Study On Microstructure Characteriztions And Properties Of Plasma Beam Remelting And Cladding On Surface Of AZ91D Magnesium Alloy

Magnesium alloy is regarded as engineering material with wide application prospect in the aviation, spaceflight and vehicle electronics due to its low density, high specific strength and stiffness, good damping shock absorption property electromagnetic interference defense capability and excellent mechanical processing property. However, low surface hardness and poor tribological properties of magnesium alloy shorten the service life. Here, the surfaces of magnesium alloy were modificated by means of plasma beam remelting and cladding. The surface of AZ91D magnesium alloys were cladded with TiB2-Al2O3 and Al+TiB2-Al2O3, respectively. The microstrctrue of remelting layer and cladding layer were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe X-ray microanalyser (EPMA). The wear and corrosion behavior of remelting layer and cladding layer were systematic researched by vickers tester, wear tester and electrochemical workstation, respectively.The research results showed that grain refinement had taken place on the remelting layers of AZ91D magnesium by plasma beam treatment. The magnesium alloy substrate and remelting layer were mainly composed of a-Mg andβ-Mg17Al12, but the content of theβ-Mg17Al12 in the remelting layer was higher than in the substrate. The micro hardness of the remelted layer increased one time and reached up to 105-125HV0.1 comparing with the substrate 60-70HV0.1. Simultaneously, the wear resistance and corrosion resistance of the remelted layer was also increased obviously.When the magnesium alloy was cladded with TiB2-Al2O3, the top of the cladding layer was comparatively compact while the bottom was round state of sinter. The cladding layer phase was composed of TiB2, Al2O3 and a-Mg. Composite ceramics did not react with magnesium alloy, but the melting magnesium permeated into the cladding layer leading to good metallurgical bonding between multiphase ceramics and magnesium during plasma cladding process. The microhardness of the cladding layer was up to 1600 HV0.1 and the weight loss of wear was only ten percent of magnesium alloy substrate, which meant that the wear resistance of the cladding layer had been enhanced to a great extent.Plasma surface cladding of AZ91D magnesium alloy with Al, Al2O3 and TiB2 powders was also studied. The phases of the cladding layer including of TiB2, MgAlO4, Mg, Al2O3 and Mg17Al12n indicated that Al+TiB2-Al2O3 powders had reacted with AZ91D magnesium alloy substrate. The cladding layer was consisted of reticulate magnesia-alumina intermetallic compounds and lumpy ceramics. The reticulate zone was minished as decrease of the content of alumina. The microhardness of the cladding layer was not even. That is to say, the microhardness of the ceramics layers was higher and that of magnesia-alumina intermetallic compounds was lower. Wear resistance and corrosion resistance of the cladding layer was enhanced with increase of the ceramics content in a certain range. However, when the content of ceramics phase was beyond the range, wear resistance would decreased. Passivation phenomenon was found during corrosion process which included earlier uniform corrosion and later pitting corrosion. The electrode potential of the cladding layer was 0.25 V higher than that of magnesium alloy substrate.