Friction stir welding: high-melting material such as low-cost welded steel plate
Honda has developed a technology for welding steel and aluminum alloys by means of friction stir welding for a quantity of products. Tohoku University and Hitachi have developed cobalt alloy mixing heads that can weld high-melting metals such as steel and titanium. Previously, the use of Friction Stir Welding (FSW), which is limited to low-melting metals such as aluminum alloys and copper alloys and magnesium alloys, has begun to expand to metals with higher melting points such as steel*1.
1. At standard atmospheric pressure, aluminum has a melting point of 660 ° C, copper is 1083 ° C, and magnesium is 651 ° C. The iron is as high as 1539 ° C and the titanium is as high as 1727 ° C.
Friction heat softens the metal to a mucus
Friction stir welding refers to the use of a stirring head with a protrusion (stirring needle) at the top of the cylinder to push the stirring head while rotating from the two plates (base metal) or the two overlapping plates that need to be welded. Pressure, the technique of welding two plates together (Figure 1).
The frictional heat generated between the rotating agitating needle and the plate softens the metal around the agitating needle into a mucus, and the rotating agitating needle is thereby inserted into the plate. In this way, the metal softened around the stirring needle will be mixed together (plastic flow occurs), so that the two metal plates are integrated, or a metal bond is formed on the joint surface of the two metal plates, so that the two become One.
Previously lacking a high temperature resistant mixing head
The advantages of friction stir welding are as follows: (1) welding (solid phase welding) is possible without melting the base metal, so there are few deformations and welding defects after welding, so that the quality of the product can be improved and the cost can be reduced; 2) Energy saving than fusion welding; (3) no protective gas is required; (4) noise is small and no dust is generated. Moreover, this technology is also very effective for the welding of dissimilar materials, and therefore has attracted the attention of the industry, and it is expected to achieve the effect of weight reduction by selecting appropriate materials for each part*2. However, as introduced at the beginning of the article, materials that previously used friction stir welding at mass production levels were limited to low melting point metals.
2. Friction stir welding and liquid phase welding of molten base metal - Compared with fusion welding, the welding temperature is low, and it is easy to control the welding input heat by setting welding conditions. Therefore, it is easy to reduce the generation of intermetallic compounds that are the main culprit in reducing the weld strength. By reducing the thickness of the intermetallic compound layer, the strength of the welded portion can be increased. When the friction stir welding is performed, if a large amount of stirring is performed, a large amount of intermetallic compound is also generated.
The use of friction stir welding was previously limited to low melting point metals because of the lack of a low temperature resistant high temperature agitating head suitable for mass production. When friction stir welding is applied to an aluminum alloy, a stirring head made of a steel material such as tool steel is generally used. When a high melting point metal such as steel or a titanium alloy is welded by such a stirring head, the stirring head is softened in a high temperature region where the metal can be softened, and the stirring head is severely worn and cannot be used. Although stir heads made of non-steel materials such as ceramics and tungsten-rhenium alloys have been developed, these stirrers are very expensive and are not suitable for mass production.
No need to stir to expose the steel to the new surface
Honda developed a technique for welding steel and aluminum alloy by friction stir welding for joining overlapping steel and aluminum alloy base materials. Its greatest feature is that it only stirs the aluminum alloy without stirring the steel (pictured below in Figure 1). This prevents the temperature of the stirring head from being too high.
Steel and aluminum alloys can be welded together without the need to stir steel because metal bonds are formed between the steel and the aluminum alloy. That is to say, the technique does not stir the steel, but removes the coating film and the oxide film on the steel surface. In this way, the new surface of the steel is exposed and a metal bond is formed between the aluminum alloy and the aluminum alloy which is activated by stirring. The presence of this metal bond combines steel and aluminum alloy*3.
3. Bimetallic corrosion (electro-erosion) occurs after water enters between steel and aluminum alloy. This technique prevents this phenomenon by inserting a silicon film between steel and aluminum alloy.
Honda used this technology in the front sub-frame of the new Accord for the North American market and achieved a variety of effects (Figure 2). Specifically, not only is the weight reduced by 25% compared to the original steel sub-frame, but the power consumption during welding is reduced by about 50%. In addition, this technology has been used to change the structure of the sub-frame and the suspension mounting part, so that the rigidity of the mounting part is increased by 20%, thereby contributing to the improvement of vehicle sports performance.
Expand application areas with new materials
On the other hand, Tohoku University, Hitachi, and Hitachi Metal Precision Instruments have developed a stirrer with a low temperature that can withstand high temperatures, thus expanding the application range of friction stir welding (Fig. 3). The material used in the mixing head was jointly developed by Tohoku University and Hitachi, and the mass production technology of this material and mixing head (casting) was jointly developed by Hitachi and Hitachi Metal Precision. The stirrer head can be used for friction stir welding of high melting point metals such as steel, titanium alloys and zirconium alloys, and is scheduled to begin sales in the fall of 2013. Although Hitachi Metal Precision Instruments stated that “price will be discussed in the futureâ€, it is estimated to be set at a price band that can be used at the mass production level.
The material developed by Tohoku University and the like for the friction stir welding head is a cobalt alloy, and an intermetallic compound which maintains high strength even at a high temperature is distributed inside. Specifically, Co3 (Al, W) is distributed in the cobalt-based alloy. The key is how to achieve a uniform fine distribution of Co3 (Al, W), and to make the distribution amount and the grain boundary shape close to the desired state. For this reason, more reasonable ingredients and heat treatment methods have been used.
Suitable materials for the mixing head currently envisaged are carbon steel, high tensile steel, titanium alloy (Ti-6Al-4V, etc.), pure titanium and copper alloy. Although some materials are difficult to use due to strength and thickness, the welding target is generally a material having a stirring head temperature (combination temperature) of about 1000 ° C when performing friction stir welding.
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