Effects of Cr-addition on ageing response of an Al–Si–Mg die cast alloy

Yiwu Xu ab1, Yuxin Zhang c1, Jinping Li d, Pan Wang e, Jianfeng Wang e, Houwen Chen c, Houqing Sun f, Hongyi Zhan e


Chromium (Cr) is rarely added to Al–Si die cast alloys as it has a strong tendency to induce the formation of coarse-sized sludge particles. In the present study, high pressure die-cast samples made of Al-7wt.%Si–Cr–Mg alloys with varied Fe contents (0.11 wt%, 0.26 wt% and 0.32 wt%) were tensile tested under as-cast and T5 heat-treated conditions, in comparison to those made of Al-7wt.% Si–Mn–Mg alloy with a constant Fe content (0.11 wt%). The results demonstrated that under the as-cast condition, the Al–Si–Cr alloys with higher Fe contents can achieve comparable tensile properties as the Al–Si–Mn alloy with strictly controlled Fe level. However, after a T5 heat treatment (205 °C for 60 min), the increment in yield strength is less for the Al–Si–Cr alloys in relative to the Al–Si–Mn alloy, regardless of Fe contents. Scanning electron and scanning transmission electron microscopic characterizations were conducted on the as-cast and heat-treated microstructures, aiming to understand the undermined ageing response caused by Cr addition.


A new emerging trend in the automotive manufacturing is to adopt highly integrated one-piece aluminum (Al) gigantic casting (>50 kg/unit, also known as giga-casting) made by high pressure die casting (HPDC) process to replace welded steel stampings in front and rear body structures [1]. Giga-casting will bring benefits including simplified manufacturing process, reduced capital investment in launching a production line as well as light-weighting effect. To guarantee an excellent fracture resistance as required by riveting with steel sheet and crash performance, giga-casting is preferably made using primary Al from electrolysis process which emits significant greenhouse gas (GHG). To contribute to zero-emission vision of the automotive industry, it is necessary to explore economical approach to reduce GHG emission of giga-casting. Applying recycled Al scrap instead of primary aluminum as raw material will significantly reduce the GHG emission of Al giga-casting as remelting recycled Al scrap only emits <5% GHG as that of primary Al production [2,3]. However, applying a high fraction of scrap may introduce undesirable impurity elements, especially iron (Fe) [4,5]. Hence, to accommodate more scrap in production, it is necessary to suitably increase Fe tolerance in the alloy used for giga-casting. However, there is a concern that a higher Fe content may introduce more brittle Fe-containing intermetallic compounds (IMCs) degrading mechanical properties [[6], [7], [8]].

Currently, the alloys used in the fabrication of high-integrity die casting (HIDC) can be categorized into Al–Si and Al–Mg alloy families. Al–Mg series alloy has a better fracture resistance and tensile ductility in the as-cast state in relative to Al–Si series alloy [[9], [10], [11], [12]]. However, the castability (e.g., fluidity and tendency to hot-cracking) of Al–Mg series alloy is inferior to Al–Si series alloy [11]. Hence, to achieve better design flexibility, Al–Si series alloy is still the workhorse in the fabrication of HIDC. In the development history of Al–Si die cast alloys, an important milestone is the invention of Al–Si–Mn series alloy in which Mn-addition of over 0.5 wt% is used to provide die-sticking resistance instead of Fe-addition of over 0.8 wt%. Researchers from Alcoa and Rheinfelden discovered the anti-die-sticking function of Mn-addition independently [13]. In comparison to Al–Si–Fe series alloys (A380, A360, ADC12, etc.), Al–Si–Mn series alloys (A365, C611, Aural5s, etc.) are more ductile and fracture-resistant as the very coarse and brittle Fe-rich IMCs of platelet morphology, which are commonly seen in the microstructure of Al–Si–Fe series alloys, are transformed to block- or script-shaped Fe-rich IMCs in the microstructure of Al–Si–Mn series alloys [13]. In addition, due to the strict control of Fe content (<0.2 wt%) in Al–Si–Mn series alloys, the total volume of Fe-rich IMCs in Al–Si–Mn series alloys is much smaller in relative to Al–Si–Fe series alloys.

