In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

Laser beam machining (LBM) is one of the most widely used thermal energy based non-contact type advance machining process which can be applied for almost whole range of materials. Laser beam is focussed for melting and vaporizing the unwanted material from the parent material. It is suitable for geometrically complex profile cutting and making miniature holes in sheetmetal. Among various type of lasers used for machining in industries, CO2 and Nd:YAG lasers are most established. In recent years, researchers have explored a number of ways to improve the LBM process performance by analysing the different factors that affect the quality characteristics. The experimental and theoretical studies show that process performance can be improved considerably by proper selection of laser parameters, material parameters and operating parameters. This paper reviews the research work carried out so far in the area of LBM of different materials and shapes. It reports about the experimental and theoretical studies of LBM to improve the process performance. Several modelling and optimization techniques for the determination of optimum laser beam cutting condition have been critically examined. The last part of this paper discusses the LBM developments and outlines the trend for future research.

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Laser beam machining—A review

The state of a cutting tool is an important factor in any metal cutting process as additional costs in terms of scrapped components, machine tool breakage and unscheduled downtime result from worn tool usage. Several methods to develop monitoring devices for observing the wear levels on the cutting tool on-line while engaged in cutting have been attempted. This paper presents a review of some of the methods that have been employed in tool condition monitoring. Particular attention is paid to the manner in which sensor signals from the cutting process have been harnessed and used in the development of tool condition monitoring systems (TCMSs).

Sensor signals for tool-wear monitoring in metal cutting operations—a review of methods

A Review on Process Monitoring and Control in Metal-Based Additive Manufacturing

Determination of the maximum temperature and temperature distribution along the rake face of the cutting tool is of particular importance because of its controlling influence on tool life, as well as, the quality of the machined part. Numerous attempts have been made to approach the problem with different methods including experimental, analytical and numerical analysis. Although considerable research effort has been made on the thermal problem in metal cutting, there is hardly a consensus on the basics principles. The unique tribological contact phenomenon, which occur in metal cutting is highly localized and non-linear, and occurs at high temperatures, high pressures and high strains. This has made it extremely difficult to predict in a precise manner or even assess the performance of various models developed for modelling the machining process. Accurate and repeatable heat and temperature prediction remains challenging due to the complexity of the contact phenomena in the cutting process. In this paper, previous research on heat generation and heat dissipation in the orthogonal machining process is critically reviewed. In addition, temperature measurement techniques applied in metal cutting are briefly reviewed. The emphasis is on the comparability of test results, as well as, the relevance of temperature measurement method to high speed cutting. New temperature measurement results obtained by a thermal imaging camera in high speed cutting of high strength alloys are also presented. Finally, the latest work on estimation of heat generation, heat partition and temperature distribution in metal machining is reviewed. This includes an exploration of the different simplifying assumptions related to the geometry of the process components, material properties, boundary conditions and heat partition. The paper then proposes some modelling requirements for computer simulation of high speed machining processes.

Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining

High efficiency of the powder deposition in coaxial laser cladding is a key subject for this new technique adopted to the industry. A catchment model was proposed in this study to characterise the particle bonding under laser heating in the cladding process. The phenomena of the particle rebound and adhesion on substrate were discussed and the catchment efficiency was mathematically modelled based on the beam spot and the stream cross-section area. The theoretical catchment efficiency was further verified with the experiments and the results give an insight of the powder stream characteristics of the coaxial nozzle.

A simple model of powder catchment in coaxial laser cladding

Numerical simulation of the focused powder streams in coaxial laser cladding

The optical attenuation of a laser beam passing through a powder stream during coaxial laser cladding has an important effect on the powder catchment efficiency for this new process. This attenuation for variations in the powder stream focus was studied both experimentally and theoretically in this article. Measurement of the laser attenuation caused by the streams was made to compare with the theoretical results. It shows that more than 50% of the laser energy could be lost in the powder stream and the focused stream gives less powder catchment than the nonfocused stream in coaxial laser cladding for 304 L stainless steel powder on mild steel with 1 kW CO2 laser radiation.

Laser attenuation of the focused powder streams in coaxial laser cladding

This paper presents a numerical analysis of the heating, melting and evaporation processes of a single spherical powder particle when irradiated by a CO2 laser beam in coaxial laser cladding. The power particle has a size ranging from 20 to 200 μm and the intensity of the laser has been varied from 500 to 3000 W . The laser energy, initial powder velocity and size have been shown to have important effects on the temperature profile of the powder stream. It has also been shown that high powder evaporation due to high power laser radiation may induce significant loss in the powder particle mass, to as much as 25% of the initial size at certain conditions in the simulation.

Thermal processes of a powder particle in coaxial laser cladding

The blown powder laser cladding process has recently been greatly enhanced by the development of a coaxial powder feed system. It provides a new route to generate the metal parts directly from CAD drawings. The performance of the coaxial powder feeder depends on various gas flow streams which significantly affect the distribution mode of the powder stream and the deposition rate in cladding. Two types of optical techniques have been adopted in this study to investigate the powder concentration mode of the coaxial jet streams. The mode of the powder stream is also mathematically modelled and compared to the experimental results of stainless steel powder. The Gaussian distribution mode in the transverse direction of the powder stream was identified by theory and experiment at cold stream conditions.

Jehnming Lin

Papers

A simple model of powder catchment in coaxial laser cladding

Numerical simulation of the focused powder streams in coaxial laser cladding

Laser attenuation of the focused powder streams in coaxial laser cladding

Thermal processes of a powder particle in coaxial laser cladding

Concentration mode of the powder stream in coaxial laser cladding