Dear friends,
We approached toward SLM of a new Al-Zn-Mg alloy using single track experiment. The initial feedstock powder was ball-milled (not atomized). More than 10 tracks fabricated at different processing parameters (170-250 W, 200-350 mm/s) were obtained (see in attachment). The markered microhardness increment is observed after increase in power and scanning speed. But the surfaces of the tracks are quiet different while the cross-sections look similar in case of penetration and connection angle. The surface of 250 W/350 mm/s track with the highest hardness is very dark and it is difficult to reveal its quality related to hardness increment (see in attachment). Moreover the cross-section width is the highest too (see in attachment). We tie the darkness with Zn and Mg evaporation, but it is not clear why it has the highest hardness.
Increase in hardness can be explained by partial oxidation of the metal during reflow. Even when melted under protective gas, aluminum and magnesium absorb large amounts of oxygen into its structure during melting (magnesium absorbs oxygen as well as nitrogen in the structure). Oxides can be finely dispersed in the melt and increase the hardness after cooling. A dark SEM image is evidence of an oxygen content in the alloy (Al2O3 and MgO appear darker in SEM than Al and Mg).
Vadim Verlotski thank you for the comment. Yes it is possible. Thus the hardness values of the whole tracks batch are to be similar. But they are different. I do not suppose that such kind of oxides reinforcement plays the key role while. Hence the question is still open as well as quality of tracks estimation.
I agree with Mr. Verlotski. It is better if you performs some kind of chemical analysis on the surface of your track and check the superficial composition. In case of oxides (most probably), I suggest you to check your powder quality for any sign of oxidation. In commercial LPBF machines, most of the oxygen enters the chamber through the feedstock material.
For quality of your scan track: multiple measurements of width and depth of these tracks through cross-sectional analysis in highly recommended. You could use that data for moving towards multi scan builds. check the tracks for any sign of balling and if positive try to move on with higher energy densities.
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