Effects of heat treatment and addition of small amounts of Cu and Mg on the microstructure and mechanical properties of Al-Si-Cu and Al-Si-Mg cast alloys

Highlights

• The effects of heat treatments on the microstructure of Al-Si-Cu and Al-Si-Mg cast alloys were studied.

• Mg addition modifies the form of the eutectic Si and causes precipitation of the M-Mg₂Si and π-Al₈Mg₃FeSi₆ phases.

• The solution heat treatment leads to fragmentation, spheroidization and coarsening of the silicon particles.

• The Al-Si-Mg alloy has a higher age-hardening response than the Al-Si-Cu alloy.

• The results of the micro-hardness and XRD analysis are in good agreement with each other.

Abstract

The present study investigates the effects of heat treatment on the microstructures and mechanical properties of Al-Si-Cu and Al-Si-Mg alloys containing small amounts of added copper and magnesium, with the aim of obtaining the maximum alloy hardness via the adjustment of the solution heat treatment and aging. The solution heat treatment was performed at 500 °C and 525 °C for the Al-Si-Cu alloy and at 525 °C for the Al-Si-Mg alloy. The samples of both alloys were solution treated for various times between 5 min and 16 h, followed by quenching and finally an artificial aging was performed at 210 °C for a time ranging from 10 min to 50 h. The microstructures of the as-cast alloys and heat-treated samples were studied using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and hardness tests. The results show that the solution treatment time significantly affects the coarseness of the microstructure, causes the dissolution of the intermetallic phases, and homogenizes the distribution of copper and magnesium in the matrix. The peak-aged Al-Si-Cu alloy was obtained after a solution heat treatment at 525 °C for 15 min followed by 2 h of aging. The peak-aged Al-Si-Mg alloy was obtained after 8 h of solution heat treatment at 525 °C followed by 40 min of aging. The Al-Si-Mg alloy with the 0.6 wt% Mg addition displayed an age-hardening response that was approximately 34% higher than that of the Al-Si-Cu alloy with a 0.3 wt% Cu addition.

Introduction

The mechanical properties of aluminum alloys depend of many factors, including the alloy composition, casting method, solidification rate, presence of iron intermetallics and Si content. A high level of Si increases the ductility of the aluminum, results in a stronger response to aging, and tends to refine the grain size of the intermetallics [1]. However, the strength of the alloy is primarily related to the content of (Mg + Cu) and the heat treatment, which both influence the precipitation strengthening and the volume fraction of Cu- and Mg-rich intermetallics [2].

In Al-Si alloys with added Fe, Mn, Cu and Mg, the following phases may be present: θ-Al₂Cu, M-Mg₂Si, β-Al₅FeSi, Q-Al₅Cu₂Mg₈Si₆, π-Al₈FeMg₃Si₆ and α-Al₁₅(FeMn)₃Si₂ [3]. The iron phases are considered the most harmful in these alloys because of their deleterious effects on ductility and corrosion resistance. Therefore, the iron content is maintained as low as possible, usually 0.6–0.7 wt% Fe. According to Mondolfo [4], if the content of (Mn + Fe) is above 0.8 wt%, then the α-Al₁₅(FeMn)₃Si₂ crystals will be dominant, giving hexagonal globules (polyhedral).

During solidification, the iron phases may solidify into three distinctly different morphologies: needle-like morphology (β phase), Chinese-script morphology (α phase), and star-like or polyhedral morphology (primary α phase) [5]. In general, the primary β phase has a harmful influence on the mechanical properties compared to the α phases. The crystallization of this phase can be minimized by rapid solidification, manganese addition, and melt superheating [6]. In fact, the choice of chemical composition (Fe/Mn ratio) and a high cooling rate have been shown to completely suppress the β-Al₅FeSi phase [7].

In addition, the primary crystals, α-Al₁₅(FeMnCr)₃Si₂, commonly termed ‘‘sludge”, possess high melting points and high densities after crystallization [8]. Moreover, when the sludge crystals are entrained into castings, they decrease the alloy fluidity [9] and appear as hard spot inclusions, which result in machining difficulties and degradation of the mechanical properties [10]. From the literature, the formation of sludge is generally controlled via the composition of Fe, Mn and Cr elements. This is given by the sludge factor (SF), which is empirically defined by the formula SF = % Fe + 2% Mn + 3% Cr ≥ 1.8 [11]. However, Shabestari [12] have shown that star-like intermetallics still form when the sludge factor is greater than 1.30, and a low concentration of 0.1 wt% Cr with more than 0.2 wt% Mn is needed to convert all of the β platelet phases into small star-like intermetallics, even with 1.2 wt% Fe. The formation and sedimentation of the sludge and its morphology are also a function of the superheating and the holding time, as mentioned in the literature [13].

The influence of the Cu content on the microstructure and hardness has been investigated by several researchers. As the Cu content increases, the hardness of the matrix and the tensile strength increase at the expense of ductility [14,15]. Shabestari et al. [16] found that in Al-Si-Mg alloys solidified in graphite molds, alloys with approximately 1.5 wt% copper have the best strength-related mechanical properties. A similar effect was found for Mg addition which improves the hardness and tensile strength [17] and modifies the silicon eutectic [18]. Moreover, increasing the Mg content to high levels (Mg > 1.0 wt%) can completely transform the β-Al₅FeSi needles into the π-Al₈Mg₃FeSi₆ Chinese-script morphology [19].

The main purpose of the solution heat treatment (SHT) is to obtain a supersaturated solid solution; however, a higher solution temperature could lead to the incipient melting of Al₂Cu phases in the Al-Si-Cu alloys. This phenomenon was studied by Samuel [20] for Al-Si-Cu-Mg alloys. The results showed that in low-magnesium alloys (0.04 wt% Mg), θ-Al₂Cu melts at 540 °C. Increasing the magnesium content (0.5 wt% Mg) reduces the incipient melting temperature of the Q-Al₅Mg₈Si₆Cu₂ phase to 505 °C. In addition, Jarfors et al. [21] reported that to avoid grain boundary melting, the restrictive solution temperature for Al-Si-Cu alloys should be limited to 525 °C for 1 wt% Cu and 495 °C for more than 2 wt% Cu; the authors also revealed that Al-Si-Mg alloys permit higher solution temperatures than the Al-Si-Cu-Mg alloys.

Artificial aging is the final stage in the development of the properties of heat-treatable aluminum alloys. Aging typically occurs at an elevated temperature of 170–210 °C [22], and its response depends primarily on the degree of supersaturation of the Mg and Cu - the main parameters expected to control the mechanical properties of the aluminum alloys - in the α-Al matrix after the SHT [23].

The present research investigates the effect of the times used for SHT and aging with the minimum Cu and Mg contents on the hardness of Al-Si-Cu and Al-Si-Mg alloys. Optical microscopy and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) were used to identify the intermetallic phases. The formation of different precipitates was studied using X-ray diffraction (XRD). The effects of the hardening mechanisms on alloys with small amounts of Cu and Mg were analyzed and discussed to determine the best times for the SHT and aging processes of the typical applications of these alloys.