Study On In-situ Mg₂Si/Al Composites Fabricated By Laser Deposition

Laser deposition technology is a high-economical technology because of its fast cooling rate,unlimited powder selection,simple process,and high laser beam energy density.Mg₂Si/Al composites,which have low density,high specific modulus,and low cost,are suitable for application in the field of lightweight components for automotive engine cylinder liner and brake disc.In this paper,in-situ Mg₂Si/Al composites were fabricated by laser deposition based on presetting powder method,the thermodynamics and dynamics of Al-Mg-Si system were analyzed,the reaction thermodynamics was calculated.Dynamic model of in-situ Mg₂Si/Al composites fabricated by laser deposition was established.The phase composition,microstructure,hardness,wear resistance and corrosion resistance of single layer and multi-layer composites under different laser process were studied.And the optimum laser parameter was obtained.The research results indicate as follows:1.Thermodynamic analysis indicates that the binary phases formed in the Al-Mg-Si system are Mg₂Si,Al₃Mg₂,Al₁₂Mg₁₇.And the formation heat generation,binding energy,and Gibbs free energy of Mg₂Si are minimum.The calculation results of the non-equilibrium solidification process of Al-Mg-Si system with different Mg₂Si contents show that,during the laser deposition process,Al-Mg phase is generated at the low temperature;in-situ Mg₂Si is dominated at the high temperature,and the Al-Mg phase can be eliminated by controlling the laser power which can affect the reaction temperature.2.The reaction kinetics mechanism of the Al-Mg-Si alloy system during laser deposition as follows:melting,diffusion,reaction,decomposition,and cooling.With the increasing of the temperature,the Al₃Mg₂ phase is formed through the diffusion.At the high temperature stage,the Al₃Mg₂ phase is decomposed and Mg₂Si phase is in-situ synthesized.Dynamics model of in-situ Mg₂Si/Al composites fabricated by laser deposition was established as follows.It can be found from the dynamic model that the rich Mg layer thickness,Si particles size,laser power?system temperature?and aluminum content are the main factors which affect the reaction rate.And decreasing the rich Mg layer thickness,Si particles size and aluminum content,increasing the laser power?system temperature?result in accelerating the reaction rate and extent.3.With the change of laser power,scanning speed and Mg₂Si content,the microstructures are different.The microstructure of both single layer and multi-layer consists of the small black Mg₂Si phase,the white bulk Si+Al+Mg₂Si phase,as well as the gray?-Al phase.The microstructural morphology of the deposition layer is columnar,dendritic and equiaxed crystals from the bottom to the top.With the increasing of laser power and scanning speed,the content of Mg₂Si is increased,and the grain size of Mg₂Si first is decreased and then increased.The minimum size of Mg₂Si phase of 3.5?m is obtained when the laser power is 1000w and the scanning speed is 350mm/min.When the Mg₂Si content is 4%and 6%,a small amount of dotted and thin Mg₂Si phases exist at the bottom of the deposited layer;When the Mg₂Si content is 8%,the Mg₂Si at the bottom increases and is turned into Chinese script;when the Mg₂Si content is increased to 10%and 15%,Mg₂Si is uniformly distributed gradually in the microstructure with various shapes such as dots,blocks,thin strips,and Chinese script.4.With the increasing of laser power,and scanning speed,the hardness of composites first is increased and then decreased,and the wear resistance is proportional to the hardness.When the laser power is 1000w and the scanning speed is 350mm/min,the hardness of the composites is up to 135HV,which is twice higher than that of the matrix 6061Al alloy.The friction coefficient is 0.52.The minimum wear is 0.18mm²,and the wear resistance is the best.The maximum self-corrosion potential is 0.2296V.The self-corrosion current is the smallest.And the corrosion resistance is the best.With the increasing of Mg₂Si content,the hardness of composites is gradually increased,and the wear resistance is gradually decreased.The main reason is that,with the increasing of the Mg₂Si content,the reinforcing effect on the composite material is increased significantly.