Study On Plasma Surface Modification Of AZ91D Magnesium Alloy

Magnesium（Mg） alloys used as the lightest metallic structural materials have anoptimistic development potential. Mg alloys are acknowledged as the21st century greenengineering material because of their low density, high specific strength and specific stiffness,high damping performance, good electromagnetic shielding characteristics, goodmachinability and easily recycled. Mg alloys have a widespread availability in automobile,aerospace, electronics industry. However, low strength and hardness, low wear resistance andcorrosion resistance have greatly restricted the application field of Mg alloys. Therefore, italso has important economic and social significance to enhance the chemical and mechanicalproperties of Mg alloys by surface modification technology.In this paper, the plasma cladding technology was employed to surface modification ofMg alloys, and carried out a study on the Mg allloys by flexible plasma remelted and cladding.The structure and properties of the remelted layer, Al-base alloy coating and TiB₂+Al₂O₃（TiC）coatings are studied systematically by scanning electron microscope （SEM）, energy dispersivespectrometer （EDS）, electron probe micro-analyzer （EPMA） and micro-Vikers hardnesstesting machine，wear test, electrochemical corrosion test. In addition, reaction process ofin-situ TiB₂+TiC coating on the AZ91D Mg alloy surface was analyed.The results of plasma remelt show that the remelted layer mainly consist of fine α-Mgand rod-shaped or granular β-Mg₁₇Al₁₂that dipersed in the α-Mg matrix and the content ofβ-Mg₁₇Al₁₂increases with the increasing of remelting current. Due to the fine grainstrengthening, solid solution strengthening, precipitation strengthening and supersaturationpoint defects additional strengthening, the microhardness, wear and corrosion resistance of the remelted layer have the significant improvement.Al-Si and Al-Si+Y alloy coatings were successfully fabricated on the surface of AZ91Dmagnesium alloy by plasma cladding processing. The results show that the Al-based coatingshave a well bonded metallurgic interface with substrate Mg alloy. As for Al-Si coating, themicrostructure is composed of Mg₂Si dendrite distributing on the Mg-Al intermetalliccompound matrix. Microstructure refinement and the existence of multiple Mg intermetalliccompounds lead to the excellent wear and corrosion resistance of Al-based coatings. Afteradding a certain proportion of rare earth element Y, besides the above intermetalliccompounds, the coating form the white Al2Y around Mg₂Si compound. Compared with themicrosturcture of Al-Si coating, the morphology of Mg₂Si is changed from coarse dendrite togranular or rod-shaped, which make the hardness, wear and corrosion resistance are superiorto those of Al-Si coating and Mg alloy matrix.TiB₂+Al₂O₃coating was prepared on the surface of AZ91D Mg alloy, and the differentration Al powder were added in cladding materials in order to combinability and compactness.The results show that those coatings form multiple phase such as TiB₂，Al₂O₃，α-Mg，Mg₁₇Al₁₂，Mg₂Al₃, in which TiB₂and Al₂O₃ceramic present the aggregation phenomenon,while α-Mg and Mg-Al compounds have a dendrite growth characteristics. Due to themulti-phase strengthening of ceramics and intermetallic, the hardness and wear resistance ofthose coatings have greatly improved, while corrosion resistance being in varying degrees ofenhancement.The obtained coating by plasma cladding pure TiB₂+TiC powder on the surface of Mgalloy, contain TiB₂、TiC and α-Mg and multi-phase ceramic distribute in matrix.Added with acertain proportion Al powder in the cladding materials according to TiB₂-TiC:Al=2:1, thecoating form a large number of Mg₁₇Al₁₂intermetallic compound due to the dilution of Mgalloy. The coating have a well bonded metallurgic interface with substrate Mg alloy, andTiB₂+TiC distribute in the matrix mainly with Mg₁₇Al₁₂. In spite of the reduction ofceramic-phase content, but the hardness and wear resistance haven’t great changes comparedwith those of clad pure TiB₂+TiC powder. which reason is because of comprehesivestrengthening of ceramic and intermetallic compound in the coating. Moreover, as the resultof the reduction of α-Mg content, the corrosion resistance of TiB₂-TiC:Al=2:1coating has enormously improved.In-situ synthesized the composite coating of multi-phase (TiAl₃，Mg₁₇Al₁₂，TiB₂+TiC)strengthening on the Mg alloy surface by plasma cladding （Ti+B₄C）+Al powder. In thecoating TiAl₃distribute the upper while TiB₂+TiC ceramics precipitate to the bottom ofcoating. From the point of view of thermodynamics, the reaction process was analyzed of（Ti+B₄C）+Al system and the formation mechanism of phase in the coating was discussed.