A major strength of the laser direct metal deposition (LDMD) rapid manufacturing technique is its ability to manufacture freeform shapes, to directly create different surface coatings on a part, and to produce parts from graded porous to fully dense solid structures. This technique is gaining popularity in the fields of aerospace and biomedical manufacture due to its flexibility and cost effectiveness. Surface coating and repair are its biggest application in industry. Previous models of surface layer coating predicted heat flow in the substrate and the resulting temperature distributions successfully, however layer geometry predictions were absent or incomplete. Here, an analytical modelling method of surface layer coating has been presented which accounts for track interactions and uses a novel powder mass affinity factor in multiple track laser direct metal deposition. The model has been validated by a series of surface layer coating experiments using Ti-6Al-4V alloy. Surface layer characteristics in terms of layer profile, layer thickness and layer roughness have been compared. The model is, then, extended from multiple tracks to multilayer structure fabrication. The multilayer porous structures have been fabricated by using the laser in continuous-wave and pulsed-wave modes for potential use in biomedical applications. In the second part of the work, a coupled analytical-numerical solution has been developed for the single track deposition model described earlier. Laser direct metal deposition is a complex process involving multiple interdependent processes which can be best simulated using a fully coupled mass-energy balance solution and the model removes the shortcoming of previous models where energy and mass balance equations are solved in a decoupled manner. The model has been applied to find out temperature distributions, track profile and microstructure scale, its experimental validation makes it convincing. The model is quite efficient as compared to finite element methods and a useful industrial aid for selecting the parameters to use for laser direct metal deposition when separate geometric and microstructural outcomes are required. In the third part of the work, a comparative study of LDMD characteristics using GA (gas-atomised) and PREP (plasma rotating electrode process) Ti-6Al-4V powders has been presented. The LDMD characteristics in terms of geometric dimensions, surface finish, microstructure, micro hardness and any defects such as intralayer porosity are compared and it is concluded that PREP powder deposition has some potential benefits when high value components are to be built, for example, aerospace applications. This study has also exposed the scientific understanding on the causes of intralayer porosity generation behaviour in laser direct metal deposition. To minimize the intralayer porosity, optimum combinations of processing parameters/ conditions and characteristics of the initial powder, have been proposed for practical engineering applications.

Modelling and analysis of laser direct metal deposition of Ti-6Al-4V alloy

This paper presents the effect of process parameters on the geometry characteristics in butt welding of 1.5 mm mild steel sheets using a I kW single mode fibre laser at a range of laser powers, welding speeds and laser focal point position. Experiments were planned using the statistical design of experiment (DoE) technique for optimizing the above selected welding parameters and statistical modelling technique was employed to understand their interactions and significance. Suitable parameters to achieve net-shape welds were identified. Experimental methodology and results are detailed and the results are discussed. Micro-hardness measurements and tensile stress testing were carried out. It has been shown that laser net-shape welded samples have superior mechanical properties.

Characterization of weld bead geometry in fibre laser butt welding of mild steel sheets by means of statistical modelling

Modeling of Transport Phenomena in Dissimilar GTA Weld Metals

Laser technologies have made distinguished contributions to modern industry. These have typically been realised through the important role played by lasers in the advancement of manufacturing technology in many areas such as welding, which has become an important joining technique and thus promoted the use of lasers in a wide variety of applications in the oil, gas, aerospace, aircraft, automotive, electronics and medical industries.A detailed review of previous work in the use of lasers for advanced manufacturing, and in particular, laser beam welding is given. The work reported in this thesis aims to develop a new method of laser welding. This is connected with investigations relating to the production of net shape welds for bead-on-plate welding and butt welding of mild steel plates. Based on the nature of its operation, use of a fibre laser was considered most suitable compared to other solid state lasers. Net-shape welds were demonstrated on mild steel plates using an IPG 1 kW single mode fibre laser with a maximum power output of 1000 W.The thesis shows results from experimental and modelling (based on finite element and computational fluid dynamic modelling) to validate the idea and the understanding of underlying scientific principles. The thesis is presented in the form of a collection of published work generated by the author during the course of this project. In addition, some results that are not yet published are also included. Design of experiments and statistical modelling has been used in the experimental work to understand the process parameter interactions. Microstructural and mechanical testing have been carried out to evaluate the performance of the welds. Net shaped (the weld bead is flat to the parent material surface) welds have been achieved and compared with standard welds. The understanding of net-shape weld formation and the effect of the laser welding parameters was enhanced by the theoretical modelling. The thesis concludes with a summary of scientific findings and an overview of future work.

Laser net shape welding of steels

The objective of this study is to investigate the effects of Heat Affected Zone produced by Plasma Transferred Arc process and residual stress in substrate steel 55NiCrMoV7. In this study, based on ABAQUS code, a sequentially coupled thermal, metallurgical, mechanical 2-D finite element model is developed. In the numerical simulations, the coating and Heat Affected Zone are used to predict the residual stresses. The simulation results revealed that the final residual stress in substrate steel do seem to be influenced by the HAZ.

Numerical Modeling of Residual Stress and Prediction of Heat Affected Zone Induced by PTA Process

Laser welding of metals and alloys is extensively used in industry due to its advantages of controlled heating, narrow weld bead, low heat affected zone (HAZ) and its ability to weld a wide range of metals and dissimilar metals. Laser welding of dissimilar metals such as carbon steels and stainless steel is still a challenging task, particularly due to the formation of brittle phases in the weld, martensitic formation in the HAZ and solidification cracking in the fusion zone. These issues can significantly deteriorate the strength of the welded joint. The aim of this work is to investigate the fundamental phenomena that occur inside the dissimilar weld zone and their effect on weld quality. In order to establish the key process variables, an initial study concentrated on the effect of different laser process parameters on dissimilar weld quality. In the second part of the work, a comprehensive study was performed to understand and subsequently control the alloying composition in laser dissimilar welding of austenitic stainless steel and low carbon steel. A dissimilar weld that is predominantly austenitic and homogeneous was obtained by controlling the melt pool dynamics through specific point energy and beam alignment. The significance of dilution and alloying elements on joint strength was established. A coupled CFD and FEM numerical model was developed to assist in understanding the melt pool dynamics and transportation processes of alloying elements. The model has been validated by a series of laser welding experiments using various levels of specific point energy. The laser welding characteristics in terms of geometric dimensions, surface morphology, alloying concentration, and dilution, were compared, and it is concluded that the specific point energy and laser beam position are the key parameters that can be controlled to obtain a weld bead with characteristics most suitable for industrial applications. In the third part of the work, a comparative study was performed to understand the significance of cooling rate, and alloying composition on the microstructure and phase structure of the dissimilar weld zone. Results show that the HAZ within the high carbon steel has significantly higher hardness than the weld area, which severely undermines the weld quality. A new heat treatment strategy was proposed based on the results of the numerical simulation, and it is shown to control the brittle phase formation in HAZ of high carbon steel. A series of experiments was performed to verify the developed thermo-metallurgical FEA model and a good qualitative agreement of the predicted martensitic phase distribution is shown to exist. Although this work is presented in the context of dissimilar laser welding of mild steel to stainless steel, the concept is applicable to any dissimilar fusion welding process.

Mohammadreza Nekouie Esfahani

Papers

Laser welding of dissimilar carbon steel to stainless steel 316L