Effects of laser-weld joint opening size on fatigue strength of Ti-6Al-4V structures with several diameters

Abstract

This study was conducted to evaluate the fatigue strength of Ti-6Al-4V laser-welded joints with several diameters and joint openings. Sixty dumbbell rods were machined in Ti-6Al-4V alloy with central diameters of 1·5, 2·0 and 3·5 mm. The specimens were sectioned and then welded using two joint openings (0·0 and 0·6 mm). The combination of variables created six groups, which when added to the intact groups made a total of nine groups (n = 10). Laser welding was executed as follows: 360 V per 8 ms (1·5 and 2·0 mm) and 380 V per 9 ms (3·5 mm) with focus and frequency regulated to zero. The joints were finished, polished and submitted to radiographic examination to be analysed visually for the presence of porosity. The specimens were then subjected to a mechanical cyclic test, and the number of cycles until failure was recorded. The fracture surface was examined with a scanning electron microscope (SEM). The Kruskal-Wallis test and Dunn test (α = 0·05) indicated that the number of cycles required for fracture was lower for all specimens with joint openings of 0·6 mm, and for 3·5-mm-diameter specimens with joint openings of 0·0 mm. The Spearman correlation coefficient (α = 0·05) indicated that there was a negative correlation between the number of cycles and the presence of porosity. So, laser welding of Ti-6Al-4V structures with a thin diameter provides the best conditions for the juxtaposition of parts. Radiographic examination allows for the detection of internal voids in titanium joints.

Fatigue performance of joints executed in pure titanium structures with several diameters

“…Radiographies of the specimens were then taken with radiographic film (Ektaspeed Plus, Eastman Kodak, Rochester, USA) to verify internal defects. The radiographic exam consisted of exposing the specimens to radiation (90 KV, 15 mA, 0.6 s, and 10 to 13 mm distance), using a periapical film 11,12) . The specimens that presented voids within the dumbbell rod's central diameter were excluded.…”

“…The jointed portions were lined up in a metal matrix in order to maintain distances of 0.0 and 0.6 mm between them 12) . The aligned specimens were fixed with acrylic resin Duralay II (DuraLay, Reliance Dental Mfg Co, Chicago, USA), respecting the welded distances (0.0 and 0.6 mm).…”

“…The metal adjacent to the gap was abraded with airborne 100 µm aluminum oxide particles at a pressure of 0.55 MPa, and the joints were welded on opposite sides of the specimen in order to stabilize them. Next, the gypsum was removed, and the welding was conducted using energy of 360 V/8 ms (1.5 and 2.0 mm specimens) and 380 V/9 ms (3.5 mm specimens), with the focus and frequency calibrated at zero using a Desktop -F (Dentaurum, Pforzheim, Germany) laser machine 12) . The welding process was performed by a trained and competent professional.…”

“…Specimens were divested with the help of a pneumatic hammer (M320, Flli Manfredi, Sofia, Italy) and airborne particles, abraded at a pressure of 0.55 MPa with 100 µm aluminum oxide particles 12) . The specimens were finished and polished using special titanium drills (Rematitan, Dentaurum, Pforzheim, Germany), rubbers n o 5001 polisher (Dedeco Dental, New York, USA), and titanium polishing paste (Tiger Brilliant Polier Paste, Dentaurum, Pforzheim, Germany).…”

“…Such characteristics include excellent biocompatibility, high corrosion resistance, desirable physical properties, low cost, low density, and low thermal conductibility 3,5,7,8) . The low density associated with the low atomic number confers titanium with a low radiopacity grade that permits radiographic analyses to visualize internal voids in titanium-cast frameworks [10][11][12][13] . Implant prostheses require more accurate fits than natural teeth prostheses do because of the lack of periodontal tissues able to compensate for minor relationship errors 8) .…”