多階段固溶熱處理對不同銅添加量AA7075鋁合金之機械性質與顯微結構影響
Other Title
Effects of Multi-Stage Solid Solution Heat Treatments on the Mechanical Properties and Microstructures of Different Cu Additions AA7075 Aluminum Alloys
Journal
2024臺大學士論文獎
Date Issued
2024
Author(s)
Tz- En CHANG
Advisor
Abstract
This study focuses on comparing the differences in mechanical properties and microstructure of AA7075 aluminum alloys with different copper contents after various solution heat treatments. By employing multi-stage solution heat treatments at different temperatures, the dissolution state of second phases within the aluminum alloy is observed using a Scanning Electron Microscope (SEM), differentiating it from aluminum alloys treated with traditional single temperature solution heat treatments. The process aims to optimize the time of solution treatment to achieve higher mechanical properties, and improve the poor properties of high copper content aluminum alloy.
Through Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) analysis, it is found that the main second phases in AA7075 aluminum alloys are the η phase, S phase, and ω phase, with melting points of 481 °C, 493 °C, and insoluble, respectively. Different copper contents change the ratio of the second phases, especially S phases. This experiment designs solution heat treatments at 475 °C and 490 °C based on DSC results to optimize the dissolution of second phases and prevent incipient melting.
The Electron Probe Micro Analyzer (EPMA) reveals that solution treatment at temperatures exceeding the melting points of second phases causes incipient melting voids, reducing mechanical properties. Single-stage solution treatment at 490°C causes the η phase to incipiently melt, leaving voids. Solution treatment at temperatures significantly below the melting points of second phases fails to dissolve them completely. The study finds that solution treatment at 475 °C leaves the S phase behind, significantly reducing mechanical properties.
The study discovers that designing two-stage solution treatments for second phases, with heat treatments at 475 °C and 490 °C respectively, can solve the problems of residual phases and incipient melting mentioned-above, significantly enhancing mechanical properties. Using Electron Backscattered Diffraction (EBSD) and Transmission Electron Microscope (TEM) find no significant differences in grain size and precipitates between single-stage and two-stage solution treatments. Thus, applying two-stage solution treatment to the aluminum alloy can improve problems caused by second phases.
To enhance the mechanical properties of high copper content aluminum alloys, a 500 °C third stage of solution heat treatment was introduced to eliminate the S phase remained and increase the density of precipitates. Finally, it effectively improves the strength and ductility of high copper aluminum alloys. Through analysis using TEM bright field, HRTEM, and atomic-resolution image, it was observed that, under the peak aging conditions of the three-stage solution heat treatment, the precipitates tend to be smaller compared to those in the single-stage and two-stage. Synthesizing the experimental results, it is observed that although the high-copper aluminum alloys initially exhibited inferior properties, through multi-stage solution heat treatments, the properties of high-copper aluminum alloys can achieve levels comparable to or better than those of low-copper aluminum alloys.
Subjects
HRTEM
HAADF
原子級影像
AA7075 Aluminium alloys
second phases
solid solution treatment
SEM
incipient melting
η precipitates
atomic-resolution image
Publisher
國立臺灣大學材料科學與工程學系
Description
指導教授：楊哲人
Type
thesis
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