Heat control and design-related effects on the properties and welding stresses in WAAM components of high-strength structural steels

Abstract

Commercial high-strength filler metals for wire arc additive manufacturing (WAAM) are already available. However, widespread industrial use is currently limited due to a lack of quantitative knowledge and guidelines regarding welding stresses and component safety during manufacture and operation for WAAM structures. In a joint research project, the process- and material-related as well as design influences associated with residual stress formation and the risk of cold cracking are being investigated. For this purpose, reference specimens are welded fully automated with defined dimensions and systematic variation of heat control using a special, high-strength WAAM filler metal (yield strength > 790 MPa). Heat control is varied by means of heat input (200–650 kJ/m) and interlayer temperature (100–300 °C). The ∆t8/5 cooling times correspond with the recommendations of filler metal producers (approx. 5–20 s). For this purpose, additional thermo-physical forming simulations using a dilatometer allowed the complex heat cycles to be reproduced and the resulting ultimate tensile strength of the weld metal to be determined. Welding parameters and AM geometry are correlated with the resulting microstructure, hardness, and residual stress state. High heat input leads to a lower tensile stress in the component and may cause unfavorable microstructure and mechanical properties. However, a sufficiently low interlayer temperature is likely to be suitable for obtaining adequate properties at a reduced tensile stress level when welding with high heat input. The component design affects heat dissipation conditions and the intensity of restraint during welding and has a significant influence on the residual stress. These complex interactions are analyzed within this investigation. The aim is to provide easily applicable processing recommendations and standard specifications for an economical, appropriate, and crack-safe WAAM of high-strength steels.

Influence of the WAAM process and design aspects on residual stresses in high-strength structural steels

“…6 shows the distribution of the analyzed longitudinal residual stresses σ rs x (x,z) (110 measurement points) on the sidewall of the marked surface on a central point specimen. Note that due to a number of repeating central point test almost identical topology of residual stress maps were observed, as can be seen compared to specimens with same test parameter/conditions in [27], indicating a high level of reproducibility of the manufacturing process.…”

“…Likewise, the larger specimen x (x,z) on the surface of the WAAM specimen wall; design variation with increased dimensions (welding parameters and heat control according to test no. V5) [27] dimensions could cause higher shrinkage forces, which can no longer be compensated to the same extent by the local effect of the transformation residual stress as was still the case with the smaller specimen dimensions. In addition, the maximum residual stresses of approx.…”

“…This phenomenon is known from joint welding, where solid phase transformations can lead to a reduction of the local [21] or even global [28] residual stress level as a result of locally or globally restraint volume expansion during phase transformation. Comparing with [27], Fig. 6 shows the distribution of the analyzed longitudinal residual stresses σ rs x (x,z) (110 measurement points) on the sidewall of the marked surface on a central point specimen.…”

“…The analysis of the measured values of all specimens shows that the effect of the interpass temperature is not significant, while the heat input has a significant effect on the longitudinal residual stress. This offers the possibility to optimize the manufacturing process via the interpass temperature with regard to residual stresses [27].…”

Review of Measuring Methods of Residual Stress in Wire Arc Additive Manufacturing Products

“…For instance, it can affect the mechanical properties, such as strength, hardness, and ductility of products, by making them different from the material's intrinsic properties. High heat supply in the WAAM process results in reduced tensile stress within the component and can lead to undesirable mechanical properties and microstructure [154,155]. Microstructural characteristics have an impact on the material properties of parts.…”