Nondestructive evaluation of micro-oxide inclusions in additively manufactured metal parts using nonlinear ultrasonic technique

Additive manufacturing (AM), commonly known as 3D printing, is an emerging technology for manufacturing metal parts. Recently, micro-oxide inclusions, which are inevitably generated during AM processes owing to the high-temperature environment, have been noted to enhance the mechanical strength of AM metal parts. However, an explicit nondestructive testing (NDT) method to assess the micro-oxide inclusions of AM metal parts has not been reported yet owing to the difficulty of sensing micro-inclusions. In this study, the micro-oxide inclusions of AM metal parts were evaluated nondestructively using a nonlinear ultrasonic technique. The uniqueness and advantages of this study are (1) the development of a micro-oxide inclusion evaluation technique for AM metal parts, (2) superior evaluation ability for micro-inclusions compared to conventional NDT; (3) applicability of the proposed method in assessing the strengthening of the mechanical properties of the AM parts by the inclusions; and (4) potential for nondestructive online monitoring. The performance of the proposed method was validated using specimens fabricated under various 3D printing conditions. The results of the micro-oxide inclusions assessed by the proposed method were consistent with the metallography and tensile testing results. Furthermore, the performance of the proposed method was better than that of conventional NDT.11Nsciescopu

Melt pool oxidation and reduction in powder bed fusion

In this work, the mechanism of melt pool oxidation and reduction during the powder bed fusion was studied. To elucidate the mechanism, the effect of process parameters such as laser power, scan speed and shield gas flow speed on the variation on oxygen, nitrogen and carbon content in the printed material was investigated. In 316L stainless steel, melt pool reduction and oxidation occurred simultaneously during the process involving CO and CO2 gas evaporation at the depression zone. An increment of energy density and shield gas flow speed led to an increase in melt pool reduction rate and a decrease in porosity. It was found that nitrogen and CO gas could be a major constituent of vapor when oxygen and nitrogen content in the powder was high. Uneven solidification texture and melt pool instability were observed when the speed of shield gas flow was similar to the laser scan speed. Porosity, melt pool dimension, and solidification texture also changed under different shield gas flow speeds. Higher oxygen content and porosity was found in bi-directional scanning pattern than in uni-directional scanning pattern. Among uni-directional scanning pattern cases, scanning along the flow direction resulted in higher oxygen content and porosity compared to opposite and perpendicular scanning. Interaction between shield gas flow and laser plume was appreciably engaged in melt pool oxidation and reduction rate.11Nsciescopu

In-situ synthesis of nitrides and oxides through controlling reactive gas atmosphere during laser-powder bed fusion of Fe-12Cr-6Al

This study endeavors to investigate the feasibility of in situ synthesis of nitrides and oxides within Fe-12Cr-6Al alloys through precise modulation of reactive gas atmosphere during the Laser-Powder Bed Fusion process. A focused study on elucidating the effects of critical process parameters, such as laser power, scanning speed, and hatch distance, was undertaken to discern their impact on the material's microstructure, oxygen/nitrogen content, and hardness. Synthesis of nano-sized AlN and Al2O3 precipitates within the Fe-12Cr-6Al matrix was achieved when nitrogen was used for a process gas. A rise in laser power from 120 W to 200 W, coupled with a reduction in scanning speed from 1200 mm/s to 400 mm/s, resulted in decreased porosity and an increase in grain size. Additionally, all printed samples showed lower oxygen content compared to the initial powder, while nitrogen ratios were notably higher. A marginal increase of the Vickers hardness value, from 240 ± 4.5 to 266 ± 3 HV, was observed as the laser power increased and the scanning speed decreased. The hardness values were higher than those obtained from different production methods with identical compositions. These compelling results suggest a positive impact of nitride precipitates and oxide precipitates to improve hardness values