Materials Science Forum Vols. 539-543

Abstract: Transient local melting can account for the remarkable decrease in the strain rate when the tool rotational speed, heating rate and cycle time produce temperatures exceeding the 475 °C in Al 7075-T6 spot welds. The calculated strain rate is about 20 s-1 in Al 7075-T6 spot welds produced using tool rotational speeds from 1500 RPM to 3000 RPM and dwell time of 4 s. However, the peak temperature is less than less than 475 °C when a low tool rotational speed setting (1000 RPM) is used; transient local melting and tool slippage are precluded and the calculated strain rate is high (about 650 s-1). It is proposed that transient local melted films directly influence formation of the stir zone microstructure in Al 7075- T6 spot welds. It is suggested that the stir zone microstructure in spot welds made using high tool rotational speed settings are determined by a combination of grain boundary sliding and the limitation of cavity interlinkages when transient local melted films form.

Abstract: The thermo-mechanical simulation of Friction Stir Welding focuses the interest of the welding scientific and technical community. However, literature reporting material flow modeling is rather poor. The present work is based on the model developed by Heurtier [2004] and aims at improving this thermo-fluid simulation developed by means of fluid mechanics numerical and analytical velocity fields combined together. These various velocity fields are investigated separately and especially according to the power dissipated during the flow. Boundary conditions are considered through a new approach based on the kinematic analysis of the thread of the pin. An equilibrium is established between the vertical motion of the bulk material dragged in the depth of the metal sheet, and its partial circulation around the pin. The analyses of the obtained velocity fields enable the understanding of the welded zone asymmetry and highlights the bulk material mixing between the welded coupons in the depth of the sheet. A regression is performed on the relative sliding velocity of the aluminium according to the surface of the tool: shoulder and pin. Two dimension flow lines in the depth of the metal sheet are then obtained and successfully compared with the results obtained by Colegrove (2004) [1].

Abstract: Tribological properties are found to change with microstructure. In Ni-P amorphous alloy, annealing conditions were varied with laser irradiation parameters such as scanning speed and laser power. The increase in hardness affected by scanning speed. A peak of hardness was observed as the function of scanning speed. This is because, the formation of nanoscopic composite structure by distribution of crystalline Ni3P compounds in amorphous matrix is the hardest structure.

Abstract: The influence of roll angle of the FSW tool rotation axis on tensile properties of joints was examined to clarify the allowance of deviation from proper tool posture for three-dimensional friction stir welding. A5083-O plates of 3 mm in thickness were butt welded by three-dimensional controlled FSW machine. Transversal inclination of the tool (roll angle of the tool) was continuously changed from 0 degrees to 10 degrees for both sides. Butt joints ruptured at base metal within the range of roll angle of the tool plus/minus 2 degrees, and ruptured at the thin part made by inclined tool shoulder entering in the range by plus/minus 7 degrees. Over the range of plus/minus 7 degrees, butt joints ruptured at elongated cavities made by tool inclination at the border of stir zone and base material.

Abstract: This paper described an experimental study on the friction stir welding of dissimilar metal between commercially pure titanium (CPTi) and magnesium alloy AZ31B (Mg). Butt-welds produced at various tool rotation speeds were evaluated by the observation of the weld surface appearance, X-ray radiographic test, tensile test, SEM and EDX analysis. The main results obtained are as follows. Butt-joint welding of the CPTi plate to the Mg plate was easily and successfully achieved. The ignition of Mg occurred during welding at the tool rotation speeds over 1200rpm. The fragments of CPTi existed in a continuous form in Mg matrix. The tool rotation speed of 1200rpm attained the maximum tensile strength of a joint, which was about 75% of that of Mg base metal. Fracture occurred at the weld interface in most joint. EDX analysis revealed that Al in the Mg diffused into CPTi through the weld interface and MgO existed at the interface. It seems that the decrease in Al concentration in the Mg and MgO formation around the weld interface caused the low tensile strength of the joints.

