Three defect types in friction stir welding of aluminum die casting alloy

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Abstract

For different tool plunge downforces, the optimum FSW conditions of aluminum die casting alloy were examined. The higher the tool plunge downforce is, the wider the range of the optimum FSW conditions is. The following three different types of defects are formed, depending on the FSW conditions. (1) A large mass of flash due to the excess heat input; (2) cavity or groove-like defects caused by insufficient heat input; and (3) cavity caused by the abnormal stirring. As for the abnormal stirring, it is clearly seen that the shape of the top part on the advancing side in the stir zone is completely different. For this type of defect, the effect of the tool plunge downforce is small, though the size of the defect due to insufficient heat input significantly decreases with the increasing downforce.

Introduction

Conventional fusion welding of aluminum die casting alloys is generally difficult due to the formation of blowholes by entrapped H2 gas and the fragile intermetallic compounds in the weld metal. Accordingly, a new welding method is required to solve these problems. In recent years, friction stir welding (FSW) was developed as a solid state joining process in 1991 at TWI [1]. This process is effective for the welding of various aluminum alloys. However, only a limited number of studies have been carried out on die casting and cast aluminum alloys [2], [3], [4], [5], [6].

FSW enables materials to be joined below their melting point TM and the cast structure to be modified because of grain refinement, the fine dispersion of intermetallic compounds, no formation of blowholes with removing cast defects and the dendritic structure of the base metal. Also, dissimilar joining with wrought alloys can be performed by FSW [2], [7], [8].

As already described, the application of FSW to aluminum die casting alloys has a significant practical concern from the view point of industrial use. Accordingly, the determination of the optimum FSW conditions is very important. The optimum FSW conditions have been discussed for several aluminum alloys [9], [10], [11], however the mechanism of the joint defect formation during FSW has not been discussed in detail.

The aims of this study are to examine the weldability of ADC12 FSW joints obtained by the load control type of the FSW machine and to evaluate the effect of the welding parameters on the joint quality. Based on these results, the mechanisms of the joint defect formation during FSW are classified.

Section snippets

Experimental procedures

Four-millimetre thick ADC12 aluminum die casting alloy plates were used in this study. The gas content of the plate was about 2.6 ml/100 g Al. Table 1 shows the chemical compositions of the ADC12 base material. These plates are 300 mm long, 100 mm wide and 4 mm thick.

To evaluate the range of the optimum welding conditions for ADC12, stir-in-plate welding was performed using load control type FSW machine. The tool has a columnar shape with a screw probe. SKD61 was used as the tool material. The

Inspection of FSW joint

Fig. 1 shows the typical appearances of the joint surfaces, X-ray radiographies and macroscopic structures of the cross-sections of the FSW joints at the constant tool rotation speed of 1500 rpm.

For the tool plunge downforce of 6.9 kN at the welding speed of 250 mm/min, it is clearly seen that a sound joint was obtained. However, an inner defect, such as cavity, is observed inside the joint at a high speed of 500 mm/min. This defect cannot be seen on the surface, though it was revealed that a

Conclusions

For each given tool plunge downforce, sound joints are obtained at appropriate tool rotation speeds and welding speeds. At the highest tool plunge downforce of 14.2 kN, the range of the optimum FSW conditions was wider than the others.

Three different types of defects are formed, depending on the FSW conditions. (1) A large mass of flash due to the excess heat input; (2) cavity or groove-like defects caused by insufficient heat input; and (3) cavity caused by the abnormal stirring.

As for the

Acknowledgement

This work was supported by Grant-in-Aid for cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from the Ministry of Education, Science, Sports and Culture.

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