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Modelling of residual stresses of the laser cladding process

Tamanna, N. (2019). Modelling of residual stresses of the laser cladding process. (Unpublished Doctoral thesis, City, University of London)

Abstract

The focus of this work is to predict residual stresses and find the key controlling factors in formation of residual stresses in the Laser Cladding process. Three types of models (conceptual, analytical and numerical) have been developed to understand the process and predict residual stresses. The conceptual model describes the simplified relation of temperature, stress and strain with time schematically using the established theoretical knowledge of generation of strain and stress after thermal treatment. A one dimensional analytical model was adopted to predict residual stresses in dissimilar clad and substrate materials. H13 steel and Ti6Al4V were used as substrate material. Four cladding materials (Al2O3, TiC, TiO2 and ZrO2) were used on the H13 steel substrate and a range of preheating temperature of the substrate, from 300 K to 1200 K, was investigated. The minimum residual stress was found for Al2O3 clad on the H13 steel substrate. For Ti6Al4V substrate, six cladding materials (Al2O3, TiC, TiO2, ZrO2, SiC and TiN) were used and, 300 K to 1000 K was the range of preheating temperature of the substrate. TiN cladding material generated the maximum tensile residual stress. Below 1000 K, SiC formed minimum residual stresses on the Ti6Al4V substrate. In both cases, the increment of preheating temperature of substrate decreased the residual stress. 2D and 3D thermo-mechanical models were developed to predict temperature distribution and residual stress distribution using commercial software (ANSYS’18). In 2D numerical model, as a substrate material H13 steel was used and H13 steel, Al2O3 and TiC were used as clad materials. Tensile residual stresses were observed in the clad, at interface of the clad and the substrate and in the substrate near the interface in all samples. It was found that Al2O3 coating on H13 steel produced lower residual stress (1220 MPa) in the clad than TiC coating on H13 steel (1359 MPa). The proposed analysis offers to select the combination of clad and substrate materials having minimum residual stresses in this process. A 3D thermo-mechanical model of deposition of Ti6Al4V clad on the Ti6Al4V substrate material was developed. This model was verified with an established model in order to establish the accuracy of mesh and the applied boundary conditions. Temperature history of this model offers the knowledge about the peak temperature at the interface (3201 K), shape and depth of melt pool (0.5 mm) and HAZ (0.23 mm), the formation time of melt pool, heating rate (2.45×104 K/s), cooling rate (1.1×104 K/s) and microstructure in the heat treated area. The residual stress distribution shows maximum tensile stress was 108 MPa which is far below the yield strength (1061 MPa) of Ti6Al4V material. Sequentially, three cladding materials (Al2O3-1, Al2O3-2 and TiC) were deposited on the surface of the Ti6Al4V substrate using similar model. The peak temperature at the interface for Al2O3-1/Ti6Al4V and TiC/Ti6Al4V samples are 3101 K and 3911 K respectively. The maximum predicted equivalent stress (SEQV) in TiC/Ti6Al4V sample (274 MPa) was higher than Al2O3-1/Ti6Al4V sample (205 MPa). Furthermore, Al2O3-2 (α-alumina) generated higher tensile residual stresses than Al2O3-1 (the refractory alumina) on the same substrate due to temperature dependant properties, Al2O3-2/Ti6Al4V sample.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
Date Deposited: 25 Jun 2020 11:50
URI: https://openaccess.city.ac.uk/id/eprint/24395
[img] Text - Accepted Version
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