Research On The Process Of Micro-plasma Fusion Welding

Since the early1950s, micro-plasma fusion welding has studied by more and more peoplebeacause it has desirable characteristics, such as low welding current, high energy density,small heat input, high arc shrinkage and stable arc, etc. Compared with laser welding andelectron beam welding, micro-plasma fusion welding has advantages of low cost, simpledevice and flexible control, etc. With the development of modern technology, the demands forsurface properties and precision welding of materials become higher. Therefore, micro-plasmafusion welding technology is widely used in preparation of coatings and welding of thinmaterials. In this paper, Ti-based composites coatings were prepared on titanium alloys byLHM-50precision micro-plasma welding machine and titanium alloy thin plates were alsowelded. The microstructure and properties of composite coating and welding joints wereinvestigated and the influence of different welding current on the microstructure and propertieswas also studied. Microstructure, constituent phases, chemical composition, microhardness,wear resistance and tensile properties were investigated by SEM, XRD, EDS, microhardnesstester, friction and wear tester and tensile testing machine.Experimental results showed that after a series of optimization runs, in situ synthesis ofTiC reinforced Ti-based composites coatings were prepared successfully and one-side weldingwith back formation of TC4thin plate with various thickness was achieved. The coatingmainly composed of TiC, α-Ti and β-Ti. With the increase of Cr3C2content, α-Ti and β-Timatrix phases in the coatings was completely transformed to β-Ti. Eutectic TiC reinforcedphases appeared in Ti-based composites coating with5%Cr3C2content and the microstructurewere granular and flaky. Primary TiC reinforced phases appeared in Ti-based compositescoating with above10%Cr3C2content and the microstructure were dendrites, rod and granular.With the increase of fusion welding current, the dendrites of TiC reinforced phases becamecoarser. The average microhardness of the coatings increased with the increase of Cr3C2content and reached1200HV in coating with20%Cr3C2, which was3.5times that of titaniumsubstrate. Due to the secondary wear of spalling TiC particles, average friction coefficients ofthe coating with15%and20%Cr3C2were higher than that of the titanium substrate. Aftercontrasting the average friction coefficients and wear morphologies of various coatings, it was found that the coating with10%Cr3C2has the best wear resistance. The microstruture of TC4thin plate weld bead (WB) was acicular martensite α’ phases, which directionally distributed inβ grain. With the increase of fusion welding current, the width and length of the acicularmartensite increased. The disorder degree of the acicular martensite also increased and the sizeof β-phase grain increased. The average hardness of the weld bead was around420HV, which80HV higher than that of titanium matrix. Maximum hardness appeared near the fusion line.Tensile strength of the three joints was95%that of the TC4titanium substrate. Position oftensile fracture was in the heat-affected zone. Fracture morphologies of the joints showedquasi-cleavage. Fracture mode was brittle and transgranular fracture. In this paper, theoptimazed processing parameters was selected for the preparation of the coating bymicro-plasma fusion welding: welding current30A, plasma gas0.5L/min and welding speed1mm/s. The optimazed processing parameters was selected for the welding of0.5,0.8,1.0mmTC4thin plate: welding current15,22,28A, plasma gas0.5-0.6L/min,0.6-0.7L/min,0.7-0.8L/min and welding speed3mm/s,2.5mm/s,2.3mm/s, respectively.