Effect of vanadium on Fe-rich intermetallics in HVDC Al-8Si-0.2Mg-0.3Fe alloys

Al-Si based alloys were widely applied to manufacturing automobile structural components such as wheel hubs and shock absorber towers because of their high strength and castability[1][2]. In the interest of environmental protection and energy conservationusing recycled aluminum alloys for automotive structural components is an effective way to lightweight vehicles. Howeverthe Fe content in aluminum alloys rises significantly with increasing recycling times[3]. Fe has low solubility (0.04 %-0.052 %) in aluminum alloys. This often leads to the formation of harmful Fe-rich intermetallicsseverely degrading the mechanical properties[4]. Based on the composition and morphologythe Fe-rich intermetallics are usually divided into β-Al₅FeSi (β-Fe) with platelet-like morphologyα-Al(FeMn)Si (α-Fe)with Chinese script or polyhedral-like morphology[5]. The characteristics of Fe-rich intermetallics in Al-Si alloys such as morphology and size were significantly dependent on the solidification process and alloy composition[6]. Platelet-like β-Fe can cause stress concentration during tensile deformationseriously harming the alloy's ductility[7]. Thusmodifying the morphology of Fe-rich intermetallics is essential to reduce their negative impact on mechanical properties. Both physical and chemical routes were applied to modify the characteristics of Fe-rich intermetallics. The platelet-like morphology of β-Fe could be modified by physical routes such as rapid cooling[5]superheat[8]and ultrasonic treatment[9]. Howeverthese processing methods are not suitable for industry settings[5][10]. Chemical methodssuch as microalloying with transition elementsare more suitable for industrial production. Adding neutralizing elements such as Mn and Cr can transform platelet-like β-Fe into Chinese script α-Al(FeMn)Si [11][12]. MeanwhileFan et al. found that the Fe content in α-Al(FeMn)Si increased with the decrease in formation temperature[13]. Howeverthe addition of a neutralizing element also leads to the amount of Fe-rich intermetallics increasing obviously. The modified Fe-rich intermetallics usually clustered together and therefore also seriously deteriorated the ductility of Al-Si alloys. Thereforeit is important to find a suitable neutralization element that can modify the morphology and limit the quantity of Fe-rich intermetallics simultaneously.

High vacuum die casting (HVDC) is highly versatilefeaturing high production efficiency and quality. As a resultit’s widely used to manufacture automobile structure components[14]. The solidification process in HVDC usually divides into equilibrium solidification in the shot sleeve and rapid solidification in the die cavity[15]. Thereforethe solidification of Fe-rich intermetallics in the HVDC process is significantly different from that in traditional gravity casting. Fe-rich intermetallics formed in the shot sleeve were defined as primary Fe-rich intermetallicswhile those formed in the die cavity were defined as secondary Fe-rich intermetallics[16]. The significantly different solidification conditions in the shot sleeve and die cavity resulted in the different characteristics of Fe-rich intermetallics. Consequentlythey have notably different effects on mechanical properties.

Vanadium microalloying is a beneficial way to modify the mechanical properties of different alloys. Wang et al. revealed the grain refinement mechanism of aluminum alloy by vanadium addition. Their findings showed that adding 0.3 wt% vanadium led to a columnar to equiaxed transition. This was because Al₁₀V particles provided high nucleation potency for aluminum grains[17]. Derya et al. revealed that vanadium microalloying could improve the ultimate tensile strength of high vacuum die casting A360 alloybut it would reduce its ductility[18]. Luo et al. also indicated that the appropriate content of vanadium addition could refine the grains of Al-Cu-Mg-Ag alloy. Howeveran excessive amount of vanadium often led to the formation of the V₂Mg₃Al₁₈ phaseultimately decreasing the ductility[19]. Kumar et al. studied the effect of vanadium addition on mechanicaltribologicaland microstructure properties of Al-Si alloy. Their results revealed that the tensile strength and elongation of the vanadium-added specimens were greater than the base Al-Si alloy[20]. The above studies concluded that the improvement of mechanical properties after vanadium addition was mainly attributed to its ability to refine aluminum grains. Howeverthe mechanical properties of cast aluminum alloys not only depended on the grain size of α-Al but also the intermetallics such as platelet-like β-Fe and Chinese script α-Al(FeMn)Si. Lin et al. studied different V additions on the Fe-rich intermetallics of A356 cast alloys. They found that 0.8 wt% addition of vanadium in A356 cast alloys could modify the morphology of Fe-rich intermetallics from needle-like to Chinese-scriptthus improving the mechanical properties[21]. Dæhlen et al. further revealed that element V accumulated in the α-Al(FeMn)Si phaseand its content increased with increasing V-content in the alloy[22]. HoweverThomas indicated that element V preferably accumulated in β-Fe intermetallics in foundry Al-Si alloys[23]. The microalloying of vanadium on the crystal structure and characteristics of Fe-rich intermetallics have not been clarified. More importantlyrecycled Al-Si alloys inevitably contain impurities such as Fe and Vand removing these impurities is costly. Thereforerevealing the interaction between Fe and other impurities elements is crucial for using recycled Al-Si alloys to produce automotive structural components.

In this studythrough a combination of experimental methods and first-principles calculationsthe impact of vanadium microalloying on the crystal structure and morphology of Fe-rich intermetallics in HVDC Al-8Si-0.2Mg-0.3Fe-xMn-(0.6-x)V (x = 0.30.40.50.6) alloys was comprehensively investigated. The three-dimensional characteristics including clustered characteristicsdistributionand size were systemically discussed to reveal the nucleation and growth mechanism of Fe-rich intermetallics during the solidification process.