Toward defect-free components in laser metal deposition with coaxial wire feeding through closed-loop control of the melt pool temperature

Abstract

Laser metal deposition (LMD) is an additive manufacturing process in which a metal powder or wire is added to a laser-induced molten pool. This localized deposition of material is used for the manufacturing, modification, and repair of a wide range of metal components. The use of wire as feedstock offers various advantages over the use of powder in terms of the contamination of the process environment, the material utilization rate, the ease of handling, and the material price. However, to achieve a stable process as well as defined geometrical and microstructural properties over many layers, precise knowledge on the effects of the input variables of the process on the resulting deposition characteristics is required. In this work, the melt pool temperature was used as an input parameter in LMD with coaxial wire feeding of stainless steel, which was made possible through the use of a dedicated closed-loop control system based on pyrometry. Initially, a temperature range was determined for different process conditions in which a stable deposition was obtained. Within this range, the cause-effect relationships between the melt pool temperature and the resulting geometry as well as the material properties were investigated for individual weld beads. It was found that the melt pool temperature is positively correlated with the width of the weld bead as well as the dilution. In addition, a dependence of the microhardness distribution over the cross section of a weld bead on the melt pool temperature was demonstrated, with an increased temperature negatively affecting the hardness.

A Review on Wire-Laser Directed Energy Deposition: Parameter Control, Process Stability, and Future Research Paths

“…Specialized optics have been developed to shape the laser beam in a way that enables wire feeding through the center of the beam to ensure a distribution of laser radiation over the wire [121][122][123]. The coaxial approach consists of feeding the wire in the center of multiple single lasers (donut shape) or inside an annular laser beam, providing diverse options for the process [54,79,124,125].…”

“…This complexity also contributes to higher sensitivity to height deviations with this head type [116,128]. Therefore, despite being directionally independent, its broader industrial adoption is hindered due to the processes' sensitivity to disturbances [125]. In contrast with lateral wire feeding, in coaxial wire feeding, the positive defocusing of the beam cannot result in a stable process as it leads to wire melting above the substrate, which is detrimental to the process.…”

“…In the W-LDED process, dilution refers to the mixing of the wire feedstock with the substrate or the previously deposited layer during the deposition process. Controlling dilution is essential for producing high-quality weld beads with the desired properties and structural integrity [125,133].…”

Studies on the Use of Laser Directed Energy Deposition for the Additive Manufacturing of Lightweight Parts

“…Lastly, the hardness measurements were performed with a fully automated hardness tester (Qness 60 A + EVO, ATM Qness GmbH, Germany). The diagonal lengths of the indentations were determined manually in the software to ensure a maximal accuracy [39].…”

Effect of Defocusing Variation on Laser Wire Additive Manufacturing Morphology and Plasma Plume Change

“…Junhui Li et al 8 deposited laser-fed powder additive manufacturing specimens without the presence of porosity by varying the amount of defocus to increase the laser spot diameter. Christian Bernauer et al 9 experimented with different processes of 316L laser fusion by varying the focal position of the laser beam to obtain a 2.75 mm diameter annular laser spot and found that the volume of the deposited weld channel increased signi cantly at higher melt pool temperatures, however, the overall hardness of the melt pool was rather lower. Yanhua Zhao et al 10 , on the other hand, argued that the variation of the defocusing amount can change the convergence and divergence of the beam, and large defocusing amount has a more uniform energy density distribution.…”