi, and Al 2 Cu with the corresponding EDS analyses in Table 3. Figure 6b shows the Cu-containing Al 2 Cu and Q-AlCuMgSi phases and a heavy phase rich in Pb, Sn, and Ca, which was most probably oxidised since the amount of oxygen was relatively high. An additional Al 2 SrSi 2 phase was determined as a result of Sr enrichment. The actual compositions of the phases determined by the EDS analyses (Table 3) were not exact but Crystals 2023,13, 757 8 of 13 varied in elements and Al was present in almost all measurements. This was due to the nature of the analysis, which also analysed the background of the sample. The phases were determined on the basis of EDS analyses and previous reports [17,18,20–23]. Crystals 2023, 13, x FOR PEER REVIEW 8 of 13 Figure 5. Cooling curve and its derivative for samples (a) 226-100; (b) 226-00. 3.4. Metallography •SEM SEM micrographs are shown in Figure 6 with the corresponding EDS analyses. The same microstructural constituents were determined in all six samples, only the fractions were changed, as shown below. Figure 6a shows phases such as αAl-matrix, βSi, α-Al15(Fe, Mn)3Si2, β-AlFeSi, and Al2Cu with the corresponding EDS analyses in Table 3. Figure 6b shows the Cu-containing Al2Cu and Q-AlCuMgSi phases and a heavy phase rich in Pb, Sn, and Ca, which was most probably oxidised since the amount of oxygen was relatively high. An additional Al2SrSi2 phase was determined as a result of Sr enrichment. The actual compositions of the phases determined by the EDS analyses (Table 3) were not exact but varied in elements and Al was present in almost all measurements. This was due to the nature of the analysis, which also analysed the background of the sample. The phases were determined on the basis of EDS analyses and previous reports [17,18,20–23]. Figure 5. Cooling curve and its derivative for samples (a) 226-100; (b) 226-00. Figure 6. SEM micrographs with marked spots of EDS analyses: ( a ) Sample 226-00; ( b ) Sample 226-100; (c) Sample 226-20. •Optic metallography Optical metallography showed similar results to SEM, but some phases and their distributions were clearly visible in the optical micrographs. Figure 7shows optic micrographs of all six samples at lower magnification. The green and red arrows show the Fe-rich phases. The green arrows show the Al 15 (Fe, Mn) 3 Si 2 phases, which were in the form of Chinese script, and the red arrows show the needle-like β -AlFeSi phases. In the sample Crystals 2023,13, 757 9 of 13 without scrap returns there were no needle-like phases, but as the content of scrap returns in the samples increased, the needle-like phases appeared and the proportions became higher due to different Mn:Fe ratios as the Mn content decreased. Very few needle-like phases appeared in samples 226-100 and 226-80, but the first representative needle-like phase appeared in sample 226-60, in which the Mn:Fe ratio was 0.38, and the amount of the phase increased as the ratio was further decreased to 0.30 in sample 226-00. Table 4shows the results of the number and measured areas of the needle-like phase on two micrographs for each sample. In samples 226-100 and 226-80, 7 and 5 needles were found, respectively, but the number of needles increased to more than 30 in samples with higher amounts of foundry scrap returns. Table 4also shows the area percentages of the needle-shaped phase in the samples. It can be seen that the amount increased from 0.11 area percent in sample 226-100 to 0.69 area percent in sample 226-00. The Chinese script-like Al 15 (Fe, Mn) 3 Si 2 phase did not show the decreasing trend as would be expected due to the inhomogeneous distribution of phases in the microstructure and the phases were not captured representatively in the optic micrographs. Table 3. EDS analyses in at% and determined phases from Figure 6. ·Phase O Al Si Cr Mn Fe Cu Zn Mg Ca Sn Pb Sr 1 Al2Cu 1.8 63.8 1.5 ···32.9 ······ 2α-Al15(Fe, Mn)3Si2·70.1 11.8 1.0 4.4 11.0 1.6 ······ 3β-AlFeSi ·65.1 20.8 ·1.5 12.7 ······· 4αAl-matrix ·97.7 1.5 ···0.5 0.3 ····· 5βSi ·27.6 72.1 ···0.2 ······ 6 Pb-rich phase 41.9 11.1 4.7 ···2.9 · · 2.3 3.2 29.8 · 7 Al2Cu 1.3 19.7 31.2 ···10.9 ·4.2 · · · · 8 Q-AlCuMgSi 1.2 78.6 1.1 ···19.1 ·36.9 · · · · 9 Al2SrSi22.1 31.8 52.9 ······0.7 · · 12.6 10 Pb-rich phase ·6.9 91.1 ·····0.6 ·0.5 0.5 0.5 11 βSi ·5.4 94.6 ·········· Table 4. The number and amount of needle-like β-AlFeSi phase in all samples. Sample Designation Area/µm2Area% No. 226-100 628.4054 0.109109 7 226-80 681.3081 0.118121 5 226-60 732.128 0.123589 11 226-40 2630.312 0.457297 31 226-20 2697.656 0.468391 32 226-00 4003.538 0.690667 33 3.5. Mechanical Properties •Tensile tests Thirty-five days after casting the samples, the four specimens of each alloy sample underwent tensile testing. According to the standard SIST EN ISO 6892-1 A224, the tensile strength (Rm), yield strength (R p0.2 ), elongation (A), and modulus of elasticity (E) were determined. The average values were calculated from four measurements and are shown graphically in Figure 8a. In addition, linear correlations were calculated, as shown in Figure 8a. It can be seen that all of the measured properties decreased with increasing amounts of scrap material, except for a slight increase in elongation, but the scatter of the results was relatively large so the increase was insignificant. The decrease in all measured