TWI Industrial Member Report Summary 610/1997
Use of carbon dioxide gas lasers for industrial surface engineering has been established for many years, but applications of the technology have been slow to develop. Development of high average power Nd:YAG lasers coupled with flexible fibreoptic beam delivery systems offers an alternative laser source with several processing advantages.
The intense, finely focused, high power heat source produced by a laser beam is ideal for localised material processing. Laser heating produces local changes at the surface of a material, whilst leaving the properties of the bulk of a given component unaffected, and introduces minimum distortion. Most metals and non-metals absorb laser beam energy within a depth of two or three atomic diameters, thus lasers are true surface heaters with no great effective depth of penetration.  In addition, the fast processing rates and atmospheric operation of lasers are beneficial for surface engineering. The principal laser surface engineering applications can be divided into three broad areas:
- heating without melting; ie transformation and shock hardening
- heating with melting; ie laser glazing, surface homogenisation, remelting
- melting with the addition of material; ie cladding, alloying, impregnation.
Solid state, neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers do not yet deliver the powers which can be achieved by CO 2 lasers. Recent commercial development of Nd:YAG lasers capable of delivering up to 4kW through fibreoptic cables places Nd:YAG powers well within the range required for surfacing applications, which has increased interest in their use for surface engineering. Nd:YAG lasers with beam delivery via a fibreoptic cable offer advantages of processing remotely from the laser power source, currently up to 200m, flexible beam manipulation using standard industrial robots and potential advantages in terms of beam energy distribution and absorption by metallic substrates.
This report describes the development of parameters for transformation hardening of low alloy, heat treatable steel and cladding of low and medium carbon steels by fusion of pre-placed powder using a 2kW average power Nd:YAG laser with beam delivery via a 1mm diameter, 10m long fibreoptic cable. Results are compared with previous studies using a 2kW average power CO 2 laser where appropriate. Fundamentals of laser operation, beam delivery and the relative advantages of Nd:YAG lasers for industrial materials processing are also discussed.
- To review current laser surface engineering applications for transformation hardening, cladding and alloying.
- To laser transformation harden medium carbon steel using a 2kW Nd:YAG laser with fibreoptic beam delivery and to assess the characteristics of the tracks produced.
- To laser clad plain carbon steels with nickel alloy 625 using a 2kW Nd:YAG laser with fibreoptic beam delivery and to assess the characteristics of the overlays produced.
- To compare the results of the present trials with those achieved using CO 2 lasers of similar average power.