Date of Award

6-23-2018

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

A380, Aluminum Alloy, Nickel, Strontium

Supervisor

Hu, Hongfa

Rights

info:eu-repo/semantics/openAccess

Abstract

With the advent of downsized engines, castable high strength aluminum alloys for lightweight engine components are urgently needed for the replacement of cast iron. The aim of this study is to develop new casting aluminum alloys and processing technologies for the production of lightweight powertrain components with as cast high strengths. The first stage of this study was to develop an appropriate casting method which was advantageous to increase the mechanical properties of aluminum alloy A380. Squeeze casting was found to be effective for the elimination of porosity in aluminum alloy A380 with a relatively thick cross section compared with the conventional high pressure die casting process. Mechanical properties such as ultimate tensile strength (UTS) and elongation to failure (Ef) was enhanced up to 215.9 MPa and 5.4% respectively over those of the conventional high-pressure die cast part (UTS: 173.7 MPa, Ef::1.0%). The analysis of tensile behavior showed that the squeeze cast A380 exhibited a high tensil toughness (8.5 MJ·m3) and resilience (179.3 kJ·m3) compared with the die cast alloy (toughness: 1.4 MJ·m3, resilience: 140.6 kJ·m3). To meet the requirement of high temperature environment for automotive application components, transition alloying element nickel (Ni) was added into the aluminum alloy A380. The results of tensile testing on the Ni-containing A380 alloys showed that Ni was an effective additive for improving mechanical properties. As the Ni addition increased from 0 to 2.0 wt.%, the ultimate tensile and yield strengths and resilience rise to 225.40 MPa, 128.04 MPa and 175.90 kJ/m3 respectively. Examination of the analyzed microstructures indicates that the complex Ni-containing intermetallic phases forms once Ni added. The influence of transition alloying element nickel addition to the solidification of squeeze cast aluminum alloy A380 was also investigated via thermal analysis. 2%Ni addition to A380 promoted the formation of the Ni-containing ternary phase at a relatively high temperature and suppressed the formation of the Al-Cu phase which took place at a relatively low temperature during solidification. One less phase formation was observed in cooling process. With alkaline earth element, strontium added into A380 as a modifier of eutectic silicon, the strength of alloy was further improved up to UTS: 241.6MPa and YS: 172.5MPa. A design of experiment (DOE) technique, the Taguchi method, was used to develop as-cast high strength aluminum alloys with various element additions of Si, Cu, Ni and Sr. For each element, three different levels of weight percentages were selected (Si: 6, 9, 12%, Cu: 3, 5, 7%, Ni: 0.5, 1, 1.5% and Sr: 0.01, 0.02, 0.03%). Tensile properties as ultimate tensile strength, yield strength and elongation at failure were selected as three individual responses to evaluate the engineering performance of the designed alloys. The alloy with the optimal composition had average UTS of 267.00 MPa, elongation at failure of 1.13% and yield strength of 210.37 MPa under the as-cast condition. The contribution on tensile properties of each element was determined by an analysis of variance. The results of the tensile testing at high temperatures up to 300oC showed that 2 wt. % Ni additions increased the UTS and YS by 27.4% and 11.7% over those of A380 alloy. The Sr addition had a mirror effect on the high temperature tensile strengths. The XRD patterns and TEM analysis revealed that the presence of Ni-containing intermetallics should be responsible for the improvements of the strengths of the tested hypoeutectic Al-Cu-Si alloy.

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