Microstructure Investigation Of WC_p/Al Composites Fabricated By Coincident Wire-powder Direct Laser Deposition For Additive Manufacturing

With the development of space technology,composite materials are difficult to satisfy the poor service environment.In this special environment,composite materials cannot only have the advantage of low density,higher strength,small thermal expansion coefficient and good abrasion resistance,but also radiation shielding prpperties.WC_p/Al,as a new composite material,has earned widely concerned.But WC_p/Al composite is difficult to process,which limited its application.Combined wire-powder direct laser deposition?WPDL?technology,combined with the advantages of separate wire feeding and powder deposition,is very suitable for fabricating aluminum matrix composites.Combined wire-powder direct laser deposition?WPDL?has been proven to be a promising method for the fabrication of Metal Matrix Composite?MMCs?.However,the process for fabricate composite material is quite different from traditional methods.Therefore,the purpose of this paper is to study the process of fabricate WC_p/4047 Al composite thin-walled parts by WPDL,and investigate the relation between microstructure and properties.In this paper,orthogonal test was used to evaluate the influcence of laser power,scanning velocity and powder feeding rate on single-layer deposition.On this basis,multilayer thin wall composite materials was fabricated.In experiment,molten pool flow downward with the increasing of deposited thin wall,which results the width of thin wall increased.The method of output gradient-descend laser power optimize the shape of thin wall effectively.The results of the phase distribution and hardness of the composites show that the laser power can not be decreased infinitely.There is a critical value of laser power.Microstructure at different areas of thin wall structure was investigated by SEM,TEM and XRD analysis.Besides Al,Si,WC and Ti,in-situ enforcement phase of Al₃ Ti,Al₄W and TiC were identified by XRD and TEM.In situ enhanced phase distributed in the matrix evenly.Al₃ Ti compounds also existed around the unmelted Ti particles.Nano-indentation test shown that the hardness between Ti and Al matrix incresed significantly due two the Al₃ Ti compound.The interface layer of WC cansists of TiC,Al₄ W and Al₃ Ti.The thickness of the interface ranges from 100 nm to 200 nm.Without the addition of enhancement particles,oriented-growth ?-Al was observed.The character of oriented growth was disappeared with the addition of WC and Ti.The condition of oriented growth was breaking.Firstly,WC and Ti particles can hinder the ?-Al oriented growth.Secondly,Al₃ Ti and liquid occurred in peritectic reaction in front of S/L interface.The occurrence of non-uniform nucleation hinder the growth of ?-Al columnar.The strength of the composite material fabricated by WPDL is between 230 and 248 MPa.After annealing,the strength of the material decreased slightly.After the solution treatment,Si particles with a size of 2?m were precipitated on the matrix.The strength of composite material decreased nearly to the level of 4047 Al.In addition to Si,microstructure of composite was not changed after heat treatment.Excessive dissolved WC boundary and low-level reaction WC boundary were the mainly crack source.The crack is inclined to propagate to the WC.The thick reaction layer of Ti particle is easy to craze and tend to propagate to matrix.The adhesion of Al₃ Ti and Al is strong.The cracks are generally generated from the inside.But the solution-treatment Al₃ Ti will separate from matrix after tensile test.In-situ tensile experment results shown that crack initiate at the WC boundary firstly.When loading to certain force,interface of Ti and Al₃ Ti phase begin to crack.Although the interface of Ti is cracked subsequently,the crack will quickly transfer to the matrix,leading to the final fracture of the composite material.Compared with 6061 Al,the coefficient of thermal expansion of the composite decreases from 27×10-6?-1 to 21.5×10-6?-1.Compared with 40471 Al,the corrosion potential of WC_p/Al composite is almost the same as that of 4047 Al.But the corrosion current density incerased from 2.99×10-6 A/cm2 to 5.05×10-7A/cm2.The corrosion resistance of the composites decreases.The results of dry friction shown that the friction coefficient of WC_p/Al composite is closed,nearly 0.4 to 0.5.However,the wear width and wear depth of the composite material are less than that of 4047 Al.The wear resistance of the WC_p/Al composite is improved.