Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al-Si-Cu Alloy.

Ruoqi Li,Makoto Kobashi,Naoki Takata,Yuji Okada,Yuichi Furukawa,Asuka Suzuki

doi:10.3390/ma14237155

Ruoqi Li, Makoto Kobashi + Show 4 more

Open Access

https://doi.org/10.3390/ma14237155

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Journal: Materials	Publication Date: Nov 24, 2021
Citations: 4	License type: CC BY 4.0

Affiliation: Nagoya University, Toyota Motor Corporation (Japan)

Abstract

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15(Fe, Mn)3Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.

Highlights

Aluminum (Al) alloys typically feature low densities, high specific strengths, high thermal conductivities, and superior corrosion resistances
Two phases with different contrasts were observed in the α-Al matrix of the solution-treated samples, whereas pores were observed around the elongated phase in the sample treated at 540 ◦ C (Figure 3d)
These results clearly indicate the necessity of accounting for the effects of Fe, Mn, and Mg as impurities in the AC2B alloy (Table 1) on the precipitation at high temperatures above 400 ◦ C

Summary

Introduction

Aluminum (Al) alloys typically feature low densities, high specific strengths, high thermal conductivities, and superior corrosion resistances. These alloys are widely used in aerospace, automobile, and mechanical engineering and their related industries. The high thermal conductivity and low density of Al–Si alloys make them favorable alternatives to cast iron in the fabrication of automotive engine components. The addition of Cu as an alloying element can improve the strength of Al–Si alloys, and Al–Si–Cu ternary alloys are generally used as conventional materials in cylinder heads fabricated via sand and gravity die casting processes [6,7,8,9,10]

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