Mechanical Properties And Corrosion Behavior Of Micro-plasma-argon-arc Welded And Laser Welded Pure Titanium: An Experimental Study

Because of the excellent combination of biocompatibility, high corrosionresistance, low density, optimum hardness and Young's modulus, goodductility, low thermo-conductivity and radiation resistance, free magneticproperty, titanium and its alloys are clinically used as the main metal inprosthodontics in the 21st century. However, some shortcomings oftitanium's castability, together with the precision required in precisedentures, make welding techniques essential in prosthodontics.Nowadays more and more advanced welding techniques are widely usedin clinic, but laser welding is very expensive and its energy is lower.Compared with laser welding, micro-plasma-argon-arc welding uses theplasma arc guns, which, by compressing the plasma arc throughmechanical, thermal and electromagnetic effects to obtain higher energydensity and electro-ionization, can produce welding pieces with smalldeformation and can have two sides formed at one welding. Although wehave applied the micro-plasma-argon-arc welding to dental cobalt-chromium alloy with quite good results, reports on applications ofmicro-plasma argon-arc welding in titanium are few.In order to provide the basic evidence of micro-plasma-argon-arc welding, in this experiment we intend to compare the mechanical properties,microstructure, corrosion resistance of laser welding and micro-plasma-argon-arc welding.There are two parts in this thesis:PartⅠ: Study on the mechanical properties of themicro-plasma-argon-arc and laser welded pure titaniumMaterials and Methods: 4 pieces of plasma welded titanium samplesand 4 pieces of laser welded samples were made by 16 pieces of 40 mm×17 mm×1 mm specimens. Other 3 pieces of 40 mm×34 mm×1 mmspecimens of titanium were cast as control. Samples of the experimentsabove were cut into the specimen. Tensile strength was tested byCSS-44100 electronic universal testing machine and microstructures ofwelds were observed by OLYMPUS BX60M-microscope. Microhardnesswas also tested. Scanning electron microscopy and energy spectroscopywere also used to observe the welds. Results of the tensile strength wereanalyzed by oneway ANOVO.Results: The weld zones and the heating affected zones ofmicro-plasma-argon-arc welding were silvery white and the penetrationswere complete. There was almost no heating affected zone in laserwelding. The large size of typical needle-like structure could be seen inmicro-plasma-argon-arc weld and the small, regular grain could be seenin laser weld through metallography and scanning electron microscopy. However, it revealed a welding stress crack in the center section of laserweld. The tensile strength of micro-plasma-argon-arc welding was betterthan that of laser welding, but it had no significant difference with control.Microhardness of weld and heating affected zones did not change inmicro-plasma-argon-arc welding specimens, but it increased in the weldof laser welding specimens. Energy spectroscopy suggested plasma andlaser welding joints only have a very small amount of Ti and O elements.Conclusion: Mechanical properties of micro-plasma-argon-arc weldedtitanium specimens were better than those of the laser welded titaniumspecimens. It's believed that micro-plasma-argon-arc welding is an idealmethod of welding.PartⅡ: Study on corrosion behavior of the micro-plasma-argon-arcand laser welded pure titaniumMaterials and Methods: Samples made the same as those in partⅠwerecutting into the specimens of 4 mm~2. An electrochemical work station(BAS-100B model) was used to measure the corrosion potential (Ecorr)and the polarization curve. Welding specimens of 40 min×20 mm×2mmwere made and immersed in artificial saliva at 37℃for 100 days. Ti ionconcentration was measured every 20 days by atomic emissionspectrometry and Ti ion release rate was calculated. Tensile strengths ofspecimens immersed for 100 days were measured. The results wereanalyzed by two-way ANOVA. Results: Electrochemical test results showed that the corrosion potentialand the corrosion rate of micro-plasma-argon-arc welding group toppedthe first, the control was in the middle, and the laser group was the lowest.At the beginning of the immersion (20 d), Ti ion concentration ofmicro-plasma-argon-arc group was the highest, and that of the controlwas the lowest. But as time progressed, the rate of Ti ion release ofmicro-plasma-argon-arc group decreased and was the highest until the80th day while the rate of control increased. On the 100th day, there wasno significant difference of Ti ion release among groups. The tensilestrength of each group decreased and that of the control decreased themost.Conclusion: Both micro-plasma-argon-arc welding and laser weldinghardly impair titanium corrosion resistance. In some respects, they caneven promote its corrosion resistance. Micro-plasma-argon-arc weldingand laser welding were both thought to be ideal methods of welding.