No abstract

Friction stir brazing (FSB) was developed for dissimilar joining to overcome the problems associated with friction stir lap welding (e.g. pin wear, narrow bonded area, hook defect) by metallurgical reaction instead of plastic flow. To enhance the thermomechanical effect of the rotating shoulder, a pin free tool with a large shoulder of 30 mm diameter was used in FSB of 5 mm thick Al/Cu plates in stepped lap configuration with Zn braze. Compared to the case using common tool of 20 mm diameter, it was found that although the intermetallic compound (IMC) layer grew thickly, it was disrupted into particles, and end crack within thick IMC layer was eliminated. As a result, the joint fractured not along interface and the fracture load increased by as much as 2271 N. This work proposed an approach to disrupting continuous IMC into particles by enhancing the mechanical effect of rotating shoulder.

In this paper, prediction and controlling angular distortion in fillet welded joint and structure were investigated. First, two methods to reduce angular distortion in fillet welded joint were investigated by experimental and numerical analysis. One was to apply a constant external force in-process and the other was rigid clamping. In numerical analysis, a new in-house finite element code has been developed based on the idea of iterative substructure method (ISM) to calculate welding distortion in rational time. During experimental analysis, the constant external force was designed and applied in-process to reduce angular distortion of fillet welded joint. The results showed that the distortion can be efficiently predicted by ISM, which were in good agreement with the experimental ones. Applying constant external force in process was a more effective method to reduce distortion than using rigid clamping. In addition, with a constant load distance from weld bead, the locations of the applied constant external force and rigid clamping along the longitudinal direction (welding direction) have little influence on the magnitude of welding angular distortion. Finally, the angular distortion of a large fillet welded structure was predicted with ISM and also controlled with applying a constant external force based on the simulation results of the fillet welded joint.

A keyhole produced by friction stir weld bead on plate (4?7 mm thick 1060 Al plate) technique was filled using a method like rotational extrusion process, in which a pin free tool and a T shaped filler bit was used. As compared with the case using a simple cylindrical filler bit, in which all of the filled samples fractured at the initial periphery interface, the beneficial effect of T cap on improving filling quality was demonstrated by higher and more stable fracture load and fracture location away from initial periphery interface, although small gaps were present at the lower periphery interface and the bottom interface. To further eliminate the small gaps mentioned above with liquid phase, Zn braze foil was preplaced at the bottom of the keyhole. As expected, with the aid of the extruding and torquing effects of the pin free tool, intimate contact was achieved well by filling and alloying effects of molten Zn.

In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and Jn-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, Ji~ • Jn-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the Jn, values is easy to use. Moreover, the mode I fracture toughness Jt, is very sensitive to the rolling direction of the test steels, but the mode H fracture toughness Ji~ is completely insensitive to the rolling direction of the steels, and the Ja~/Ji, ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.