Solidi cation and Microstructure-Mechanical Behavior of Al-Si-Mg(Cu) Alloys with Ti, Zr, V, Cr and Cu Additives

Abstract

Most secondary aluminium alloys have varying chemical compositions due to presence of various elements that are added deliberately (such as through alloying) and or accidentally (such as through tools or equipment used). The desirable or undesirable e ects of these elements make it inevitable to determine how secondary alloy properties are a ected by their individual and combined additions. It is through such researches that secondary aluminium alloys, characterized by light weight, excellent castability and formability, and recyclability, can also be considered the de-facto standard for the manufacture of premium components. In this research, the e ect of some transition elements on the mechanical performance (Ti, Zr, V and Cr)and the solidi cation characteristics (Ti, Zr, V, Cr and Cu) of 356 Al-Si alloys that are HIPed and T6 heat treated and as-cast respectively are investigated. The base alloy, alloy 356, was developed followed by preparation of ve of its variants through adjustment of transition elements, whose e ects were under investigation. The alloy variants were named 356X, 356XCr, 356X0.5Cu, 356X3.5CuCr, and 356TiSr based on their chemical composition. X denoted 0.15%Ti + 0.15%Zr + 0.25%V + 0.015%Sr, Cr ≡ 0.15%Cr and TiSr = 0.15%Ti + 0.015%Sr. The tests performed were: microscopy (optical and scanning electron microscopy), thermal analysis, fatigue, and tensile. Microscopy was used to: identify the best statistical probability plots for the alloys' particle area, determine the relationship between various particle shapes characteristics and in the prediction of microstructure-mechanical performance properties on combined addition of some transition elements to alloy 356. Microcopy tests revealed that (AlSi)x(TiZr)Fe intermetallics together with other phases containing Zr, Ti, V, Zr, and Cu additives were present. Fractography conducted on tensile specimens showed that fracture occured in a brittle-ductile manner as characterized by occurence of both dimples and cracked intermetallics. It was clear that the 3-parameter Lognormal plot gave the best distribution plot both for the entire particle area and for 0.1% of the largest particle areas. Quantitative microscopy revealed that: the relationship between maximum Feret dimensions and deq is not perfectly linear, a strong correlation existed between circularity and maximum Feret dimension/or area and that aspect ratio and maximum Feret dimension/or area had no clear relationship. These microscopy results can help in the prediction of future related properties. Thermal analysis tests were conducted on all the six alloys at a cooling rate of 0.87 oC/s so as to identify solidi cation characteristics of the alloys (α-Al, DCT, Al-Si, Al-Si-Cu and solidus temperatures). The range of these temperatures was typical to most 356 alloys. It was also evident that X and ZrV additions decreased solidus temperatures while all other additions (XCr, TiSr, X0.5CuCr and X3.5CuCr) increased it. Also, X, ZrV and TiSr decreased α-Al nucleation temperature and DCT with the decrease by X being greatest followed by ZrV and nally TiSr. XCr increased: α-Al nucleation temperature, Al-Si and solidus temperatures while X+Cu+Cr addition decreased DCT with a higher decrease registered at lower Cu contents. From these results, optimum process temperatures such as solution heat treatment temperatures (500 oC) and xv pouring temperatures informed by start of solidi cation temperatures were recommended. Fatigue and tensile tests were also conducted on alloys 356, 356X and 356XCr at room (25 oC) and at both room and high (237 oC) temperature respectively. Fatigue strengths were obtained at 107 cycles (60 Hz and R = -1). High fatigue strengths obtained (>65 MPa) were associated with hipping, heat treatment, grain re nement, modi cation and alloying using strength forming precipitates such as Cr-related ones in α-Al matrix. Generally, addition of XCr to 356 improved tensile and fatigue performance of 356 alloys than TiSr or sole X. X addition to alloy 356 was considered unfavourable for cases where high ductility is a prerequisite while XCr was termed unsuitable for high temperature elongation applications. Lastly, X and XCr additions would be ideal for high strength applications both at room temperature and at 237 oC. From these tensile and fatigue test results, e cient designs can be made out of alloys with a similar chemical composition to those investigated. Overall, this research contributes to thermal analysis softwares by providing data speci c to the tested set of alloys whose composition has not been tested before. Additionally, it also gives tensile and fatigue performance of unique alloy-compositions, as far as the intentional combination and quantities of Ti, Zr, V, and Cr are concerned. This research thus adds to the body of knowledge.