Shinobu Satonaka

Mechanical properties and microstructures of magnesium alloy AZ31B joint fabricated by resistance spot welding with cover plates

Abstract The authors welded magnesium alloy AZ31B sheets using the technique of resistance spot welding with cover plates, and investigated the effects of welding parameters on the tensile shear strength of joints and shape characteristic of nugget. The joints with high tensile shear strength were obtained under relatively low welding current. The equiaxed grains with the many intragranularly precipitated particles Mg17Al12 in the nugget were observed.

Effects of Electrode Force on the Characteristic of Magnesium Alloy Joint Welded by Resistance Spot Welding with Cover Plates

Magnesium alloy AZ31B sheets were welded using the technique of resistance spot welding with cover plates. The effects of electrode force on the joining performance and pore formation during welding were investigated. The results reveal that the enhanced electrode force is an effective way to inhibit pore formation and improve magnesium alloy resistance spot welding performance.

Prediction of Tensile-Shear Strength of Spot Welds Based on Fracture Modes

Assessment of the strength of spot welds is a key process in the design of spot welded structures. Moreover, the prediction method to be used to evaluate the strength of specimens is not always clear, even for static loading. This is because the strength of spot welds depends upon various factors such as nugget diameter, plate width and thickness, material strength, number and location of spot welds and different fracture patterns. In this study, three formulae were proposed for the prediction of the tensile-shear strength of spot welds, which are based upon the relationship between the maximum stress and material strength in three fracture modes; plug type fracture, shear type fracture and fracture across the base metal. In these formulae, two parameters were used for the assessment of maximum stress in the uneven stress distribution around a spot weld. Assuming that fracture takes place at the weakest portion of the structure, the tensile-shear strength is given by the minimum value of the fracture load evaluated for the three fracture modes. It was shown by comparing the predictions and experiments that the predicted fracture load and real fracture mode are in close agreement for both single- and multi-spot welds.

Trends and new applications of spot welding for aluminium alloy sheets

With recent transport machinery and appliances including vehicles, measures to reduce energy consumption and to cope with environmental issues are important tasks to consider whilst the enhancement of functions and comfort is also sought. Technologies to reduce weight are an effective means to meet this demand and its requirements and, therefore, a variety of methods have been proposed. Representative methods include (1) those using thin-sheet structures of high-strength materials, (2) those using lighter materials such as aluminium alloys and magnesium alloys, and (3) those using multi-materials in which macromolecular materials, lighter material, highstrength steels, etc. are combined to make use of individual properties of materials. Among these methods, those which use high-strength materials can be employed with conventional welding methods and designing methods for steels, and so, helped by a steady supply of high tension steels and the development of ultra-high tension steels, they have already been applied to many service production sites. The methods using lighter materials and/or using multi-materials are the areas expected to see their range of application grow but they will require similar or dissimilar material welding processes of lighter materials such as aluminium alloys. Although welding technology and design methods for aluminium have nearly been established, there are still problems remaining to be solved in service production sites which are hindering the expansion of their application range. In different material welding between aluminium alloys and steel materials, a large difference between them in their thermal and physical properties affects their weldability, and hence various devices and measures have been tried out when usual welding methods are employed. Those welding methods tried include MIG welding, diffusion welding, laser welding, ultrasonic welding, friction welding and friction stir welding. At present, however, there have not been sufficient studies which take account of the service application of the welding methods. On the other hand, resistance spot welding, which has been employed for the assembly processes of vehicles and carriages, has advantages such as that it enables the use of existing equipment and apparatus, and that it can readily be automated for mass-production. Hence, it has been hoped that resistance spot welding can be applied to different material welding. Some methods have been proposed to make this possible. One of them is a method of using cladding materials. This method has problems caused by the insertion of the cladding materials, such as a reduction in productivity and in the life of electrodes owing to high current. Hence, its applicable range is restricted. Another similar method proposed is the method of using pure aluminium as the insert material to prevent a reaction phase from being created at the weld interface between different materials. To deal with these problems, the authors proposed a new resistance spot welding method of using a cover plate for the similar or dissimilar material welding of aluminium alloy sheet materials. This method is to enhance the heating and keep the temperature of the fused area in the spot welding of aluminium alloys by executing welding with the cover plates inserted between the materials to be jointed and the electrodes. In this report, this method was applied to the welding between aluminium alloys as well as the different material welding of aluminium alloys to mild steel and to stainless steel, to evaluate the effectiveness of this welding method from the aspects of the mechanical properties of the welded joints and the microstructure at the weld interface. The study will be described in the following sections.

Non-parametric effects on pore formation during resistance spot welding of magnesium alloy

Abstract The effects of faying surface condition, pre-existing pores in base material and cover plate on the pore formation during resistance spot welding of magnesium alloy were investigated. The results reveal that the hydrogen rejection and the pre-existing pores in base material have little effect on the pore formation, and that the larger pore formation is mainly caused by shrinkage strain during resistance spot welding of magnesium alloy.

