Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

Abstract

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg2Si‐containing eutectic structure, along with Al8(Fe,Mn)2Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.

Cyclic Deformation Behavior of A Heat-Treated Die-Cast Al-Mg-Si-Based Aluminum Alloy

“…Three pole figures (i.e., {100}, {110} and {111}) are calculated as shown in Figure 5c. A fairly random texture was observed in the material after heat treatment, being similar to that in the as-cast state [16]. Figure 5 shows the EBSD orientation maps of the alloy in the heat-treated condition.…”

“…This is mainly attributed to the spheroidization of both types of intermetallic phases (Mg 2 Si and Al 8 (Fe,Mn) 2 Si) in the heat-treated condition, being similar to the spheroidization phenomenon of cementite (or iron carbide Fe 3 C) in pearlitic steels to form the softer and more ductile spheroidite. The EDS analysis of the as-cast alloy can be found elsewhere [16]. Magnesium content in general is primarily responsible for high strength-to-weight ratio, as Mg combines with other elements to form second-phase particles, e.g., Mg 2 Si participles [29,30].…”

“…Typically, Al-Mg-Si alloy contains primary α-Al, magnesium silicide (Mg 2 Si) and AlFeSi phases during solidification [15]. Recently, the authors presented the low-cycle fatigue (LCF) characteristics of cast AlMgSiMnFe alloy [16], but it is unknown how heat treatment would affect the microstructure and fatigue behavior. The non-equilibrium nature of solidification (i.e., high cooling rates, pouring velocities and nucleation) leads to micro-segregation and formation of a wide range of intermetallic phases [15,17].…”

Quantitative relationship between microstructure characteristics and fatigue parameters of A319 casting alloy

“…However, as implied in Figure 6B,C, the difference of hysteresis loops between samples solidified at different cooling rate is not obvious for both Srmodified or unmodified conditions; this may be due to limited plastic deformation under 0.3% total strain amplitude. 22 The spacing between fatigue striations in the sample at high cooling rate was observed to be much smaller than the one at low cooling rate, indicating dislocations encountered more obstacles in samples with smaller SDAS due to more frequent interaction between dislocations and grain boundaries. Three regions can be clearly distinguished on fracture surface of all samples, namely the fatigue-crack-initiation region (FCI), the steady-crack-propagation region (SCP), and fast-fracture region (FF).…”

“…Fatigue striations are productions of a repeated blunting-sharpening process because of dislocation slipping in plastic zone in front of the fatigue crack tip in face-centered cubic materials, and the spacing between fatigue striations represents the rate of crack propagation. 22 The spacing between fatigue striations in the sample at high cooling rate was observed to be much smaller than the one at low cooling rate, indicating dislocations encountered more obstacles in samples with smaller SDAS due to more frequent interaction between dislocations and grain boundaries.…”