Abstract: The microstructure of the friction-bonded interface of Al alloys to low C steel has been investigated by TEM observations to reveal the controlling factor of the formation and growth of the IMC (Intermetallic Compound) layer, which caused the premature fracture at the interface even when its thickness was less than 1 μm, as reported in a previous paper. The thickness of the IMC layer observed at the interfaces of Al-Mg alloy/steel and pure-aluminum/steel joints increased almost in proportion to the friction time, but did not obey the parabolic law a characteristic kinetics of the diffusion-controlled process. Analyses of SAD patterns from the IMC layer indicate that it consisted of Fe2Al5, Fe4Al13, (Fe, Mn)Al6 and FeAl2, depending on the alloying elements. These IMCs were granular and distributed almost randomly within the IMC layer, suggesting that mechanical mixing of the steel with the Al alloy occurred at the interface. In the low C steel adjacent to the IMC layer, a zone of much finer grains than those of the base metal was observed. Its width increased with friction time and pressure, and with the growth of the IMC layer, as well. These results suggest that the superficial region of the steel underwent a heavy plastic deformation during the friction process and it had a close relation with the growth of the IMC layer.

Abstract: In the manufacturing thick structural pipes recently a new process has been developed. This Paper describes this newly discovered process called Laser Hybrid welding. In laser hybrid welding two processes i.e. laser welding and Gas Metal Arc (GMA) welding are combined together to create synergistic effects. In laser GMA hybrid welding of plates with thicknesses of up to 20 mm, the problem of pore formation and of centre rib defects occurs frequently which have, up to now, not yet been subject to detailed scientific analysis. It was the objective of this research work to reduce these weld defects by weld parameter investigations (of a geometrical and also metallurgical nature) or to even avoid them completely. It was investigated to determine the extent to which shielding gas composition, (root) gap width and misalignment of the joining partners affect the techno-mechanical properties. For this purpose, the welds were subjected to non-destructive test methods (viz. visual inspections and X-ray examinations) and also to destructive test methods (viz. transverse sections hardness measurements, tensile tests and notch bending tests). The synergistic effects have been discussed in the light of the mechanical and metallurgical characterisations of the weldments. Optimum process parameters have been evolved which could tolerate up to 0.4mm root gap and 0.8mm of misalignment without causing any centreline cracking. Combinations of helium and argon gas shielding have been found to produce porous free welds. It has been concluded that it is possible to replace submerged arc welds by laser GMA hybrid welds in the manufacturing of longitudinally welded pipes.

Abstract: This paper gives an historic overview and new developments of research activities in the field of the oscillatory behaviour of liquid metal in arc welding. Early work focused on the oscillation behaviour of the weld pool in Gas Tungsten Arc Welding (GTAW). Agitated weld pools exhibit specific modes of oscillation, the frequency of which can be measured from the arc voltage data and is conditioned by the geometry of the weld pool and the properties of the liquid metal. Of technological interest is the alteration of the oscillation behaviour for partially and fully penetrated situations, which can be used for penetration control during welding. A logical extension of the research activities was related to the influence of filler wire addition on the oscillation behaviour. An intermediate step towards the description of Gas Metal Arc Welding (GMAW), is the situation of GTAW with cold filler wire supply. It was found that both the liquid weld pool and the pendant liquid droplet at the tip of the filler wire experience an oscillation, which obscures the influence of the individual contributions of both liquid masses on the voltage data. It was shown that online penetration control is still possible, provided that the metal is transferred in an uninterrupted way, i.e. the filler wire flows smoothly into the weld pool. For GMAW, in which detached droplets collide with the weld pool surface, the difficulties are even more prominent. Recent work is related to this issue. Monitoring of the phenomena occurring at the weld pool and the pendant droplet become problematic by means of the voltage data. Observations by means of high-speed video imaging will be discussed. Apart from the experimental studies, efforts are undertaken in numerical simulations of the processes. A good correlation is obtained between experimental data and the results of the numerical models.

Abstract: Pulsed electric-current sintering (PECS) was applied to the bonding of SiC (pressureless-sintered silicon carbide) to Cu (oxygen-free copper) using a mixture of Cu and Ti powders as an intermediate layer. The influences of the intermediate powders on the bond strength of the joint were investigated by observation of the microstructure. The bonding was carried out at carbon-die temperatures from 973 to 1173 K at a bonding pressure of 10 MPa for 3.6 ks. The application of intermediate layers of 100% Ti, 95% Ti + 5% Cu, and 5% Ti + 95% Cu remarkably improved the bond strength as compared with direct bonding without an intermediate powder. SEM observations of the joint with the intermediate powders revealed that a Cu solid-solution layer, a TiC layer, and a Ti5Si3 layer had covered most of the interface, similar to those observed in the friction-bonded and pulsed-electric current bonded joints of SiC to Cu in which the application of a Ti foil as an intermediate layer remarkably improved the bond strength.