Study On Surface Nanocrystallization Induced By High-energy Bombarding And Laser Alloying In Magnesium Alloy

Magnesium alloy currently used as the lightest metallic structural materials has high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability and is easily recycled. It is an important way for equipments to achieve lightweight and portable, and acknowledged as the 21st century green engineering material. Unfortunately, magnesium has a number of undesirable properties including poor corrosion and wear resistance and high chemical reactivity that have hindered its widespread use in many applications. It has realistic significance for promoting wider application and giving full play to its unique performance advantages of magnesium alloys to strengthen the research on improving the wear and corrosion resistance. In this paper, the techniques of self-surface nanocrystallization induced by high-energy bombarding and laser alloying to achieve the purposes of improvement on surface properties of magnesium alloy. The effect of a steel shot size, impact distance and handling time on self-surface nanocrystallization induced by high-energy bombarding was investigated systematically.The process parameters were optimized to applicable to self-surface nanocrystallization of a magnesium alloy as follows: the pressure of N₂ and O₂ is 1.5 MPa, nitrogen and oxygen flow ratio of 7:5, kerosene flow rate of 4 L/h, bombing particle diameter ofφ0.5 mm, impact distance range of 290³20 mm, handling time in the range of 180²40 s. The surface nanocrystallization on magnesium alloy can be obtained successfully using the above technological parameters. Cross-sectional microstructure after nanocrystallization treatment reveals that the deformation layer presents gradient variation obviously, including the topmost surface layer of severe plastic deformation, deformation twins-based subsurface layer and a transition layer with slight deformation near the substrate. The inherent refinement mechanism of surface nanocrystallization on magnesium alloy was deduced through TEM / HRTEM microstructure observation and analysis. Meanwhile, the grain refinement mode of coarse grains converted into nano-grains was abstracted under the condition of severe plastic deformation, i.e. the deformation mode is dominated by mechanical twinning in the initial stage of the surface nanocrystallization for magnesium alloy, and accompanied with the dislocation movement of basical plane and prismatic plane. The coordination and competition between the deformation twinning and dislocation motion dominated the deformationprocess in the mid-stage of the severe plastic deformation. Subsequently, cross slip can be activated owing to a certain degree of grain refinement and temperature rising derived from high-energy bombarding, Dislocation movement dominates the competition in the later stage, and resulting in further fragmentation of the residual twins and micro-banded substructure. Then, dynamic recrystallization occurred under the sufficient driving force that coming from high-energy substructure with the increasing distortion, deformation storage elevated, dislocation multiplication, annihilation and rearrangement. Eventually, the nano-grains formed with clear grain boundary, homogeneous distribution and random orientation.The behaviors of nano-layer were studied systematically. The microhardness of the most top surface layer is about twice comparison with the substrate, and it is gradually decreasing with the increase of the depth from the surface. The results of friction and wear experiment shows that the friction coefficient and wear weight loss were reduced significantly, and adhesive wear and abrasive wear is the main wear mechanism, meanwhile, coupled with oxidation wear. It is found that the deterioration of corrosion resistance occurred in acid, alkali and salt 3.5% NaCl solution with different PH values after surface nanocrystallization treatment through the investigation on corrosion behavior. Thermal stability experiments show that the critical temperature of stabilized existence for nanocrystalline of magnesium alloy is 330°C. The vacuum microwave oven was used for experiments of diffusion alloying Al, Si and Al-12Si alloy in magnesium alloy before and after the surface nanocrystallization treatment. It's worth noting that the diffusion alloying of magnesium alloy utilizing microwave has not been reported. The results showed that the thickness of alloyed layer increasing with the processing temperature increase. Moreover, the thickness of nano-treated alloying layer is more 2 to 3 times than that without nano-treatment.The optimized process parameters of laser surface alloying for Mg alloys are obtained through experimental study as follows: pulse duration of 0.8 ms, frequency of 45 Hz, spot diameter of about 1.0 mm, current of 220 A and the scanning speed is 350 mm/s.It is found that in situ reaction produced in the alloyed layer between the element Al and Nb, and the matrix elements Gd, Y, and generated a high-temperature hard-phase, such as Al₂Gd, Al₂Y and Al₃Nb intermetallic compounds. Furthermore, strengthening phase TiB2 and (W, Ti)C did not decompose by XRD diffraction analysis of the Al-Nb / Al-TiB2 / Al-(W, Ti)C system alloyed layer. Surface macro-morphology shows that there has been some degree of nodulation phenomenon on the quality of laser surface alloying with the content of alloying element Al tapering off in the mixed powders. The entire cross-section is divided into three parts: alloyed layer, transition layer and matrix. The SEM and TEM observations show that the grain refinement is significant and uniform distribution, furthermore, the new phases such as Al₂Gd, Al₂Y, Al₃Nb, etc. of in situ synthesis and strengthening phase TiB2 and (W, Ti)C are uniformly dispersed in the alloyed layer, and most present nearly spherical shape, only a small part of that shows the quadrangle blocks like, and the size of most of intermetallic compounds is about 100 nm. It is worth noting that the morphology of larger size is not a single precipitates, but agglomeration of a number of fine particles by further observation.Hardness test results show that the hardness of alloyed layer is to improve up to 4⁶ times compared with the substrate. The results of dry friction and wear reveal that friction and wear performance of the alloyed layer with different alloying elements and the mass ratio can be improved significantly, and the friction coefficient changed from about 0.52 to 0.25⁰.35. Analysis of wear morphology indicated that the main form of wear for the alloyed layer is the adhesive wear, abrasive wear and oxidation wear. Electrochemical test results show that corrosion resistance of magnesium alloy in 3.5% NaCl solution can be improved by laser surface alloying.