Effect of Mn and heat treatment on improvements in static strength and low-cycle fatigue life of an Al–Si–Cu–Mg alloy

Abstract

Tensile and low cyclic fatigue properties were used to assess the influence of Mn additions and heat treatment on the performance of the Al–7Si–1Cu–0.5Mg (wt%) alloy. As a reference, the Mn-free base and the currently used automotive alloys A380 and A319 were used. In the as-cast state, the Mn-modified alloy consisted of the α-Al, eutectic Si, and Cu- and Mg-based phases including Al_2.8Cu, Al_14.9SiCu_4.9 and Al_13.8Si_4.5CuMg₄. In addition, the Mn-rich phases of Al_59.2Si_9.2CuFe_3.4Mn_6.3, and Al₂₆Si_4.1Cu_1.1MgFe_1.4Mn_2.6 were present after casting. During solution treatment, Cu-based phases were completely dissolved, while the eutectic Si and Mg-rich phases experienced partial dissolution. Some of the Mn-rich phases were partially dissolved while the Al_59.2Si_9.2CuFe_3.4Mn_6.3 compound transformed to Al₁₅(FeMn)₃(Si)₂. The Mn-modified alloy after T6 heat treatment achieved an elongation of 13.5% with an ultimate tensile strength of 355 MPa, which is ~18 MPa higher than the Mn-free base and ~30% higher than the A319 alloy after similar heat treatment. The fatigue life of the Mn-modified alloy was also substantially longer than that of the reference base and the A319 grade. After failure, the Mn-modified alloy exhibited ductile fracture with major cracks propagated through inter-dendritic regions of the primary Al-phase with fragmented intermetallics. It is believed that the Mn-rich dispersoid precipitates, formed as a result of T6 heat treatment, improved performance of the Mn-modified alloy.

Introduction

Cast aluminum-silicon alloys are widely used in the automotive industry, due to their high strength to weight ratio, excellent castability, good corrosion resistance and recycling properties [1], [2], [3], [4]. However, the properties of presently used alloys are not sufficient for future applications aimed at fulfilling the Corporate Average Fuel Efficiency (CAFE) regulations. To further reduce the engine size and make lightweight vehicles, higher strength Al–Si alloys are necessary. The common approach to improving alloy strength is addition of alloying elements which form dispersoid intermetallics leading to the modification of its microstructure [5]. Further improvements may be achieved through changes of alloy phase constituents by heat treatment or additions of modifiers such Sr, Na, Ca, Mn etc. [6], [7], [8], [9]. It is often reported that Al–Si alloys contain the detrimental impurity of Fe which forms brittle and complex needle-shaped intermetallics that significantly increase the tendency to form porosity, thus diminishing mechanical properties [10], [11]. As the solubility of Fe in the α-Al matrix is very low, during solidification it forms the Fe-rich β-Al₅FeSi needle-like phase which has sharp edges, causing severe stress concentrations leading to the reduction of the tensile properties, especially ductility of the alloys [12]. It is shown that additions of transition metals such as Ni, Cr or Mn change the morphology of the β-Al₅FeSi needle-like phase, improving properties of the Al–Si alloy [13], [14], [15]. At the same time, they form dispersoid intermetallics which hinder the movement of dislocations causing an improvement in alloy strength. Also, additions of Cu and Mg to the Al–7Si–0.3Mg–0.5Cu alloy improve mechanical properties [16].

Many researchers investigated the role of alloying elements such as Cr, Fe, Mn, Ni, Ti, V and Zr in cast Al–Si–Cu or Al–Si–Mg alloys in modifications of the microstructures and tensile properties [17], [18], [19], [20], [21], [22], [23], [24], [25], [14], [26], [27]. Additions of Ni to the Al–Si–Cu–Mg alloy improve the alloy strength by forming dispersoid phases of ε-Al₃Ni, δ-Al₃CuNi, γ-Al₇Cu₄Ni and T-Al₉FeNi [28]. The presence of Ni in the Al–Si alloy neutralizes Fe and reduces the tendency to form the Fe-rich β-Al₅FeSi needle-like phase [29]. At the same time, the Cu content in the α-Al matrix is reduced by forming phases between Ni and Cu, which leads to reduced precipitation hardening [18], [30]. On the other hand, the presence of brittle Al₉FeNi phases and other phases are seen as negative effect of Ni additions to the Al–Si based alloys [28]. It was reported that additions of Mn to the Al–Si alloy formed thermally stable dispersoids, improving creep properties [31], [5], [28]. However, there are no studies reporting on the role of Mn in changing the low cycle fatigue behavior, which is of key importance in the performance of automotive parts. Although studies on strain-controlled low-cycle fatigue (LCF) provide important input into the design of engineering components [32] most of the literature data are focused on tensile properties and high-cycle fatigue (HCF) behavior of Al alloys. To date, there have been no studies on the LCF resistance of the cast Al–Si–Cu–Mg alloys, in particular the Al–Si–Cu–Mg alloys modified with Mn.

Thus, the aim of this study was to investigate the influence of Mn additions on the deformation behavior of the Al–Si–Cu–Mg alloy under tensile and cyclic loading conditions. The fatigue mechanism related to various strain amplitudes under LCF conditions was also analyzed in detail and compared with the Mn-free base alloy and the A380 and A319 reference grades.