Abstract
The mechanical properties, formability and corrosion resistance of aluminium are directly influenced by the alloy’s chemical composition and microstructure. The physical, chemical and mechanical characteristics of aluminium alloys can differ substantially from those of the base metal in its pure state.
The addition of magnesium to aluminium, for instance, produces an alloy with behaviour different from pure aluminium. This alloy exhibits higher mechanical strength, excellent formability and enhanced corrosion resistance, particularly in coastal environments.
In this study, the same alloy produced at different industrial plants was examined. Due to the specific characteristics of each facility, slight variations in chemical composition were identified, especially in the levels of residual elements. These differences had a direct impact on the final mechanical properties after the rolling process and the development of the specified temper.
Introduction
The aluminium manufacturer had three industrial plants. Plant one had the refining stage, primary aluminium reduction (smelter) and remelting operations for producing ingots for rolling. Plant two had reduction and remelting processes. Plant three was equipped with recycling units, remelting facilities, hot and cold rolling lines and annealing furnaces.
Although all plants produced ingots, only plants one and two manufactured commercially pure alloys due to their operational characteristics. All rolling operations from initial processing to the finishing of coils and sheets were performed exclusively at plant three.
At a certain point, for strategic reasons, the company temporarily transferred production of the AlMg2.5 alloy to plant two. The initial ingots were hot rolled and subsequently cold rolled. The coils then underwent thermal stabilisation following the same furnace practice previously used. However, the first batch was rejected because the mechanical properties fell below the minimum values required by the standard.
Root cause analysis
Process engineering initiated an investigation to determine the root cause of the rejection. A detailed assessment of the entire production sequence was conducted, ranging from remelting to the finishing process. The only significant difference observed between plants two and three was the chemical composition of the alloy, particularly the levels of residual elements.
At plant two, the alloy was produced by adding only the specified principal alloying elements, within the nominal limits for the AlMg2.5 alloy. In contrast, the alloy produced at plant three exhibited higher concentrations of residual elements due to the production of other alloys with different chemical compositions in the same remelting furnace. Although the levels of residual elements were low, this variation directly influenced the mechanical properties. The higher total content of alloying elements generally results in greater mechanical strength, assuming all other process parameters remain unchanged.
As a result, the alloy produced at plant three could tolerate more aggressive furnace practices without compromising final performance, due to its more robust chemical composition. When production shifted to plant two, this processing margin was no longer available.
Given this scenario, two possible routes were evaluated to compensate for the difference in chemical composition: Adjusting furnace practice or modifying the cold rolling rate.
Corrective actions: Elimination of rejections
At that time, the best option was to adjust the furnace practice to conditions compatible with the chemical composition of the ingots produced at plant two, ensuring that the heat treatment achieved the minimum mechanical properties required by the standard.
Conclusion
This study was fundamental in demonstrating that, in metallurgy, even minimal variations in chemical composition can lead to significant changes in the mechanical properties and formability of aluminium.
Also read: The effect of inclusions on the aluminium lid forming process
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