Influence of Heat Control on Properties and Residual Stresses of Additive-Welded High-Strength Steel Components

Abstract

Advanced high-performance filler metals for wire arc additive manufacturing (WAAM) exist on the market already. Nevertheless, these high-strength steels are not yet widely used in industrial applications due to limited knowledge of cold-cracking susceptibility, welding residual stresses, and therefore sufficient safety in terms of manufacturing and operation. High residual stresses promote cold-cracking risk, especially in the welding of high-strength steels, as the result of a complex interaction between the applied material, process conditions, and component design. The focus of the present investigation was the determination of the influence of the process parameters on the ∆t8/5 cooling time, mechanical properties, and residual stresses to correlate, for the first time, heat control, cooling conditions, and residual stress for WAAM of high-strength filler materials. This contributed to the knowledge regarding the safe avoidance of cold cracking. In addition to a thermophysical simulation using a dilatometer of different high-strength steels with subsequent tensile testing, reference WAAM specimens (open hollow cuboids) were welded while utilizing a high-strength filler metal (ultimate tensile strength >790 MPa). The heat control was varied by means of the heat input and interlayer temperature such that the ∆t8/5 cooling times corresponded to the recommended processing range (approx. 5 s to 20 s). For the heat input, significant effects were exhibited, in particular on the local residual stresses in the component. Welding with an excessive heat input or deposition rate may lead to low cooling rates, and hence to unfavorable microstructure and component properties, but at the same time, is intended to result in lower tensile residual stress levels. Such complex interactions must ultimately be clarified to provide users with easily applicable processing recommendations and standard specifications for an economical WAAM of high-strength steels. These investigations demonstrated a major influence of the heat input on both the cooling conditions and the residual stresses of components manufactured with WAAM using high-strength filler materials. A higher heat input led to longer cooling times (∆t8/5) and approx. 200 MPa lower residual stresses in the surface of the top layer.

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

“…This adjustment is made by controlling the values of wire feeding rates and travel speed (TS). With higher heat input the lower RS is observed on the surface of the top layer by long-time cooling durations [67]. Welding RS was measured using five experimental methods: X-ray diffraction (XRD), neutron diffraction (ND), incremental deep hole drilling (iDHD), incremental center hole drilling (iCHD), and contour method (CM) [68].…”

“…As researchers delve deeper into WAAM, they have discovered significant enhancements inside the grain arrangement and material properties of the produced components [69]. The transformation of large, elongated grains into smaller, uniformly shaped grains in both the inner and intermediate layers is accomplished by adjusting the amount of heat applied [66,67]. The influence of heat input on the RS, macroscopic structure, microscopic composition, and mechanical characteristics of the components fabricated through the WAAM procedure is studied in [70,71]…”

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

“…For AM, the path coordinates are essentially derived from a CAD model using slicers and interpreted into Fig. 1 Block diagram for the signal and material flow of the robot-based WAAM welding system; welding parameters (welding speed v T , volume flow per time V/t), which determine the layer width b S and height h S [23] executable welding programs. The robot controller interprets the programmed lines and controls the traversing movement and the welding machine.…”

“…Fig. 2 a Welding parameters and build-up strategy for a 9 mm thick wall for WAAM experiments, b specification of the reference geometry (open hollow cuboid); CAD model and weld result, c welding parameters[23] …”

Active and Passive Thermal Management in Wire Arc Additive Manufacturing

“…Interestingly, the distortion is much more significant when the number of layers is small [16]. The geometry of the built component also affects the residual stress; for instance, the corner of the rectangle shows extremely high residual stress [17]. The adaptive heat flow to the substrate or the workpiece is one of the latest approaches to controlling residual stress and distortion; for instance, Doumenc et al [18] used a cooling system to keep the substrate at 20 • C such that after every layer deposition, 20 • C is reached in 40 s. Differently, Fan et al [19] suggested trailing cooling of argon gas to −15 • C, followed by depositing a subsequent layer, which reduced residual stress.…”