Analytical Description of the Influence of the Welding Parameters on the Hot Cracking Susceptibility of Laser Beam Welds
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NTRODUCTION
DURING laser beam welding, oriented dendrites grow from the side of the weld towards the centerline. In case of laser beam welding of aluminum alloys, when the value of the power per unit depth PDepth ¼ Ps, where P is the laser power and s the welding depth, or the Peclet [1] v where criterion PeRyk ¼ wweld 4j as defined by Rykalin, wwled is the width of the welded seam, v the feeding rate, and j the thermal diffusivity, exceeds an alloy-specific threshold, an equiaxed dendritic grain structure forms in the center area of the weld.[2] Centerline hot cracks are a fatal welding defect and may propagate along the grain boundaries between the equiaxed dendritic grains in this central area of the weld. The grain structure of the weld has a major influence on the susceptibility of the welds to hot cracking.[3,4] The horizontal section in Figure 1 shows a centerline crack within an equiaxed dendritic grain structure of a weld seam in AA6016 alloy. The section was etched according to Barker[5] and illuminated by polarized light.
CHRISTIAN HAGENLOCHER, DANIEL WELLER, RUDOLF WEBER, and THOMAS GRAF are with the Institut fu¨r Strahlwerkzeuge, University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany. Contact e-mail: [email protected] Manuscript submitted April 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
According to the RDG criterion of Rappaz,[6] the solidification shrinkage and the thermomechanical deformation of the work piece lead to a pressure drop in the melt between the growing grains during the solidification process. A hot crack starts to form when this pressure drop exceeds the value of what was termed a critical depression.[6] Rappaz derived a critical strain rate limit from this criterion to quantify the hot cracking susceptibility (HCS) of aluminum alloys. The major influence of strains and strain rates on hot crack formation has been proven in recent work by means of optical measurement techniques.[7–10] Figure 2 sketches the typical processing zone that is present during laser beam welding. The origin of the coordinate system was chosen to be located in the centre of the vapor capillary, with (positive) x pointing in the direction of the moving laser beam. The solidification occurs at the trailing end of the melt pool as indicated by the large white arrow. The zone between the liquidus isotherm TLiquidus and the solidus isotherm TSolidus is referred to as the solidification zone. The solidification starts at a solid fraction of fs = 0 (all the material is liquid) and is completed at a solid fraction of fS = 1 (all the material is solid). The liquidus and solidus isotherms cross the centerline of the weld (y = 0) at xL and xS, respectively. The condition of coherency, where fS = fS,coh, is defined to be the solidification condition at which the growing grains build a coherent network. At the centerline this
Fig. 1—Horizontal section of a laser beam weld, welded with a beam diameter of 600 lm, a laser power of 4.5 kW, and a welding velocity of 6 m/min in a distanc
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