Effect Of Heat Treatment On Mechanical Properties Of Dental Pure Titanium Castings

Nowadays,owing to the excellent mechanical properties,favourable corrosion resistance and biocompatibility,titanium has became the best prosthodontics material with comprehensive performance.However,after fininshed,titanium has the work hardening and its ductility decreases.Currently in dental field,the way of titanium processing is casting.But casting may take significant effects on mechanical properties of titanium.Especially,as the ductility decreases,the clasp is to break into two easily.The typical constitution of titanium is grossus Transformedβafter casting.The elongation and tensile strength are infaust because of the grossus grains.There are three ways to improve casting grains:heat-treatment,high intermediate pressure and thermal chemistry treatment.One of the ways is used in industry as a working procedure in titanium castings.Now there isn't any way used to improve cast structure in dental titanium castings.They usually put the casting moulds into the water with the temperature of 23℃for cooling.In order to choose the best parameters and modes of heat treatment,this study adds heat treatment process in titanium castings to observe the influence of heat treatment on the tensile properties of dental titanium castings.Then to investigate the effect of different annealing parameters and modes on the metallographical microstructures and the appearances of incision.The dumbbell metal samples of government standard GB6397-86 were made with dental pure titanium.In accordance with the common cooling way of dental cast titanium,some casting moulds were put into water with the temperature of 23℃for cooling after casting as WC group(control group),while the others were put in air for natural cooling as AC group(experiment group). After sand blast,etching and X-ray check,chose the samples which have no pore internal,no defect on the surface to elongate by universal testing machine,with the elongating speed of 0.5mm/min.Then the ultimate tensile strength(σ_b),0. 2%offset yield point strength(σ_0.2),and elongation to fracture(δ) were surveyed.The microstructures of the samples without etching and breaking were determined by metallurgical microscope.The appearances of incision of the broken samples which had the max of elongation to fracture(δ) were investigated with scanning electron microscope.The results showed that the ultimate tensile strength(σ_b)and0.2%offset yield point strength(σ_0.2) of WC group were higher that of AC group,while the elongation to fracture(δ) of WC group was lower.It indicates that the casting moulds should be natural cooling after casting to reduce the cast work hardening.Moreover,heat treatment with different temperature and different holding time were added to investigated the influence of annealing parameters on dental titanium castings.The tensile properties of dental titanium castings showed that the ultimate tensile strength(σ_b),0.2%offset yield point strength(σ_0.2),and elongation to fracture(δ) of the samples all enhanced at the annealing temperature of 650℃,700℃and 750℃.With the annealing temperature raising,the tensile strength and yield strength had the downtrend.All the mechanical performance index cut down when the temperature over 800℃.As the holding time was over 0.5h,the tensile strength and yield strength of dental cast titanium didn't change obviously,while there was downtrend in elongation to fracture(δ).When the annealing temperature was 700℃and holding time was 0.5h,dental titanium castings had the best tensile properties in all the single annealing groups.In different way of the heat treatment,the tensile properties of dental titanium castings showed that after vacuum heat treatment,the ultimate tensile strength(σ_b),0.2%offset yield point strength(σ_0.2),and elongation to fracture (δ) of the samples were better than contral group.As the annealing temperature increased,the mechanical properties enhanced.Though the surface reaction layer of the vacuum annealing groups were shinner than any other groups,their tensile properties were lower than common annealing groups for their gross grains.After secondary annealing,the samples kept high-intensity while the ductility enhanced,and had the best tensile properties in all groups.However,holding time of it was too long and the surface reaction layer was too thick.The center microstructures showed that all the groups were principally composed of transformedβwhich were grosses.When the heat treatment temperature was 700℃(include secondary annealing),the grains were relatively diminished and uniform,and the arrangement had the equiaxed tendency.But when it was 900℃,massive basket net structures and some needle-shaped structures were clearly shown. Scanning electron microscope showed that all the appearances were mixed pattern incision and had the performance of tenacious break.Both of the secondary annealing and 700℃annealing groups apparently increased shear lip area and plastic fibre zone,and had bigger tenacious microbores,which were also deeper,than other groups.The tensile properties of dental titanium castings could be enhanced by heat treatment.After the annealing of different parameters and modes,the tensile properties,the metallographical microstructures and the appearances of incision of the dental cast titanium may chang correspondingly.However,owing to the thick surface reaction layer which effect on the precision,heat treatment couldn't be used in dental crowns or bridges except the way of vacuum annealing.It may be used in removable dentures,for their need of the precision was relatively lower.