Comprehension of chip formation in laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo-mechanical simulations were undertaken. A thermo-mechanical model for chip formation was also undertaken. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the simulation. The temperature fields allow us to explain the reduction in the cutting force and the resulting residual stress fields in the workpiece.Contrat Plan Etat Région (CPER) Pays de la Loir

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Supersonic Laser Deposition and LaserForge: Process Mechanism Coating Characteristics

LaserForge is a commercial coating process, that uses a pulsed laser to deposit flat sided wire onto a substrate with minimal heat input. Supersonic Laser Deposition (SLD) is an emerging coating technology that can be used as an alternative to existing thermal spray processes. It has the benefit of low temperature, allowing the deposition of nanostructured and temperature sensitive coatings, which is not currently possible with existing thermal spray. This main aim of this work was to undertake an experimental study aimed at identifying the process mechanism used in the LaserForge process. The understanding of the process mechanism could then be applied to process improvements for SLD coatings. As part of this study the bonding mechanisms of both LaserForge and SLD were studied. Initially a laser system to enable the exploration of the LaserForge parameter space was specified and a system set up to enable investigation of LaserForge. The LaserForge process parameter space was characterised using a pulsed laser with Ti-64 on CP aluminium. Successful bonding was achieved with parameters of 10 ms pulse length, 1400 W per pulse and 0.8 mm spot diameter. The process was determined to be a form of welding-based laser cladding, a melt-based process. Following discovery that LaserForge was a melt-based process, the direction of work was changed to focus on the SLD process mechanisms. Several WC-17Co coatings were deposited as a single layer (0.5 mm thick) on carbon steel. The coating cross section morphology was characterised using an optical microscope and scanning electron microscope. A tensile pull off test was used to measure the coating adhesion, and a four-point bend test with acoustic emission was used to monitor the failure of the coating. Plastic failure of the coating was identified, and a test limited adhesion strength in excess of 70 MPa measured. The coating was shown to have a stress-to fracture of approximately 550 MPa in tension, and a reinforcement effect of approximately 100 MPa when compared to the uncoated substrate. The problems with the deposition of the coatings with SLD were investigated and characterised, with the thermal effect from the laser during deposition found to be significant. This work has characterised the mechanism behind the commercial LaserForge process and the deposition challenges of depositing WC-17Co using Supersonic Laser Deposition. The benefit of these advancements will provide guidance for the direction of future work into LaserForge, and Supersonic Laser Deposition of nanostructured and advanced materials.Funded by EPSR

An experimental and theoretical study of heat transfer effects during a laser-cutting process

The heat transfer effects during laser cutting of AISI 1018 steel were studied both experimentally and theoretically. The quality of the cut as defined by the kerf width, the oxidation heat affected zone (HAZ), and the heat-treated heat affected zone were measured at all points along the cut, both for the upper and lower surfaces. The oxidation heat affected zone was found to predict the heat-treated region fairly well. The kerf width and the oxidation HAZ variation with the laser beam velocity, power, and oxygen gas pressure were also studied. The results showed that of the above variables, the beam velocity had the most effect on the cut quality;Temperatures were measured adjacent to the cut with thermocouples mounted intrinsically on the sample surface. Temperatures in and around the cutting region were also measured with an infrared sensor. The temperatures were affected by the process variables and the cut quality. Attempts at predicting the cut quality from the recorded temperature data were fairly successful;Two models of the cutting process were also developed. A simple thermodynamic model and a two-dimensional finite element model were quite successful in predicting the cut quality. The models also included the effects of the oxidation reaction and the convective cooling of the oxygen gas jet

The printing of laser-- generated heat images in cobalt oxides on glass substrates

Imperial Users onl