Showing papers by "Paul A. Colegrove published in 2007"

Model for predicting heat generation and temperature in friction stir welding from the material properties

Abstract: This paper describes a simple numerical model for predicting the heat generation in friction stir welding (FSW) from the material hot deformation and thermal properties, the process parameters, and the tool and plate dimensions. The model idealises the deformation zone as a two-dimensional axisymmetric problem, but allowance is made for the effect of translation by averaging the three-dimensional temperature distribution around the tool in the real weld. The model successfully predicts the weld temperature field and has been applied with minimal recalibration to aerospace aluminium alloys 2024, 7449 and 6013, which span a wide range of strength. The conditions under the tool are presented as novel maps of flow stress against temperature and strain rate, giving insight into the relationship between material properties and optimum welding conditions. This highlights the need in FSW for experimental high strain rate tests close to the solidus temperature. The model is used to illustrate the optimisat...

The micro-welding ability of 100W high brightness fibre laser on 316L stainless steel

Abstract: Fibre lasers are an attractive option for laser micro-welding applications because of their high beam quality which gives a small spot size. In this paper, the results of an investigation into the micro-welding of stainless steel using a 100 W fibre laser are reported. Continuous and pulsed modes of operation were investigated in terms of penetration depth, top bead width and focus position sensitivity. For the continuous mode, penetration depths of 550 µm were obtained at welding speeds of 5 mm/s. When the welding speed was increased to 30 mm/s the penetration depth was still greater than 400 µm. In the pulsed welding mode, the pulse duration and frequency were varied to give duty cycles of 10% and 50%. There was a significant increase in the penetration depth when the pulse duration was less than 0.5 ms (with the 50% duty cycle) due to the pulse enhancement of the laser at the very short pulse durations. At the same time, the top bead width reduced by over 50% for the lowest pulse duration (0.05 ms) giving a weld with an aspect ratio of 11:1. The fast cooling rate in the short pulse weld changes the final microstructure to pure austenite rather than austenite with ferrite which was obtained with the continuous mode laser welds. In all cases, the deep penetration conditions were very sensitive to focus position.Fibre lasers are an attractive option for laser micro-welding applications because of their high beam quality which gives a small spot size. In this paper, the results of an investigation into the micro-welding of stainless steel using a 100 W fibre laser are reported. Continuous and pulsed modes of operation were investigated in terms of penetration depth, top bead width and focus position sensitivity. For the continuous mode, penetration depths of 550 µm were obtained at welding speeds of 5 mm/s. When the welding speed was increased to 30 mm/s the penetration depth was still greater than 400 µm. In the pulsed welding mode, the pulse duration and frequency were varied to give duty cycles of 10% and 50%. There was a significant increase in the penetration depth when the pulse duration was less than 0.5 ms (with the 50% duty cycle) due to the pulse enhancement of the laser at the very short pulse durations. At the same time, the top bead width reduced by over 50% for the lowest pulse duration (0.05 ms) giv...