Microstructure Changes and Improvement in the Mechanical Properties of As-Cast AlSi7MgCu0.5 Alloy Induced by the Heat Treatment and ECAP Technique at Room Temperature

Miloš Matvija,Miroslav Glogovský,Ondrej Milkovič,Róbert Kočiško,Marek Vojtko,Martin Fujda

doi:10.1155/2018/5697986

Abstract

The changes in the microstructure and improvement in the mechanical properties of as-cast AlSi7MgCu0.5 alloy induced by the heat treatment and technique of equal channel angular pressing (ECAP) were investigated. The heat treatment of as-cast alloy performed before the ECAP technique was required to increase the plasticity of the alloy. Therefore, the samples of analysed alloys were solution annealed at optimized temperature of 823 K for 4 hours to dissolve the particles of intermetallicπ(Al8FeMg3Si6) phase and to spheroidize the Si particles. Subsequently, water quenching and artificial ageing at optimized temperature of 573 K for 5 hours was used to obtain an overaged alloy state. The microstructure of alloy was consisted ofα(Al) solid solution, eutectic Si particles, and intermetallicβ(Mg2Si), Q-Al4Mg5Si4Cu,α-Al12(Fe,Mn)3Si, and/orα-Al15(Fe,Mn)3Si2phase particles. The crystal structure of present phases was confirmed by hard X-ray diffraction at Deutsches Elektronen-Synchrotron (DESY) in Hamburg and by the selected area electron diffraction (SAED) performed inside the transmission electron microscope (TEM). The heat-treated alloy was processed by ECAP at room temperature following route A. Repetitive ECAP of alloy homogenized the heterogeneous as-cast microstructure and formed the ultrafine subgrain microstructure with elongated subgrains of 0.2 µm in width and 0.65 µm in length and the high dislocation density. Microstructural changes in alloy induced by both heat treatment and ECAP led to the high strain hardening of the alloy that appeared in an improvement in strength, ductility, and microhardness of alloy in comparison with as-cast alloy state.

Highlights

Hypoeutectic aluminium-silicon alloys are well-known casting materials used in automotive industry due to their excellent castability, high strength-to-weight ratio, good mechanical properties at elevated temperature, good wear resistance, and low thermal-expansion coefficient [1]
E morphology of eutectic Si particles was brous and coral-like. e particles of intermetallic phases were rod-like and of irregular script morphology. e phase analysis by hard X-ray di raction (XRD) showed that the α(Al) solid solution and eutectic Si particles were predominant phases in as-cast alloy state (Figure 3(a)). e presence of π(Al8FeMg3Si6), β(Mg2Si), Q-Al4Mg8Si7Cu2 and α-Al12(Fe,Mn)3Si, and/or α-Al15(Fe, Mn)3Si2 phases was detected by di raction analysis
Severe plastic deformation by the equal channel angular pressing (ECAP) technique at room temperature in combination with pre-ECAP heat treatment of foundry heat-treatable AlSi7MgCu0.5 alloy led to a significant improvement of its mechanical properties through the homogenization and refinement of structure

Summary

Introduction

Hypoeutectic aluminium-silicon alloys are well-known casting materials used in automotive industry due to their excellent castability, high strength-to-weight ratio, good mechanical properties at elevated temperature, good wear resistance, and low thermal-expansion coefficient [1]. E heat treatment including solution annealing, quenching, and ageing is the most widely used method to improve strength properties of these alloys due to the formation of precipitates from supersaturated solid solution during applied natural or Advances in Materials Science and Engineering. During the initial stage of the arti cial ageing, the strength and hardness of alloys increases up to the maximum value. It is a result of the formation of coherent and/or semicoherent strengthening phase particles. Besides the heat treatment of Al-Si-Mg-Cu alloys performed to improve their strength and plasticity, the techniques of severe plastic deformation (SPD) have been often used in the last decades. The nal results of this technique depend above all on geometry of die, processing route, number of passes, and temperature of processing [7]

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