Applications and examples of light and electron micrographs illustrating microstructures, which describe metallurgical phenomena in 3D printing/additive manufacturing of metal and alloy products and components, are presented along with extensive process and processing parameter descriptions and review. Examples include microstructures that have defined turbine blade fabrication and optimization over the past half century, including contemporary electron beam melting fabrication of turbine blade alloys and other novel microstructures and architectures, which result from layer by layer, non-equilibrium melt solidification and epitaxial growth involving powder bed laser and electron beam fabrication. Phase transformations and second-phase formation by rapid cooling in metal and alloy components fabricated by laser and electron beam melting technologies are illustrated for a range of high-temperature materials. Using a range of examples, the advantages of fabricating complex (especially porous) biomedical and related commercial products are described. Prospects for future developments of direct 3D metal and alloy droplet printing, as a key component of the digital factory of the future, are described. This technology is compared with more conventional solidification and powder bed layer building thermo-kinetics, especially in the context of large structure and component fabrication.

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A Metallographic Review of 3D Printing/Additive Manufacturing of Metal and Alloy Products and Components

While commercial additive manufacturing processes involving direct metal wire or powder deposition along with powder bed fusion technologies using laser and electron beam melting have proliferated over the past decade, inkjet printing using molten metal droplets for direct, 3D printing has been elusive. In this paper we review the more than three decades of development of metal droplet generation for precision additive manufacturing applications utilizing advanced, high-temperature metals and alloys. Issues concerning process optimization, including product structure and properties affected by oxidation are discussed and some comparisons of related additive manufactured microstructures are presented.

3D metal droplet printing development and advanced materials additive manufacturing

Surface hardening by inducing phase transformations through the heating effect of a laser beam was one of the first laser based fabrication methods to be commercialised, in the early 1970s. However...

Laser Transformation Hardening

Laser surface hardening of AISI 4130 carbon steel was conducted with a high power diode laser using Response Surface Methodology. Scanning speed, laser power and focal plane position were considered as the input process variables and cross sectional geometry of the hardened area, average micro-hardness and the ferrite phase percentage were considered as process responses. The effect of parameters on the responses variations were investigated using analysis of variance. Microstructure evaluation of the laser hardened zone was performed using optical and field emission scanning electron microscopy. Results indicated that by increasing the laser power and decreasing the scanning speed and focal plane position, higher surface hardness with more penetration in depth, higher average micro-hardness and minimum ferrite percentage will be achieved. Finally, the process was optimized by desire ability approach based on the applied statistical analyses. Minimum value of percentage of the ferrite and maximum value of the other responses are considered as optimization criteria. The recommended optimized results were validated using the experimental tests. The results show that the hardness of the diode laser hardening process is 3 times of the hardness of the base metal. Laser-overlapping scanning is performed in the optimum setting and effect of overlapping percentage is investigated.

High power diode laser surface hardening of AISI 4130; statistical modelling and optimization

An analytical solution for the temperature variation in a semi-infinite workpiece heated by repetitive laser pulses has been derived and the effects of various process parameters, viz. laser power, beam diameter, scan speed, pulse duration, repetition frequency and duty cycle on the surface transformation hardening have been investigated. The study shows that the soaking time of the top surface layer at a temperature above the phase transformation temperature, on which the homogeneity of microstructure and the depth of hardening depend, can be increased by repetitive laser pulse heating in comparison to continuous wave laser heating. The depth of hardness increases with the number of incident laser pulses at low frequencies in the range of Hz. This has been demonstrated by comparing the surface hardness characteristics of AISI 1055 steel specimens resulted by the continuous wave and repetitive laser pulse hardening processes.

Theoretical and experimental study on laser surface hardening by repetitive laser pulses

Surface treatments practicable with multi-kilowatt, continuous laser beams are considered. In particular, the mechanism, parameters, and advantages for the surface hardening of steels are outlined and test results described. The uses of lasers for de-scaling and the bonding of powders to metal surfaces are also proposed.

J. W. Hill

Papers

Surface treatments by laser