Influence of heat treatment and its sequence on elevated-temperature properties of Al-Mn-Mg 3004 alloy

Abstract

Various heat treatments with different sequences between the peak precipitation treatment (375 °C/48 h, referred to as “P”) and the high temperature treatment (450–600 °C, referred to as “H”) were performed in Al-Mn-Mg 3004 alloys to simulate their thermal treatments during thermomechanical fabrication processes in the present work. Their influences on the dispersoid precipitation and elevated-temperature properties (yield strength and creep resistance at 300 °C) were investigated. The results indicate that the size of dispersoids increased while the volume fraction of dispersoids decreased with increasing “H” temperature when “P” was performed either prior to or after “H” (denoted as “PH” and “HP”, respectively), which results in the reduction of the elevated-temperature properties. However, the yield strength and creep resistance after “PH” treatments were always higher than those after “HP” treatments at the same “H” temperature. When the “H” temperature during “PH” treatments is controlled below 500 °C, the materials can still maintain the appropriate yield strength and creep resistance, providing the possibility for Al-Mn-Mg 3004 alloy to adapt to the conventional fabrication processes but still maintain high elevated-temperature properties.

Introduction

Due to their excellent formability, corrosion resistance and high strength, Al-Mn-Mg 3xxx alloys have been widely used in the fabrication of containers and packaging materials [1]. Though Al-Mn-Mg 3xxx alloys have traditionally been classified as non-heat-treatable aluminum alloys, the precipitation of a number of dispersoids during heat treatment has been discovered in 3003 alloys, resulting in an improvement of the mechanical properties at room temperature (RT) [2], [3], [4], [5]. Li's group [4] found that the yield strength (YS) of 3003 alloys at RT increased from 52 MPa after homogenization (600 °C/24 h) to 80 MPa after the precipitation treatment at a relatively low temperature (375 °C/24 h), in which a large amount of dispersoids precipitated and contributed to the improvement in strength. Recently, Liu et al. [3], [6], [7] systematically investigated the influence of the dispersoid precipitation on the elevated-temperature strength and creep resistance in 3004 alloys and found that the YS and creep resistance at 300 °C increased remarkably after the precipitation treatment at 375 °C/48 h. Furthermore, the precipitated dispersoids were confirmed as α-Al(MnFe)Si phase dispersoids and shown to be thermally stable at 300 °C [3], which results in the stable elevated-temperature properties of 3004 alloy and makes the 3004 alloy a promising candidate for elevated-temperature applications.

In the conventional fabrication processes of Al-Mn-Mg 3xxx alloys, a high-temperature homogenization treatment (~ 600 °C) is typically performed before extrusion and rolling. In addition, the pre-heating of extrusion billets and rolling ingots up to ~ 500 °C is often required prior to extrusion and rolling. Those temperatures are much higher than the formation temperature of dispersoids (~ 342 °C [3]) and thus may have a significant influence on the dispersoid precipitation and the final mechanical properties [8], [9], [10]. It was reported that dispersoids tended to precipitate during the homogenization of 3xxx alloys, but their size and distribution were greatly dependant on the temperature and holding time [2], [10], [11], [12]. However, there has always been controversy in the literature about the influence of various homogenizations on the mechanical properties. In the investigations of Muggerud et al. [2] and Es-Said et al. [10] on 3003 alloys, the strength at RT was reported to decrease with increasing temperature from 375 °C to 621 °C, while the highest strength was reportedly obtained after 630 °C/9 h by Huang et al. [11].

With respect to maximizing the dispersoid precipitation and strengthening effect, a relatively low temperature treatment at 375–400 °C can be considered as the peak precipitation treatment to obtain the best elevated-temperature properties [3], [4], [6]. However, there are limited studies available on the evolution of dispersoids and elevated-temperature properties during different thermal processes for Al-Mn-Mg 3xxx alloys, such as the temperature and sequence of various thermal treatments. Therefore, optimizing the thermal treatment to adapt to thermomechanical fabrication processes but also maintain as high as possible elevated-temperature properties is a major concern during the industrial production for elevated-temperature applications of Al-Mn-Mg 3xxx alloys.

In this work, different heat treatments and sequences between the peak precipitation treatment and high-temperature treatment for the Al-Mn-Mg 3004 alloy were explored to easily simulate the possible scenarios of thermal treatments during conventional fabrication processes. The evolution of the dispersoid precipitation and elevated-temperature properties (the YS and creep resistance at 300 °C) during various heat treatments was investigated.