Investigation of the compressibility and sinterabilty of AZ91 powder production and particle production by gas atomisation method

Abstract

This study intends to determine the pressability and sinterability of AZ91 powder production by gas atomisation method and that of the produced powder for partial production. Therefore, first, a gas atomisation unit has been designed and manufactured in the laboratories of the Karabük University, Department of Manufacturing Engineering. Atomised powder production has been achieved at a temperature of 795 °C, with nozzle diameters of 2 and 4 mm and four different gas pressures (5, 15, 25, 35 bars). Argon gas has been used for atomisation and as a protective gas atmosphere. Scanning electron microscope (SEM) is used to determine the shape of the produced AZ91 powder, and a laser particle size analyzer is used to analyze the powder size. Additionally, a microhardness (HV0.025) measurement has been conducted to determine the hardness of the produced powders.To achieve a homogeneous distribution, the produced powders are mixed in a three-dimensional moving turbulator for 30 min. Mixed powders have been pressed at 300, 400, 500 and 600 MPa and have been sintered at 500 °C, 550 °C and 600 °C. Additionally, the density values have been determined before and after sintering of the materials. SEM images have been obtained from the fractured surfaces of the samples before and after sintering. XRD and EDX analyses have been performed to determine the chemical composition. Further, microhardness (HV0.5) is obtained from the pressure surfaces of the samples to determine the effects of the pressing pressure and the sintering temperature on the hardness.As a result of the experimental studies, it has been observed that the powder size decreases with the increase in gas pressure and that the powder shape generally changes from ligament and complex shape to droplet and spherical shape. From the XRD, XRF and EDX results, it has been determined that the structure comprises an α phase (Mg main matrix) and Mg17Al12 interphase, which is β phase, and very small amounts of MgO have been observed. The hardness of the produced powders increased based on the increase in gas pressure.The densities of the samples increased with both increasing pressing pressure and sintering temperature. It has been observed from the fractured surface SEM images that the number of pores formed in the samples decrease with an increase in the pressing pressure. It has been determined that the post-sintering structure exhibits a typical dendritic structure. In addition to the α-Mg matrix phase, β (Mg17Al12) intermetallic and α + β eutectic were formed in the structure. The microhardness values of the samples decreased depending on the sintering temperature; the highest hardness value was measured as 64,02 HV0.5 at a pressing pressure of 300 MPa and a sintering temperature of 500 °C, whereas the lowest hardness value was measured as 54,86 HV0.5 at a pressing pressure of 600 MPa and a sintering temperature of 600 °C. Keywords: Gas atomisation, AZ91 alloy powder, Gas pressure, Nozzle diameter, Pressing, Sintering