Resistance Spot Welding Of Magnesium Alloy Sheets with Cover Plates

Lightweight materials such as high-strength steels and aluminum alloys are widely used in automotive components for the purposes of weight reduction and consequent higher fuel efficiency. As new alloys have been developed, magnesium alloys have become attractive for the production of automotive components due to their unique properties, such as high specific strength, damping capacity, and recyclability. In the assembly process of these materials, resistance spot welding (RSW) is used to construct thin sheet structures. Although RSW of steel sheets with high weld quality has already been established, the following problems remain with regard to RSW of magnesium alloys: high welding current, degeneration of electrodes, and blowholes and expulsion in or around the nugget. In this study, RSW with cover plates was applied to magnesium alloy sheets to investigate the performance of spot-welded joints. The weld quality was evaluated based on the tensile shear strength of the joints, the shape and the size of the nugget, and the appearance of blowholes and expulsion. Since RSW with cover plates is similar to conductive heat resistance seam welding (CHRSMW), the results showed that RSW with cover plates enabled the spot welding of a magnesium alloy with a large nugget and high tensile shear strength in the joint. It was also shown that blowhole-free spot welds with a large nugget and high tensile shear strength were obtained for welding currents above 8 000 A with a long down-slope time of welding current and a high electrode force.

Ultrasonic Evaluation of Spot Welding Nugget Diameter with a Line-Focused Probe

Resistance spot welding has been widely used for the construction of thin sheet metal structures. The structures’ reliability depends upon the weld quality as well as the weld size. Therefore, various non-destructive inspections have been applied to the spot welds. Among them, the ultrasonic evaluation is widely used for the detection of defects and for the measurement of weld size. However, this method is too difficult for the detection of the fusion zone or nugget non-destructively, in spite of the high demand for their evaluation. Due in part to the recent application of high-strength steel sheets to automobiles, this problem has become a crucial subject in spot welding. The discrimination of the nugget from the solid state-welded zone or the corona bond is now strongly demanded. This study focused on the non-destructive discrimination between the nugget and the corona bond; a new, ultrasonic measurement with a line-focused probe was proposed. In the experiment, spot welded specimens both with and without surface indentations were prepared, in order to determine the affecting factors on the ultrasonic measurement of spot welds. Moreover, two types of special, thin sheet specimens were prepared to determine the acoustic phenomena in the fusion zone. The first type was the resistance spot-welded specimen and the other was the gas tungsten arc-welded specimen. The measurement of these two special specimens showed that the sound velocity in the nugget differed from the base material and was dependent upon the inhomogeneous microstructures. The different sound velocity caused a different amplitude of reflected waves in the nugget, the change of which also depended upon the measuring conditions of the water path. From these results, it was shown that the measurement based on the different amplitude and the dependence on the water path enabled the non-destructive discrimination between the nugget and the corona bond.

Resistance Spot Welding between Aluminum Alloy and Low-Carbon Steel

Aluminum alloy A5052 and low-carbon steel (SPCC) was welded using resistance spot welding with a cover plate. The interfacial microstructure was observed using electron microscopy. A thick two-layered reaction layer contains Fe2Al5 and FeAl3was observed at the welding interface, respectively. Mechanical property analysis suggested that the reaction layer has no effect on the tensile shear strength of the A5052/SPCC joint and affects the cross tension strength of the joint.

Development of New Ultrasonic Inspection Technique for Spot Welds with Matrix Arrayed Probe and SAFT

A portable type of 3D ultrasonic inspection system, named “Matrixeye”, was applied to the spot welds, in which a matrix-arrayed probe was used as a sensing unit, and the welding zone in the spot welds was visualised 3-dimensionally. The matrix-arrayed probe, in which 64 piezoelectric elements are embedded as 8 × 8 matrix arrangement, can acquire more than 4 000 ultrasonic echo data points due to their transmission and reception in 0.2 seconds. This system can synthesise 3-dimensional ultrasonic images by SAFT (Synthetic Aperture Focusing Technique) installed in the parallel signal processing board. This system can also inspect the weld quality of spot welds with additional data; e.g., an averaged diameter, area, major axis, minor axis, thickness, and indentation depth of the welded region. In addition, this system has an automatic measurement function, in which the measurement automatically starts by sensing the probe contact with the welding surface, and automatically stops by sensing the position and tilt of the probe within a setting level. The results measured by this system have good correlation with the weld size on the fracture faces and the tensile-shear strength in the destructive tests. It was shown from the evaluated performance of the system that the developed system is applicable to the inspection of spot welds.

Microstructure of Bonding Interface in Explosively Welded Metal/Ceramic Clad

Bonding interface in aluminum (Al) and silicon nitride (Si3N4) clad fabricated by explosive welding has been investigated by transmission electron microscopy (TEM). The nanocrystalline region was clearly observed at the interface between Al and Si3N4. Electron diffraction pattern and energy dispersive X-ray spectroscopy (EDS) measurements across the interface revealed that this nanocrystalline region consist of the only aluminum.