Showing papers by "Paul A. Colegrove published in 2010"

Energy and force analysis of linear friction welds in medium carbon steel

Abstract: The linear friction welding process is rapidly developing into an important manufacturing technology for high quality joining of engineering materials. The energy required for linear friction welding is an important issue due to economic and environmental reasons, but is not currently fully understood. This paper describes a comprehensive evaluation of the energy input during linear friction welding of a medium carbon steel with different process parameters. This calculation is based on an analysis of force and displacement data from the machine, which takes momentum into account. The analysis shows that energy input to the weld is minimised with high frequencies and rubbing velocities; however, there is a considerable amount of energy lost in oscillating the machine tooling under these conditions. Furthermore, analysis of the force indicates that a peak load occurs just before the samples being aligned, which is probably caused by ploughing of the samples during welding.

Rolling to control residual stress and distortion in friction stir welds

Abstract: Considerable residual stress and distortion can be produced by friction stir welding, impeding industrial implementation. Finite element analysis has been used to develop three innovative rolling methods that reduce residual stress and distortion in friction stir welds. Of the three methods, post-weld direct rolling where a single roller is applied to roll the top surface of the weld after the weld metal has cooled to room temperature proved the most effective. The residual stress predictions from the model compared favourably with residual stress measurements reported in an accompanying paper. Finally, the effectiveness of using post-weld direct rolling is illustrated with an industrial example of a large integrally stiffened panel, where the distortion was virtually eliminated.

An Investigation into Cracking in Nickel-Base Superalloy Repair Welds

Abstract: The nickel-base superalloy Rene 80 is considered very susceptible to liquation and strain-age cracking. Material in the solutionised condition is welded using the Cold Metal Transfer, or CMT process (with ductile filler alloy) and autogenously using a laser. Grain size is shown to have a significant effect on cracking. Using the CMT, welding power is shown to have high significance on the level of cracking, whereas welding speed has little effect. When welding using the laser, it is shown that the power and spot size are more crucial to the material cracking than the travel speed. It is indicated that the weld bead geometry has high significance over the occurrence of cracking, with a relationship between welding power, weld bead geometry, and stresses controlling the occurrence and magnitude of cracking. Further, some laser welds are analysed after post-weld heat treatment, and there is a significant increase in cracking after this. However, 34% of samples contained no cracking in both the as-welded and post-weld heat treated state.

Material Constitutive Behaviour and Microstructure Study on Aluminium Alloys for Friction Stir Welding

Abstract: The material constitutive behaviour and microstructure of Aluminium alloys 6082 and 7449 were studied with the Gleeble hot compression test. The novel aspect of the work is that the testing was done at high strain-rates and at temperatures within 5 K of the solidus. The results indicated that the strength was maintained up to near solidus temperatures, with no dramatic strength reduction being observed. There was however, a distinct change in the slope observed with the 7449 results around 720 K. The experimental results were then fit to the Zener-Holloman equation, which describes the relationship between the material flow stress, temperature and strain-rates. The material microstructure of the hot compression test samples was analysed, and the averaged grain size was calculated to compare with friction stir weld nuggets. This will be used to infer the processing conditions that exist in the dynamically recrystallized weld nugget. Finally, a simple model was used to understand how processing conditions affected the deformation behaviour.