Materials Science Forum Vols. 539-543

Abstract: Joining by forming of magnesium alloys is restricted by the limited forming capability of magnesium at room temperature. For this reason heating of the parts to temperatures of 220 °C or more is required to form joints without cracks. State-of-the-art joining by forming methods (such as clinching or self-pierce riveting) are usually working with a contoured die as a counter tool. Researches on these joining methods have shown that a minimum heating time of 3 to 6 seconds is needed to achieve joints of acceptable quality. In this paper two new clinching methods “dieless clinching” and “dieless rivet-clinching” shall be introduced. Both methods work with a flat anvil as a counter tool, thus offering important advantages for the application in joining of Mg/Mg, Al/Mg or Fe/Mg joints. In joining by forming with a flat counter tool the proportion of crack inducing tensile stresses in the bottom part during the joining process is very low. Moreover the heat transfer between the heated anvil and the parts is comparatively fast. That makes it possible to decrease the heating time in joining by forming with a flat counter tool to only one second or less. The dieless clinching process was simulated using the Finite Elements Method (FEM) to analyze the influence of geometrical parameters of the punch and the process parameter clamping force. Furthermore the limitations of the new dieless joining method were investigated.

Abstract: Braze Pressure Welding (BPW) with high frequency induction heating is a newly developed pressure welding technique using interlayer metals for welding the general steel pipes for pipe arrangement in buildings. BPW enables to make joints by solid-state welding in air using a flux sheet which is also developed for this technique. In this method, the interlayer metal is expected to play the primary role in making high performance joints. At a joining temperature, the melted interlayer removes contaminations from the joining surfaces of the base metals. Then the liquid filler is discharged from the joining interface by the joining pressure, and forms fillets at the gaps around the joint. In this stage, the joining pressure is relatively small, so the welding deformation can be restrained to a minimum. The fillets have the effects both on relaxing the stress concentration at the joint and on increasing the joining area, which contribute to the strengthening of the joint. Thus, the high quality pipe joining without the dispersion in joint properties due to welder’s skill can be performed. In this report, the concept and merits of BPW are explained, and the results of the investigation on the cleaning effect of the interlayer metal and the effects of fillet formation on strengthening the joint are discussed. The practical application of BPW in Japan is also introduced.

Abstract: Resistance spot welds (RSW) have been widely used in the sheet metal joining process due to high productivity, low cost and convenience. The application of aluminum alloys in automobile manufacturing has resulted in the reduction of weight in vehicle, fuel economy and improvement of drivability and driving performance. Therefore, the resistance spot weld (RSW) process has attracted attention to fabrication of aluminum structures. However, applications of Al RSW are limited due to the unreliable durability of the weld-bonded joints. This study presents an experimental investigation on the use of a post-weld cold working process to improve the fatigue strength of Al RSW. The post-weld cold working process includes a pair of uniquely shaped indenters that are pressed or driven into the structure in order to induce the compressive residual stresses around the RSW. The mechanical properties of the post-weld cold worked Al RSW were investigated, including the experimental results of fatigue. The effect of the post-weld cold working process parameters on the fatigue strength of the Al RSW was also investigated. Comparisons of the mechanical properties and qualitative results between the as-welded RSW specimens and the post-weld cold worked RSW specimens of each post-cold working parameter have been carried out. Also, discussions on the experimental results using the finite element method (FEM) for the post-weld cold working parameters are performed.

Abstract: The characteristics of Nd:YAG laser welded 600MPa grade TRIP(transformation induced plasticity) and DP(dual phase) steels with respect to hardness, microstructures, mechanical properties and formability was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. In the case of TRIP steel, the hardness was the highest at the fusion zone(FZ) and HAZ near the FZ and decreased as the base metal was approached. The maximum hardness increased with increasing welding speed to 3.6m/min and then remained. The microstructures of FZ and HAZ near the FZ consisted of ferrite and bainite for all welding speeds. In the case of DP steel, the maximum hardness was obtained at the HAZ near the FZ. It increased rapidly to 2.1m/min and then showed nearly the same value. The difference between the maximum hardness of HAZ and that of FZ increased with decreasing welding speed. The microstructure of FZ was composed of acicular ferrite but the HAZ near the FZ contained bainite and ferrite at a low welding speed. Both steels had similar tensile properties and formabilities. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the strengths were somewhat higher than those of raw metals. In a parallel tensile test, the strengths of the joints were higher than those of raw materials but the elongations were lower. Formability was determined to be approximately 80% as compared with raw material at the optimum conditions. Differences in hardness near the welded zone was dependant on the relative contents of ferrite.

