Direct metal deposition melt pool temperature distribution control through novel holographic beam shaping, allowing improved mechanical and corrosion properties

Abstract

When a laser beam is scanned a temperature profile is induced in the workpiece. The temperature distribution will be a function of the beam shape and intensity profile. The use of a circular beam leads to an uneven temperature distribution even when a top hat profile is used. This is caused by the material on the centreline of the beam getting irradiated for longer than an area towards the edge. The uneven temperature distribution will lead to a difference across the weld or deposition in terms of the temperature reached and the rate of cooling. Typically the weld will see a slower cooling rate and maximum temperature along the centreline. This leads to large columnar crystal growth, similar to a cast microstructure with an associated reduction in mechanical performance. A novel method for controlling time-temperature distribution in the material is through the use of a Holographic Optical Element (HOE). The HOE is a computer generated kinoform diffraction pattern that redistributes the beam intensity and shape into any customer designed profile. Through the use of a predictive non-linear thermal model the desired temperature distribution throughout the interaction region was input, and then propagated to the surface. This surface temperature profile was then converted to be the output of the kinoform beam shaping algorithm. This beam shape forms a complex and non intuitive beam profile allowing a controlled time-temperature profile, that can only be achieved in this way. The weld deposition process has been analysed using colour high speed video and optical pyrometry based techniques. This has shown the levels of material transport in the melt pool to be drastically reduced by the DOE redesign of the input beam shape. This is indicative of a reduced thermal gradient in the melt pool, leading to greater control of the deposition microstructure.When a laser beam is scanned a temperature profile is induced in the workpiece. The temperature distribution will be a function of the beam shape and intensity profile. The use of a circular beam leads to an uneven temperature distribution even when a top hat profile is used. This is caused by the material on the centreline of the beam getting irradiated for longer than an area towards the edge. The uneven temperature distribution will lead to a difference across the weld or deposition in terms of the temperature reached and the rate of cooling. Typically the weld will see a slower cooling rate and maximum temperature along the centreline. This leads to large columnar crystal growth, similar to a cast microstructure with an associated reduction in mechanical performance. A novel method for controlling time-temperature distribution in the material is through the use of a Holographic Optical Element (HOE). The HOE is a computer generated kinoform diffraction pattern that redistributes the beam intensity and...