Materials Science Forum Vols. 794-796

Abstract: Effect of friction stir welding (FSW) on mechanical properties and microstructure of Al-5.4Mg-0.2Sc-0.1Zr sheets with ultra-fined grained (UFG) structure was studied. The UFG-sheets were produced by equal-channel angular pressing (ECAP) followed either by cold or hot rolling. FSW was found to be very effective for retaining the UFG microstructure as well as constituent coherent nano-scale dispersoids in the welded material. Despite the preservation effect, however, the essential material softening was observed in the weld zone. This was attributed to the recrystallization occurring during FSW. The joint efficiency for yield strength of the obtained friction stir welds was found to be 81% in the hot rolled condition and only 55% in the cold rolled state. The relatively low joint efficiency was associated with the recrystallization softening as well as with the formation of a specific “kissing bond” defect in the stir zone. The joint efficiency is believed may be improved by adjusting of welding conditions and/or tool design.

Abstract: The Friction Stir Welding (FSW) is a dynamically developing version of the pressure welding processes. High-quality welded joints can be created using this process for different engineering applications (e. g. automotive parts). Nowadays, the knowing of the properties and the behaviour of the welded joints is an important direction of the investigations, especially under cyclic loading. The research work aimed to demonstrate the behaviour of the friction stir welded joints under cyclic loading conditions. Fatigue Crack Propagation (FCG) experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their friction stir welded joints. The CT type specimens were cut parallel and perpendicular to the characteristic directions of the base materials and the welded joints, and the notch locations in the specimens of welded joints were different, too. Therefore, the propagating cracks represent the possible directions of the fatigue cracks both on the base materials and on the welded joints. The results of the fatigue crack propagation tests on the welded joints clearly demonstrate the different characteristics of the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ), and the advancing (AS) and retreating sides (RS) of the weld nugget (WN). The investigations and their results were compared with each other and with the results can be found in the literature.

Abstract: The Friction Stir Welding (FSW) differs from the conventional welding process inter alia that the joint properties are not symmetric to the axis of joint line. Basically, it follows from the principle of the method because a rotary tool is used for welding. This asymmetry is due to created temperature field during welding, and the pressure force distribution along the thickness, so the investigation of these effects is important to create good quality welds. Experiments were performed on FSW joints made of non-heat treatable (5754-H22) and heat treatable (6082-T6) aluminium alloys, because there are clear difference between the reacts to these effect and show different changing on the microstructure. A special FSW tool was developed for the experiments and lowest inhomogeneity. Dynamic recrystallization and dynamic recovery can occur during welding so these were investigated by Gleeble material simulating system. During the experiments the grain sizes and grain shapes were also investigated.

Abstract: The impact welding was performed for several kinds of metal plate couples. The joint interface exhibited a sinusoidal wave form when two metal plates with the same or similar density (e.g. Al/Al, Cu/Cu and Cu/Ni) were impact-welded by high-speed oblique collision. In contrast, as for dissimilar metal plate couples with large density difference such as Al/Cu, an asymmetric wavy interface was obtained. In order to make clear the reason for morphological difference, a computer simulation of the collision behavior was performed using SPH (Smooth Particle Hydrostatic) method. The simulation results revealed that the wave form was controlled by the interaction between the emitted metal jet and metal plate surfaces ahead of the collision point. For Al/Al and Cu/Ni, the emitted metal jet hit each surface alternatively and this resulted in symmetrical wavy interface formation. While, for Al/Cu, the metal jet was emitted to the direction parallel to the Cu plate, and the interaction took place between the metal jet and the Cu plate surface. The metal jet emission and wavy interface formation mechanism were also investigated.

