Optimization of A354 Al-Si-Cu-Mg Alloy Heat Treatment: Effect on Microstructure, Hardness, and Tensile Properties of Peak Aged and Overaged Alloy

Stefania Toschi

doi:10.3390/met8110961

Abstract

The aim of the present work is the study of T6 heat treatment of A354 (Al-Si-Cu-Mg) casting alloy. The heat treatment was optimized by maximizing mechanical strength of the alloy while keeping the treatment cost effective, reducing treatment time and temperature. Due to the presence of low melting compounds, a double stage solution treatment was proposed. The first stage was aimed at the homogenization and dissolution of the low melting phase while a second stage at a higher temperature was evaluated to foster dissolution of Cu/Mg rich intermetallics and keep the solution time and temperature as low as possible. Microstructural investigations were performed through optical and electronic microscopy, which allowed the assessment of the evolution of intermetallic phases during the solution treatment. Artificial aging was studied at different temperatures from 160 °C to 210 °C where the peak aging condition was identified. Over aging of the heat treated alloy was evaluated by soaking T6 samples at 200, 245, and 290 °C for up to 168 h. Tensile behavior of the T6 and over-aged alloy (i.e., after soaking at 210 °C for 50 h) was evaluated at room temperature. Results showed that the proposed treatment allowed the enhancement of mechanical properties of the alloy in comparison to industrial practice treatment, maintaining a good level of ductility and conferring a superior resistance to long-term high temperature exposure.

Highlights

Al-Si cast aluminium alloys are commonly used in the transport sector for the production of a large variety of components
The concomitant presence of both copper and magnesium in Al-Si-Cu-Mg alloys enables the precipitation of more stable Cu-based intermetallic phases, which confers to the T6 alloys an increased thermal stability in comparison to Metals 2018, 8, 961; doi:10.3390/met8110961
The microstructure of the as-cast A354 alloy consists of α-Al dendrites surrounded by eutectic silicon particles (Figure 1a)

Summary

Introduction

Al-Si cast aluminium alloys are commonly used in the transport sector for the production of a large variety of components. Al-Si-Mg alloys, characterized by excellent castability, enable the production of complex-shape parts with high specific strength due to their response to aging treatment. The application field of such alloys is, strongly influenced by the component operating temperature: mechanical properties of heat treatable cast Al-Si-Mg alloys (e.g., A356). As a result of the coarsening of strengthening precipitates generated during artificial aging, a noticeable reduction of hardness and tensile strength occurs at temperatures close to or higher than 200 ◦ C. Aiming to ensure a superior thermal stability to cast aluminium alloys for applications such as diesel engines, copper is added to the traditional Al-Si-Mg alloys [2,3,4,5,6,7,8,9]. The concomitant presence of both copper and magnesium in Al-Si-Cu-Mg alloys enables the precipitation of more stable Cu-based intermetallic phases, which confers to the T6 alloys an increased thermal stability in comparison to Metals 2018, 8, 961; doi:10.3390/met8110961 www.mdpi.com/journal/metals

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Conclusion