In fact, the total volume of Fe-containing IMCs is not only affected by Fe content but also dependent on Mn content [4]. A strategy for developing scrap-compatible Al–Si die cast alloy is to apply another element which can impart Al–Si alloy stronger die-soldering resistance than Mn. And hence a smaller amount of such element can replace Mn-addition, which will allow a higher Fe content without increasing the total volume of brittle Fe-rich IMCs. It was reported that Cr addition has a stronger capability than Mn addition in reducing die-sticking tendency of Al–Si die cast alloys [14]. Hence, Al–Si–Cr series die cast alloy [15] is promising to tolerate a higher impurity Fe content while achieving the same mechanical properties as Al–Si–Mn series die cast alloy.

The influence of Cr-addition on the microstructure and mechanical properties of Al–Si alloy castings produced by low pressure die casting (LPDC) process has been studied by the authors [16,17]. During LPDC process, Cr-addition tends to transform Fe-rich IMCs located in the inter-dendritic region from platelet-shaped Al–Fe–Si phase to block-shaped Al–(Cr, Fe)–Si phase. In addition, Cr solutes supersaturated in the Al matrix in the as-cast microstructure tend to precipitate out in the form of Cr-containing dispersoids during solution heat treatment. Though Cr-containing dispersoid itself hardened the Al matrix moderately, coarse Mg–Si particles nucleated on it during the quenching process resulted in a reduced supersaturation of Mg and Si solutes in the Al matrix. Hence, the presence of Cr-containing dispersoid undermined the ageing response to T6 heat treatment. For HIDC, T6 heat treatment does not apply as solution heat treatment conducted at elevated temperatures (>500 °C) may induce severe distortion of thin-walled casting [12]. Artificial heat treatment conducted at relatively low temperatures (<250 °C) following casting process (i.e., T5 heat treatment) is usually applied to further improve strength property of HIDC. Whether Cr addition will affect ageing response of the alloy in T5 heat treatment or not remains unclear. Hence, the present study aims to explore the effects of Cr-addition on the ageing response of Al–Si–Cr die cast alloy in T5 heat treatment.

Section snippets

Casting process and heat treatment

Table 1 lists the nominal chemical compositions of the alloys prepared in the present study and the corresponding actual chemical compositions measured by spark optical emission spectrometer (Spark-OES). A LK DCC400 cold chamber die casting machine with a locking force of ∼400 ton was employed to produce 30 castings for each composition. During the casting process, melt temperature in the holding furnace was controlled at 700 ± 5 °C. Key processing parameters including slow shot speed, fast

Mechanical properties

Tensile stress-strain curves for the alloys in the as-cast (F) and T5 heat-treated states are shown in Fig. 1a and b, respectively. According to Fig. 1, the averaged 0.2% proof stress (RP0.2), averaged ultimate tensile stress (UTS) and averaged elongation at fracture (EL) of the alloys have been summarized in Table 2. To visualize the difference in tensile properties of the alloys, bar charts of average RP0.2, UTS and EL are plotted in Fig. 1c–e, respectively. It is found that RP0.2 and EL of


Based on SEM characterization results (Fig. 3, Fig. 4, Fig. 5), the overall as-cast microstructures of AlSiCr0.3Fe and AlSiMn0.1Fe alloys are similar. The main microstructural difference was that the total area fraction of block-shaped primary Fe-rich IMCs was larger in AlSiCr0.3Fe alloy in relative to AlSiMn0.1Fe alloy. To understand this phenomenon, Scheil solidification paths of AlSiCr0.3Fe and AlSiMn0.1Fe alloys have been calculated using PANDAT software with the PanAl2021 database as shown 


A systematic study on microstructures and tensile properties has been conducted for as-cast and T5 heat-treated Al–Si–Cr die cast alloys with varied Fe contents, as compared with a baseline Al–Si–Mn die cast alloy. The main conclusions can be drawn as follows:

  • 1.Under as-cast condition, the Al–Si–Cr alloys with much higher Fe contents achieved comparable tensile properties as the baseline Al–Si–Mn alloy with a tight control on Fe content, despite of slightly higher volume fractions of primary

CRediT authorship contribution statement

Yiwu Xu: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft. Yuxin Zhang: Data curation, Formal analysis, Investigation, Methodology, Writing – review & editing. Jinping Li: Data curation, Investigation, Methodology. Pan Wang: Investigation, Methodology. Jianfeng Wang: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. Houwen Chen: Investigation, Methodology, Writing – review & editing. Houqing Sun: Methodology, Writing – review &

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


The authors acknowledge the support from General Motors Global Research and Development, Warren, MI, USA.

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