Mechanical Properties Of The Thin Film Of Laser Welded Cast Titanium

Pure titanium is used more and more widely for fabricating restorationbecause of its good biocompatibility, excellent mechanical properties andcorrosion resistance. It is used not only for the bracket of removable partialdenture and base of complete denture, but also for the fixed partial denture andimplant denture. However, when pure titanium is used for the long span frameof removable partial denture or fixed prosthesis, the frame is usuallymanufactured partly first, and then weld into a whole restoration to compensatefor the unavoidable errors caused by the casting process. In addition, the metalwelding technique is also be used to compensate the denture defects related tocasting and repairing the fractured denture frame.Now, laser-welding technology has been used widely in welding puretitanium, because the laser energy can be concentrated on a small area resultingin fewer effects of heating and oxidation on the area surrounding the weldedspot. Past researches have reported the changes of mechanical properties of thelaser welding of pure titanium. As the laser-welding spot is small and thewelding seam is narrow, the width of welding zone is only tens to hundreds of microns. So, the weld material has a mechanic property of thin film materialnaturally, and this property could be researched accurately only employing themethod to study the property of the thin film material there. Only in this way, wecan accurately describe the mechanical properties of the weld thin film materialsand analyze the factors that impact the effect of welding. We can also obtainstress-strain curve of weld film material which is important for producingtitanium prosthesis.Accordingly, the mechanical properties of the thin film welded titaniumwere investigated with the method of laser interferometer Strain/Displacementgauge to determine the tensile properties and the stress-strain curves in thisstudy. The experimental results were compared with finite element simulationanalysis, in order to study the factors that affect the welding performance. Thefracture surfaces were inspected using scanning electron microscope and EDS toevaluate the defects for both laser-welded and control groups, to provide theexperimental basis for the laser welding performance.All the study can be divided into four parts.1. The plexiglass plates with specification of60mm×20mm×1mm and60mm×40mm×1mm were fabricated as investment patterns of pure titaniumspecimens. Every two cast titanium plates of defect-free castings with X-raydetection with dimension of60mm×20mm×1mm were welded togetherseparately at the side of60×1mm according to the weld widths (0.00mm、0.25mm and0.50mm), and was separated into three different test groups finally.And the contrast was the titanium plate of specifications60mm×40mm×1mmwithout treatment anymore. Each plate of the welding groups and the controlgroup were wire cut out six small samples.2. Tensile testing of specimens prepared in experiment one was operated with a micro-tensile machine using ISDG method. In the testing, ultimate tensilestrength were recorded and the stress-strain curves were drawn. Analyzing theultimate tensile strength, Yang’s modulus, yield strength, plastic deformationand PE of weld thin film and base mental, to evaluate the mechanical propertyof laser welded titanium.3. In finite element analysis, models of specimens with different weld widthand base mental were built. Through the process of finite element simulation oftensile test, stress, deformation and stress-strain curves of different width weldthin film were gained, and then compared with experimental results.4. The fracture surfaces were inspected using stereomicroscope and scanningelectron microscope to determine the type of fracture and evaluate the defectsfor both laser-welded and control groups. In order to determine the compositionof the weld zone, the main elements in base metal and solder were analyzed andtested by EDS also.The results of this study showed as following.1. The results show that the UTS of0.00mm,0.25mm and0.5mm widthwelding groups were492.16±33.19,488.09±43.18, and558.45±10.80MPa.The UTS of the control group was532.15±11.62MPa. One–way ANOVAdemonstrated there was no significant difference.2. The mean value of PE in base metal was19.60%, and the PE means in allwelding groups was increase, ranging from1.95%to10.92%, which were muchsmaller than in base metal.3. The stress-strain curves of welded groups were similar to the base metal,but the welded groups led to a reduction in the plastic deformation. Thestress-strain curves obtained from base metal were smooth and continuous at thestrain rate of plastic deformation period. It is obvious to note that the base metal is a typical plastic material.）4. Finite element analysis was used to simulating tensile process ofspecimens of different weld distance, the result shown that strain of plasticdeformation period was significantly greater than the experimental test results.5. Fracture surfaces of0.00mm and0.25mm width welding groups wereobserved under the stereomicroscope, showing that there were some non-weldedregions. However, fracture surfaces of0.50mm width welding group showedwelding well. SEM analysis revealed that base metal possessed its ductilecharacter with the cup and cone phenomenon，the fracture surfaces of weldinggroups showed ductile and brittle characteristics at the same time.6. The result of EDS shows that the concentrations of the three mainelements (Ti, O and C) are similar in base metal and solder. It indicates that thewelding process under a well controlled flow of insert argon gas has noformation of oxides, and the reliability of the laser welding technique used inthis study.Conclusion:1. There is no significant relationship between the UTS of laser welding andthe welding width. But the plastic deformation and the percentage elongationincrease with the increasing of the welding width.2. There are incomplete penetration region which will affect properties oflaser welded titanium in the welding groups with small weld width.3. Because of reduction in the PE and the plastic deformation, welding withsmall weld width may leads to brittle fracture which reduces service life. If theprosthesis weld width maintains large weld width, it will cause the weld zonewith a large deformation which will affect the welding accuracy. It indicates thatlaser welding should maintain appropriate welding distance.