Abstract: In Gas shielded Metal Arc Welding process, the molten drop at the electrode wire tip is detached and transferred into the weld pool by various driving forces, which have been discussed mainly focusing on gravity force, electro-magnetic force, surface tension force and plasma drag force. In this paper the oscillation phenomena of metal drop and their numerical model are described. The numerical model developed is an axial symmetrical 2D model which enables to calculate and visualize time-change of the drop shape and the flow in the drop using VOF-CSF method. The validity of the model is verified through the comparison between calculations and experiments using water
drop.

It is shown that the natural frequency of the pendant drop decreases with increase of the drop size. And the numerical simulation predicts that it is possible to detach the molten drop from the electrode wire by exciting the forced oscillation around the natural frequency with pulsed current.

Abstract: Super duplex stainless steels were welded using new flash butt welding technology of temperature controlling system. The super duplex stainless steel (329J4L) and conventional duplex stainless steel (329J3L) were used. The samples were mounted in the dies using a Gleeble thermal simulator and flash but welding was made. The specimens were heated up to 1373K for 10sec, 20sec and 30sec. Flash butt welding has consisting of a two stage processes of a flash action and a contact resistance. First stage was a flash welding process and second stage was a solid state bonding process. The cross sectional microstructure of the weld bond region showed two types of a deposited fine particles region and a solid state bonding region. The grain growth was hardly observed in the weld region and the heat-affected zone. For further increasing joining efficiency of solid state bonding at the second stage, the welding time at 1373K was increased from 5 sec to 180sec. The bonding area increased with increasing welding time at 1373K and successfully welded for conventional duplex stainless steel.

Abstract: There is an ongoing drive to reduce the operating costs of aero-engines and this may be achieved partially via an increase in engine efficiency. To achieve this, industry needs to utilise new materials that can withstand higher operating temperatures and stresses. Many of the nickel-based alloys suitable for these applications, eg MAR-M-002, are difficult to join using conventional welding techniques. This paper describes a technique used to successfully weld 2.5 mm thick plates of cast MAR-M-002. The technique used FEA modelling to analyse stresses during welding and multiple axis electron beam deflection to alter the temperature and stress distribution around the fusion zone to prevent the initiation or propagation of cracks. FEA modelling of the process has been used to reduce the total number of practical tests, and hence, to conserve the limited material supply. This technique has made it possible to produce crack free welds in what is usually classed as an 'impossible to weld' alloy.

Abstract: Laser welding is a well known process, so is GMAW. But the joining of the processes, the so called laser hybrid welding is not that known, yet. Looking into the laser welding we realise, after many years use that there are some limitations to the process. One of the most significant is that the process deals poorly with the ability to bridge gaps between plates that are to be joined. This has its drawbacks on welding economy, the tolerances on ingoing parts has to be very high, the laser process can not tolerate more than 0,1 mm gap between the plates. The GMAW process on the other hand has the ability to deal with the joint tolerances in a better way, the backside is of course the productivity and the penetration properties. In the laser hybrid process where the two processes are joined in the same welding head, we are experienceing a great improvement in several properties. Gaps are no longer a big problem, the process can deal with gaps up to 2,0 mm with the present technology, (it is possible to coop with larger gaps using an oscillation of the laser beam, but this is only on experimental stage yet). This now means that costs for ingoing parts will be reduced. Process stability at high welding speeds is acchived. There are many advantages in high strenght steels that are sensitive to heat input; better mechanical properties are possible to obtain. Looking at the stainless materials the process has many advantages, one of the most important is the joint volume reduction in thicker materials, another is the ability to weld in duplex stainless steels.

Abstract: Monte Carlo (MC) and finite difference (FD) hybrid method is applied to numerically model the growth of austenite grains caused by welding thermal cycle in the heat affected zone (HAZ) of steel welds. The grain growth behaviors and pinning effects by dispersed particles are modeled by MC method while steep changes in temperatures with time and over space are by FD method. The connection of the two methods is made possible by correlating MC step to the real time through the grain growth theory. The model results are in a good agreement with experiments as well as previous experimental results published, and it is indicated that the developed model can be used to obtain a better understanding of the austenite grain structure development in HAZ and even to improve the grain refinement technology using dispersed particles.