Structural Aspects of Remelting of the AZ91 Magnesium Alloy Surface Layer

What is it about?

In this study, modification of the AZ91 magnesium alloy surface layer with a CO2 continuous wave operation laser has been taken on. The extent and character of structural changes generated in the surface layer of the material was being assessed on the basis of both macro- and microscopy investigations, and the EDX analysis. Considerable changes in the structure of the AZ91 alloy surface layer and the morphology of phases have been found. The remelting processing was accompanied by a strong refinement of the structure and a more uniform distribution of individual phases. The conducted investigations showed that the remelting zone dimensions are a result of the process parameters, and that they can be controlled by an appropriate combination of basic remelting parameters, i.e. the laser power, the distance from the sample surface, and the scanning rate. The investigations and the obtained results revealed the possibility of an effective modification of the AZ91 magnesium alloy surface layer in the process of remelting carried out with a CO2 laser beam.

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Why is it important?

Observations with the use of both light and scanning electron microscopy revealed considerable structural changes in the surface layer of the remelted magnesium alloy, the rate and character of which resulted from the applied treatment parameters. Therefore, the combinations of basic remelting process parameters, i.e. laser power, the distance between the focus and the sample surface, as well as the scanning rate provide the possibility to form the remelting zone dimensions and the extent of structural changes. Strong refinement of the structure and a more uniform distribution of individual phases constituted the main components of the changes triggered by the laser treatment. Regardless of the adopted remelting parameters, the presence of fine dendrites of the α­Mg phase was found in the structure of the surface layer, and the presence of the γ­Mg17Al12 phase in the interdendritic spaces was observed. The structural and morphological changes found in the analysed alloy are a consequence of rapid crystallization caused by a high temperature gradient and a very short time of the heat source influence. The EDX investigations, covering the remelting zone and the material of the core revealed an evident regularity. Regardless of the applied remelting parameters, an increase of aluminium content parallel to a decrease of the magnesium content was observed on the linear layouts of the main alloy components, i.e. magnesium and aluminium in the remelted area. It should be thought that this regularity might be caused by a more intense evaporation of magnesium than that of aluminium from the remelting zone during the processing. For zinc and oxygen, no differences in the concentration of those elements were found between the remelted and non-remelted areas. The linear distributions of magnesium, aluminium, and zinc are evidence of a more uniform distribution of the components of the structure after remelting. The conducted investigations and the obtained results showed the possibility of an effective modification of the AZ91 magnesium alloy surface layer in a remelting process carried out with the use of a CO2 laser beam.