Estimating effects of processing conditions and variable properties upon pool shape, cooling rates, and absorption coefficient in laser welding

Abstract

This paper examines the role of traverse speed, Beer–Lambert absorption coefficient β, surface reflectivity, and changing liquid thermal conductivity upon the shape of the melt pool and the cooling rates that occur. The dependence of β upon processing conditions is also examined. A three-dimensional variable property, moving heat source, quasi-steady-state, finite difference model for heat conduction into the substrate during laser welding is used. With an increase in traverse speed, the pool flattens out and is swept back, and cooling rates increase. An increase in β sharply decreases the depth of penetration. With the onset of melting, changes in reflectivity did not change pool shape significantly. An increase in effective liquid metal thermal conductivity increases melt pool aspect ratio. Cooling rates increased as the energy density in the pool decreased. A dimensionless melt front velocity Φ is defined such that cooling rates exceed 1000 K/s as Φ approaches unity. The product βz, where z is the depth of penetration, is shown to vary linearly with the natural logarithm of Φ. These results imply that β affects depth of penetration more than the width, that an upper bound for β may be deduced from Φ, that variations in surface reflectivity are less critical in laser welding, that the maximum thickness that can be welded in a single pass decreases as fluid flow becomes more dominant in the melt pool, and that cooling rates increase as pool energy density decreases, especially for values of Φ<100.