A Study on Welding Deformation in Fiber Laser Welding of 9% Nickel Steel through Finite Element Analysis Part I: Implementation of Welding Heat Source Model

Due to various environmental regulations, the demand for natural gas, i.e., a clean energy, is expected to increase continuously. In terms of efficient storage and transportation of natural gas, liquefied natural gas has an advantageous volume of 1/600 compared to natural gas, but the materials that can be used at a cryogenic temperature of −163 °C are limited. A 9% nickel steel is a material recommended by IMO through IGC. It has excellent mechanical properties compared to other cryogenic materials, but its use has been limited due to its disadvantages in arc welding. Therefore, the main topic of this study is the automatic welding of 9% nickel steel using fiber laser and its purpose is to predict the welding deformation during fiber laser welding. First, an investigation was conducted to find the fiber laser welding heat source. A model that can cover all the models in prior studies such as curve, exponential, conical, conical-conical combination, and conical-cylinder combination models was proposed and the heat source model was constructed in a multi-layer format. Heat transfer analysis was performed using the ratio of a heat source radius and heat energy of each layer as a variable and the pass or failure of a heat source was determined by comparing the analysis results to the experimental results. By changing the variables in conjunction with the optimization algorithm, the main parameters of a passed heat source model were verified in a short period of time. In addition, the tendency of parameters according to the welding speed was checked.

State-of-the-art review on laser cladding process as an in-situ repair technique

The present work reviews the laser cladding process as a repairing technique and the conventional repairing techniques and different heat source models. In this review work, the authors have tried to address the various traditional methods studied for repairing and surface modification. The dominantly used heat source model for numerical modelling of the repairing techniques and the mechanism of the laser cladding process, along with its advantages over the conventional repairing techniques, is also reviewed. This paper also focuses on the predominantly used laser of high power for the cladding process and the effect of process parameters on the quality of the clad layer. The different materials used as clad materials for repairing purposes during the laser cladding process have also been discussed briefly. In this paper, the authors have surveyed literature from different regions of the globe and considered the literature since 1969. This review discusses the various conventional repairing techniques used for repairing, heat source model, process parameters, and different materials used in the laser cladding process. The authors have also briefed the advantages, disadvantages, and application in each of the sections. The use of laser cladding for in-situ repairing process, development of a precise model, use of low-power laser, and application of laser cladding for actual engineering components was also considered in future research work.

Novel Calibration Strategy for Validation of Finite Element Thermal Analysis of Selective Laser Melting Process Using Bayesian Optimization

Selective laser melting (SLM) produces a near-net-shaped product by scanning a concentrated high-power laser beam over a thin layer of metal powder to melt and solidify it. During the SLM process, the material temperature cyclically and sharply rises and falls. Thermal analyses using the finite element method help to understand such a complex thermal history to affect the microstructure, material properties, and performance. This paper proposes a novel calibration strategy for the heat source model to validate the thermal analysis. First, in-situ temperature measurement by high-speed thermography was conducted for the absorptivity calibration. Then, the accurate simulation error was defined by processing the cross-sectional bead shape images by the experimental observations and simulations. In order to minimize the error, the optimal shape parameters of the heat source model were efficiently found by using Bayesian optimization. Bayesian optimization allowed us to find the optimal parameters with an error of less than 4% within 50 iterations of the thermal simulations. It demonstrated that our novel calibration strategy with Bayesian optimization can be effective to improve the accuracy of predicting the temperature field during the SLM process and to save the computational costs for the heat source model optimization.

An innovative heat source model for calculating wheel/rail contact temperature

Modeling an accurate and effective heat source is a challenge when describing the thermal behavior of wheel/rail contact, conventionally modeled using Goldak’s heat source. This Gaussian distributed model transplanted directly from the welding field is quite different from the ellipsoidal heat source on the contact patch. An innovative heat source model which reflects the shape and size of contact patch was created based on the energy equivalence in this study. The thermal behavior of the wheel/rail contact was described by using this heat source moving along the rail. Numerical simulations based on a three-dimensional (3-D) finite element (FE) model validate that the proposed heat source model is valid as the relative errors of the peak temperature between the proposed and previously published heat source model are less than 4.45%. The distribution of the contact temperature on the rail surface reflects the shape and size of the contact patch. The semi-ellipsoidal distributed temperature obtained by the proposed elliptic cylinder model is more precise than the Gaussian distributed temperature obtained by the conventionally used Goldak’s model. The proposed elliptic cylinder model has the advantages of simple structure and flexible model variables than the ready-made Goldak’s model in commercial software.

A NEW HEAT SOURCE MODEL FOR KEYHOLE MODE LASER WELDING

Laser welding is a common technique for joining metals in many manufacturing industries. Due to the heat input and the resulting melting and solidification, the parts deform causing residual distortion and residual stresses. To assure the geometrical and functional quality of the product, Computational Welding Mechanics (CWM) is often employed in the design phase to predict the outcome of different design proposals. Furthermore, CWM can be used to design the welding process with the objective of assuring the quality of the weld. However, welding is a complex multi-physical process and in a design process it is typically not feasible, for example, to employ fluid simulation of the weld pool in order to predict deformation, especially if a set of design proposals is under investigation. Instead, what is used is a heat source that emulates the heat input from the melt pool. However, standard heat sources are typically not flexible enough to capture the fusion zone for deep keyhole mode laser welding. In this paper, a new heat source model for keyhole mode laser welding is presented. In an industrial case study, a number of bead on plate welds have been employed to compare standard weld heat sources and develop the new heat source model. The proposed heat source is based on a combination of standard heat sources. From the study, it was concluded that the standard heat sources could not predict the observed melted zone for certain industrial application while the new heat source was able to do so.