Effect of Casting and Homogenizing Process Conditions for Aluminum Alloy 6xxx on the Microstructure, Microhardness and their Response to Artificial Aging

Abstract

The proportions of the chemical elements present in the alloy are the main influence on its structure, as the microstructure is the most important factor affecting the final mechanical properties of aluminum alloys.

In this research, samples of aluminum ingots were cast by the direct cooling method using a cylindrical mold of structural iron ST37 molten aluminum was poured into it, Scraps of aluminum alloys 6xxx with known chemical composition were used.

 Casting operations were carried out according to three cases:

- Case (1): The mold is cold and coated with a layer of synthetic oil.

- Case (2): The mold is preheated to  before the casting process.

- Case (3): The mold is preheated to before the casting process.

 In the first stage, casting operations were carried out for the three previous cases, and then samples were prepared from the three castings to conduct chemical analysis using mass spectrometry, as the first sample showed that it contained 0.61% silicon, 0.53% magnesium, 0.53% iron, 0.234% copper, and 0.356% chromium, second sample contained 0.6% magnesium, 0.51% silicon, and 0.22% iron, while the third sample contained a very small percentage of magnesium less than 0.1%, 0.53% silicon, 0.338% manganese, and 0.153% chromium.

The microstructure was detected, It was found using a light microscope, as it was found to contain the unstable needle-like intermetallic phase (β-Al Fe Si) and a magnesium silicate  precipitate, when measuring the microhardness of these samples for the casting condition, the hardness values ranged for the first case 67HV, for the second case 68HV, and for the third case 69HV.

In the second stage: heat treatment processes were carried out by homogenization in order to obtain a better homogeneous structure, where the samples were subjected to heating to  and kept for 6h, after which they were cooled with water to room temperature, and the microstructure was detected using light microscopy, where a part transformation appeared from intermetallic phases to more stable α-AlFeSi phases, the microhardness values ranged for the first case HV96, for the second case 90HV, and for the third case 102HV. And in the third stage: for verify the response of the samples after treatment by homogenization to hardening by artificial aging, they were subjected to a hardening by aging experiment at  and kept for 6h, after which they were left to cool to room temperature without any cooling medium, and then their microstructure was detected using light microscopy, and the hardness values ranged of the first case was 72HV, for the second case it was 76HV and for the third case it was 64HV.