Development of surface composite based on Mg-Al-Ni system on AZ31 magnesium alloy and evaluation of formation mechanism

The in-situ synthesis of Mg-Al-Ni composite on the surface of AZ31 plate by friction stir processing (FSP), has been investigated in this article. The unprocessed AZ31 plate consisted of grains of 25 μm size. By increasing the number of FSP passes from one to five, the grain size of the AZ31 plate decreased to 7.5 and 3 μm, respectively. A uniform distribution of the reinforcements was also obtained by increasing the number of FSP passes. Based on the results of X-ray diffraction (XRD) and Energy dispersive spectrometry (EDS) analyses, Mg2Ni and Al3Ni2 intermetallic compounds are in-situ formed by a single-pass FSP of the composite specimens. By increasing the number of FSP passes, the amounts of Mg2Ni and Al3Ni2 compounds are dramatically reduced and AlNi and MgNi2 intermetallic compounds take their place. Thermodynamic and kinetic of interfacial solid state reactions were studied to determine the reactive mechanisms and phase evolutions during different passes of FSP. The maximum amount of hardness (∼106 Hv), was obtained for the composite sample after five passes of FSP.