Abstract: A friction stir welded A3003 aluminum alloy /SUS304 stainless steel dissimilar lap joint was successfully produced. A sound joint that fractured at the base metal was obtained in the center region of the joint through the reaction layer of aluminum-rich intermetallic compounds with nanoorder thickness. The microstructural changes at the interface of the joint was examined by studying the hole left by the extracted welding tool produced at the end of the friction stir welding (FSW) bead using transmission electron microscopy (TEM). Mixed layers consisted of ultra-fined intermetallic compounds and stainless steel were observed. The stirred aluminum alloy flows onto the mixed layer after the tool transit and the joining was achieved. Based on the TEM observations, the joining process of the lap joint was also discussed.

Abstract: It was difficult to join dissimilar materials such as metallic material and polymer. Conventional joining processes of these materials were mechanical fastening, using adhesion, thermal pressing, laser welding and so on. These processes had disadvantages such as expensive apparatus, restriction of dimension of products and lack of anti-weather resistance.Friction Stir Welding (FSW), which was one of the solid state joinings, was available as a joining process for dissimilar metals. However, in case of joining metal and polymer, it was not available to use the tool for FSW. So we proposed Friction Lap Process to join a metallic material with a polymer and investigated mechanical and metallurgical properties of this dissimilar joint. Itwas described in this paper that joining mechanism is discussed with evaluation of microstructure at the interface between aluminum alloy and polymer. High density polyethylene was not able to be joined for as received aluminum alloy. Anodizing was effective to join with these materials.

Abstract: The present study investigated the laser welding performance of Al-Fe aluminum alloy sheets with different contents of intermetallic compounds. Under the same welding parameters, the alloy of higher intermetallic compounds content has wide and deep weld pools with uniform sizes. The alloy of lower intermetallic compounds content has narrow and shallow weld pools with nonuniform sizes. The higher content of intermetallic compounds results in higher laser absorptivity and lower thermal conductivity, and then increases the effective absorbed energy during welding, which is beneficial to the formation of wide and deep weld pools. The distribution uniformity of intermetallic compounds influences the size uniformity of weld pools. In the alloy with lower content of intermetallic compounds, the nonuniform distribution of intermeallic compounds results in the formation of abnormal weld pool, leading to the nonuniform size of the weld pools. In the alloy with higher content of intermetallic compounds, uniform distribution of intermetallic compounds make the size of weld pools more uniform.

Abstract: Static Shoulder Friction Stir Welding (SS-FSW) is a modification to conventional FSW that was originally developed to improve the weldability of titanium alloys by reducing through thickness temperature gradients. Surprisingly, to date, there have been no published systematic studies comparing SS-FSW to FSW for aluminium welding. This may be because the high conductivity of aluminium means the heat input produced by the shoulder is thought to be beneficial. In the work presented when welding a high strength 7050 aluminium alloy, even in a relatively thin 6 mm plate, it is shown that SS-FSW has several advantages; including a reduction in the heat input, a massive improvement in surface quality, and a more uniform through thickness temperature distribution, which leads to narrower welds with a reduced heat affected zone width and more homogeneous through thickness properties. The reasons for these benefits are discussed.

Abstract: An experimental study on the behaviour and fracture of flow-drilling screw connections was carried out. Cross test specimens were used to study the deformation and fracture of the connections under different loading modes. For comparison, tests were also carried out on self-piercing rivet connections.

Abstract: A critical issue when joining aluminium alloys to dissimilar metals such as magnesium, titanium, and steel is to control the formation of brittle intermetallic compounds (IMCs) that occur due to the reaction at the joint interface. It is demonstrated that once the IMC exceeds a critical thickness, failure of welds always occurs in a brittle manner, with cracks propagating through the IMC. One approach to minimize IMC thickness is to use a solid state joining process, such as friction stir or ultrasonic welding. However, even using these processes, an IMC that exceeds the critical thickness can either form during welding or post-weld heat treatment. In this paper, a number of approaches are discussed to control IMC formation in welds between aluminium alloys and magnesium alloy. Modelling predictions indicate that interfacial reaction rates and grain size of the IMC phases are two critical factors to control layer growth. Experimental results demonstrate that the grain size of IMC layers changes as the layers grow. These modelling predictions and experimental results offer new design strategies to optimize dissimilar metal welding involving aluminium.