An initial study of diffusion bonds between superplastic Ti-6Al-4V for implant dentistry applications.

Statement of problem Production of precisely fitting fixed partial denture implant superstructures with titanium alloys is limited by casting techniques that introduce distortion. After alignment of the framework with existing implants, the remaining misfit may generate stresses that cause screw loosening and adversely affect the implant/bone interface. Purpose The purpose of this study was to prepare diffusion-bonded joints between superplastic (SP) Ti-6Al-4V plates and indenters (representing analogs to implant-supported fixed frameworks and abutments) and determine if this process has potential for producing strong, dimensionally precise prostheses. Material and methods Seven sets of trial indenter and plate specimens with dimensions of 6.4 mm × 6.4 mm × 5 cm and 8.5 mm × 1.5 cm, respectively, were prepared. Several indenter designs were used (35- or 45-degree half-angle, presence or absence of a notch, and SP versus no SP condition for Ti-6Al-4V); all plates were prepared from SP Ti-6Al-4V. For the results-guided experimental design, there was 1 trial indenter/plate combination for each design/processing condition. Diffusion bonding was performed at 10 −6 Torr, while the temperature was increased 10°C/min to 900° or 920°C. Following 10 minutes of equilibration, the indenter was pressed 2.5 mm into the plate at 0.13 mm/min. Joint strength was evaluated in tension, and the ductile or brittle character of fracture surfaces was assessed by the presence or absence of a dimpled rupture surface from secondary electron SEM observations. Fractured specimens were cross-sectioned and examined with an optical microscope to evaluate overall joint integrity and quality, and used for Vickers hardness measurements to gain insight into the variation in mechanical properties of the indenter and plate with distance from the joint. One-way ANOVA (α=.05) was used to compare hardness at the joint for the trial specimen with highest joint strength with hardness values for adjacent regions at 125-μm intervals in the indenter and plate. The Ryan-Einot-Gabriel-Welsch (REGW) multiple range test was used to identify any specific location having significantly (α=.05) different hardness. Backscattered electron SEM observations were also performed on the cross-sectioned specimens to investigate whether a layer of alpha-stabilized titanium, which would decrease joint strength, was present. Fits of Ti-6Al-4V implant analogs prepared by this diffusion-bonding process were assessed qualitatively from visual observation. Results The maximum joint strength of 820 MPa was achieved for a diffusion bonding temperature of 900°C for an SP Ti-6Al-4V indenter with a 35-degree half-angle and no notch. This joint strength is nearly 90% of the maximum tensile strength of the parent Ti-6Al-4V, which can range from approximately 930 to 1015 MPa. The hardness at the joint was significantly higher ( P P Conclusions A 900°C processing condition for diffusion-bonding an SP Ti-6Al-4V indenter with a 35-degree half-angle and no notch to a Ti-6Al-4V SP plate yielded a joint with nearly the same strength as the parent alloy. Use of this processing temperature with a 0.13 mm/min rate of pressing the indenter into the plate yielded minimal distortions for implant prosthesis analogs when observed visually.