Effects of Deposition Strategy and Preheating Temperature on Thermo-Mechanical Characteristics of Inconel 718 Super-Alloy Deposited on AISI 1045 Substrate Using a DED Process

Thermomechanical characteristics are highly dependent on the deposition strategy of the directed energy deposition (DED) process, including the deposition path, the interpass time, the deposition volume, etc., as well as the preheating condition of the substrate. This paper aims to investigate the effects of the deposition strategy and the preheating temperature on thermomechanical characteristics of Inconel 718 super-alloy deposited on an AISI 1045 substrate using a DED process via finite element analyses (FEAs). FE models for different deposition strategies and preheating temperatures are created to examine the thermomechanical behavior. Sixteen deposition strategies are adopted to perform FEAs. The heat sink coefficient is estimated from a comparison of temperature histories of experiments and those of FEAs to obtain appropriate FE models. The influence of deposition strategies on residual stress distributions in the designed model for a small volume deposition is examined to determine feasible deposition strategies. In addition, the effects of the deposition strategy and the preheating temperature on residual stress distributions of the designed part for large volume deposition are investigated to predict a suitable deposition strategy of the DED head and appropriate preheating temperature of the substrate.

Estimation Method of Interpass Time for the Control of Temperature during a Directed Energy Deposition Process of a Ti–6Al–4V Planar Layer

Directed energy deposition (DED) provides a promising additive manufacturing method to fabricate and repair large metallic parts. However, it may suffer from excessive heat accumulation due to a high build rate, particularly during a wire feeding-type DED process. The implementation of interpass time in between two depositions of beads plays an important process role to passively control the interpass temperature. In this study, a method to estimate the proper interpass time using regression analysis from heat transfer finite element analysis is proposed for maintaining the interpass temperature during a wire feeding-type DED deposition of a planar layer. The overlapping beads of a planar layer are estimated using a polygonal-shaped bead profile in the finite element model. From the estimated proper interpass time, a selected proper interpass time scheme (PITS) is suggested for practical implementation. The selected PITS is applied in a thermo-mechanical finite element model to evaluate the temperature distribution and its effects on the depth of the melt pool, the depth of the heat-affected zone (HAZ), displacement, and residual stresses. By comparing the predicted results with those using a constant interpass time scheme (CITS), the selected PITS shows better control in reducing the depths of the melt pool and HAZ without severely inducing large displacement and residual stresses.

Numerical Analysis of Longitudinal Residual Stresses and Deflections in a T-joint Welded Structure Using a Local Preheating Technique

In this paper a numerical analysis of a T-joint fillet weld is performed to investigate the influences of different preheat temperatures and the interpass time on the longitudinal residual stress fields and structure deflections. In the frame of the numerical investigations, two thermo-mechanical finite element models, denoted M2 and M3, were analyzed and the results obtained were then compared with the model M1, where the preheating technique was not applied. It is concluded that by applying the preheat temperature prior to the start of welding the post-welding deformations of welded structures can be significantly reduced. The increase of the preheat temperature increased the longitudinal residual stress field at the ends of the plates. The influence of the interpass time between two weld passes on the longitudinal residual stress state and plate deflection was investigated on two preheated numerical models, M4 and M5, with an interpass time of 60 s and 120 s, respectively. The results obtained were then compared with the preheated model M3, where there was no time gap between the two weld passes. It can be concluded that with the increase of interpass time, the plate deflections significantly increase, while the influence of the interpass time on the longitudinal residual stress field can be neglected.

Numerical Studies on Effect of Interpass Time on Distortion and Residual Stresses in Multipass Welding

Weld distortion and residual stresses are two major issues in the fabrication process. Numerical techniques are being tried out to accurately predict the structural integrity of the welding. Interpass time in the multipass welding is an important parameter which influences the weld distortion and residual stresses. In this study two pass tungsten inert gas (TIG) welding of 6 mm mild steel plates has been analyzed using Finite element analysis (FEA) software Sysweld and parametric study is conducted with different interpass time. The temperature distribution, distortion and residual stresses are calculated using three dimensional finite element model (FEM) considering phase transformations in the material. The transient thermo-metallurgical analysis followed by elasto-plastic analysis is carried out using temperature dependent and phase dependent material properties. The material deposition in the multipass welding is numerically simulated using chewing gum method, where dummy phase and dummy material are assigned for the element activation. The phase proportions are calculated by assigning suitable phase kinetics parameter extracted from continuous cooling transformation (CCT) diagram of a given material. Experiments are conducted for validation after given edge preparation and using same material as filler wire. The FEM analysis is carried out for eight cases with different time interval between passes, starting from 30 s to 240 s in the steps of 30 s. FEM results are verified with experimentally measured values. It is found that the time interval between passes has less influence on the residual stresses but significantly affects the distortion and phase proportion due to the first pass preheating effect on second pass and second pass postheating effect on first pass.

Determining the No-Recrystallization Conditions for Industrial Hot Strip Rolling of Steels Using Laboratory Simulations

Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products. The present work focuses on computation of incubation time tinc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting tinc equal to interpass time. Simulations allow for prediction of the onset of austenite static recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Determination of the Non-Recrystallisation Temperature (Tnr) of Austenite in High Strength C-Mn Steels

In the present study non-recrystallisation (Tnr) and Ar3 temperatures have been determined for the C-Mn steels from multi-pass hot torsion experiments with continuous cooling in the temperature range of 1260°C to 600°C. Results show that Tnr decreases with increasing strain/pass, strain rate or interpass time. An alternative approach based on the work-hardening rate is proposed for the determination of Tnr and is shown to be more suitable in case the usual mean flow stress method does not provide a clear Tnr value.

Evaluating Microstructural and Hardness Variations in Time Dependent Multipass Welded Mild Steel

This paper investigates the microstructural induced hardness variation in multirun welded plain carbon steel at different interpass time. Beveled 16mm thick mild steel samples were welded in 2 and 4 passes at interpass time of 90, 120 and 240s respectively via manual metal arc. The result showed that the differences in hardness values of the fusion zone and heat affected zone reduce as interpass time increases for both 2 and 4 runs. The effect was however quite distinct in the 4 runs welding cycle. In the 2 run cycle, the fusion zone and heat affected zone merge at 100 seconds; while in the 4 runs cycle, the merging occurred at 25 seconds; indicating that the higher the multipass, the shorter the time required to produce uniformity in hardness and structural homogeneity. Thereby increasing the resistance of the weld to crack susceptibility and failure. At these instances, the microstructure revealed fine grained pearlite interpassed in martensite.

Hot Deformation Analysis of a 2 wt.-% Si Electrical Steel by Means of Torsion Tests

Steels with high amounts of silicon are used in electrical applications due to their low mangectoestriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. A fundamental study of the workability of such materials using torsion testing may help to understand and to optimise its production. Single deformation torsion tests were carried out on a steel containing 2 wt.-% Si in a temperature range of 800 to 1100°C and strain rates in the range of 0.01 to 2 s-1. A value of 299 kJ/mol was found for the apparent activation energy for hot working after applying the hyperbolic-sine equation to the mean flow stress (MFS) values computed from the test. Multiple deformation torsion tests under continuous cooling conditions were carried out in the same temperature range at strain rates from 0.2 to 1 s-1, the strain per pass and interpass time (determining the cooling rate) were varied. Different critical temperatures, which are of importance for processing this alloy, can be calculated from the dependence of MFS with the inverse absolute temperature; such a method was used to determine the temperature at which recrystallisation stops (Tnr). It was found that this temperature depends on strain rate, pass strain and interpass time. Results of the microstructure analysis of quenched samples are in good agreement with the values of Tnr.