Effect Of Cryogenic Treatment On Properties Of Laser Welding Zone In Dental Titanium

Objective:Laser welding is an important technique in dental restoration. The quality of welding is directly related to the success or failure of restorations. Studying on welding process to improve the comprehensive properties of welding zone of laser welded titanium is the direction of dental profession for many years. In order to improve the overall performance of welding area of laser welded titanium, cryogenic treatment was used on welded samples in a simulated oral environment. Mechanical properties were studied and analyzed to optimize the technology of cryogenic treatment. Compared with no treatment and heat treatment group, the samples after cryogenic treatment was preliminary studied on corrosion resistance and biological properties to explore the feasibility of cryogenic treatment on the clinical application.Methods:Comlaser-4laser welding machine made in German was used to weld the samples, which are length in6mm and thickness in0.5mm.1. To optimize the technology of cryogenic treatment:The laser welded titanium standard specimens were randomly divided into6groups, including4groups with cryogenic treatment which immersed in liquid nitrogen for2hours,12hours,24hours,48hours respectively, one group with traditional heat treatment and the last one as control group without any treatment. The best treatment method was selected by studying the tensile strength, elongation and micro-hardness of the six groups of specimens.2. Evaluation of mechanical properties:Specimens were divided into four groups (parent material group, cryogenic parent material group, laser welded Ti group, cryogenic laser welded Ti group), and then the samples were immersed in artificial saliva for90days after1000thermal cycles. At last, the mechanical properties and the microstructure were tested and the fracture morphology was observed.3. Evaluation of fatigue performance:Comparing with non cryogenic treatment group, the fatigue strength of cryogenic laser welded Ti group were tested in a simulated oral physiological environment by using Instron E10000electronic static and dynamic universal material testing machine. The two S-N curves were obtained and the fatigue characteristic values were calculated. Besides, the morphology of fatigue fracture samples was observed.4. Evaluation of corrosion resistance and antibacterial properties:The anodic polarization curves of four groups (parent material group, cryogenic parent material group, laser welded Ti group, cryogenic laser welded Ti group) were measured and the characteristic parameters were calculated; the electrochemical impedance spectroscopy of four samples in artificial saliva were fitted and the antibacterial effect of four samples were observed by SEM.5. Evaluation of friction and wear performance:Two body abrasion experiments were carried by MMV-1vertical universal friction and wear testing machine, the amount of wear volume loss was calculated and the morphology of surface abrasion was observed.Results:1. The tensile strength and elongation were greatly improved by Cryogenic treatment of24hours, but the hardness of laser welded region was not significantly reduced, and better mechanical properties were obtained in this group.2. In a simulated oral environment, the tensile strength and the elongation of parent Ti after cryogenic treatment were increased by6.24%and2.2%, but increased by20.82%and150%in laser welding zone of Ti. The parent Ti showed a ductile fracture, and the laser welded Ti without cryogenic treatment showed a brittle fracture. The cryogenic laser welded Ti showed a mixed quasi-cleavage fracture. The grain size was significantly reduced and a more uniform structure was obtained by cryogenic treatment.3. Under different stress range, the fatigue cycles of cryogenic laser welded Ti were all higher than the control group. The fatigue strength characteristic value△σk of the cryogenic laser welded Ti group was121.884MPa corresponding to the slope(m) of S-N curve was5.94. Both of them were higher than the control group (97.000MPa,5.40), and the fatigue strength increased by25.654%with cryogenic treatment. The second layer strip fractography of the control group were more obvious than the cryogenic group under the same stress. The final sections of cryogenic group showed numerous and deep dimples and the ones of control group showed quasi-cleavage sections with a small quantity of dimples.4. The breakdown potential, the△E and the Rp of the cryogenic laser welded Ti were0.455V,1.365V and282110Ω·cm2. All of them were significantly higher than laser welded Ti (0.047V,0.629V and6654Ω·cm2). It can be seen that less bacterial gathered on the cryogenic laser welded Ti than on the welding zone of group without cryogenic treatment.5. The amount of wear volume loss of cryogenic laser welded Ti was less than the one of laser welded Ti. The wear mechanism of cryogenic laser welded Ti was abrasive wear but the mechanism of the laser welded Ti was mainly adhesive wear accompanied with abrasive. Conclusions:1. Optimization of cryogenic treatment technology for laser welded Ti: Immersed in liquid nitrogen (-196℃) for24hours.2. Cryogenic treatment improved the tensile strength and elongation of laser welded Ti in a simulated oral environment, but the effect of cryogenic treatment on laser welded Ti was larger than the parent Ti.3. Cryogenic treatment significantly improved the fatigue strength of laser welded Ti.4. Cryogenic treatment improved the corrosion resistance and the antibacterial properties of laser welded Ti.5. Cryogenic treatment improved the friction and wear properties of laser welded Ti.