Surface chemical and geometrical properties of pure copper powder intended for binder jetting and sintering

One novel important application of sinter-based additive manufacturing involving binder jetting is copper-based products. Three different variants of nominally pure copper powder having particle size distributions with D90 < 16, 22, or 31 mu m were investigated in this study. The packing behavior and the flow properties using dynamic test and shear cell, as well as specific surface area were evaluated. The analyses employed illustrate the multidimensional complexity. Because different measurements capture different aspect of the powder, it is imperative to apply a characterization approach involving different methods. Surface chemical analysis by means of X-ray photoelectron spectroscopy (XPS) showed that all powder variants were covered by Cu2O, CuO, and Cu (OH)(2) , with Cu2O being dominant in all cases. The finest powder with D90 < 16 mu m tended to have higher relative amount of copper in divalent state. The average apparent oxide thickness estimated by XPS depth profiling showed that the two coarser variants had similar overall average oxide thickness, whereas the finest one possessed smaller oxide thickness. The surface chemistry of the powder grades is found to be related to their rheological behavior in dynamic condition. Considering the specific surface areas in combination with the average oxide thicknesses, the amount of surface bound oxygen was estimated to be about similar to 220 ppm for all three variants. Specific concerns need to be taken during the sintering of powder to keep oxygen level below that of electrolytic pitch copper (400 ppm)

Machining of bi-metallic aluminium-grey cast iron engine block - Process optimisation by means of FEM

Bi-metallic design concept has been introduced in automotive industries to meet the increasingly tighter standards on carbon emission. Yet, machining of bi-metallic engine blocks is accompanied by several challenges like short tool life and long cycle times. This study presents a novel methodology that combines the concept of experimental design with Finite Element (FE) to optimise the tool performance and enhance the productivity when finish face milling of aluminium-grey cast iron engine blocks. This simulation-assisted approach led to an approximately 32% decrease in machining cycle time as compared to that of a reference cutting condition

Contact Formation on Silicon Carbide by Use of Nickel and Tantalum in a Materials Science Point of View

The advantageous electrical, thermal and mechanical properties make silicon carbide (SiC) a promising semiconductor for high temperature, high power and high frequency applications. Nickel (Ni) and tantalum (Ta) can be used to form both ohmic and Schottky contact. Since metallization represents one of the most important steps in the fabrication of electronic devices, the knowledge of the interaction between Ni, Ta and SiC are of primary importance for understanding and optimising the device performance. In this chapter, an introduction of thermodynamics in Ni (or Ta)-Si-C system is given. The reaction process and mechanisms of Ni-SiC during annealing are reviewed. The phases existing in the film or at the interface and the distribution of elements in-depth are clarified. The impact of pre-treatment on SiC substrate and Ni layer thickness on phase distribution is summarized. The nature of the thermally induced solid-state reactions between Ta or Ni/Ta bilayer and SiC substrate over a wide temperature range is also discussed

Effect of precipitation kinetics on microstructure and properties of novel Al-Mn-Cr-Zr based alloys developed for powder bed fusion – laser beam process

This study investigates the precipitation kinetics and resulting effect on microstructure and property of a family of novel high strength Al-Mn-Cr-Zr alloys designed for powder bed fusion – laser beam process. The alloys have been shown to be printable without solidification cracking along with high supersaturation of solutes in as-printed state. Upon direct ageing, two families of precipitates namely Al-Mn and Al-Zr are observed. Al-Mn containing precipitates, which are observed in as-printed condition as nanometric precipitates decorating special regions are seen to grow preferentially at grain boundaries, which is followed by growth in bulk of the sample. A possible explanation is suggested to be a higher diffusivity at grain boundaries leading to faster growth while depleting solutes around grain boundary region quickly. The Al-Zr precipitation, which normally follows bulk precipitation is observed to co-precipitate with Al-Mn precipitates. Optimised heat treatments are seen to achieve peak hardness of 143 HV at 623 K for 24 h and 142 HV at 648 K for 14 h as compared to 102 HV in as-printed condition for one of the alloys. This overall hardening effect is attributed majorly to Al3Zr nanoprecipitates along with semi-coherent Al12Mn precipitates

Laser powder bed fusion of an al-mg-sc-zr alloy: Manufacturing, peak hardening response and thermal stability at peak hardness

This study shows a rapid and systematic approach towards identifying full density and peak hardness for an Al-Mg-Sc-Zr alloy commonly known as Scalmalloy\uae. The alloy is tailored for the laser powder bed fusion process and has been shown to be printable with >99.8% relative density. The microstructure suggests Al grain refinement in melt pool boundaries, associated with formation of primary Al3 (Sc, Zr) particles during solidification. Peak hardening response was identified by heat treatment tests at 573,598 and 623 K between 0 and 10 h. A peak hardness of 172 HV0.3 at 598 K for 4 h was identified. The mechanical properties were also tested with yield and ultimate strengths of 287 MPa and 364 MPa in as-printed and 468 MPa and 517 MPa in peak hardened conditions, respectively, which is consistent with the literature. Such an approach is considered apt when qualifying new materials in industrial laser powder bed fusion systems. The second part of the study discusses the thermal stability of such alloys post-peak-hardening. One set of samples was peak hardened at the conditions identified before and underwent secondary ageing at three different temperatures of 423,473 and 523 K between 0 and 120 h to understand thermal stability and benchmark against conventional Al alloys. The secondary heat treatments performed at lower temperatures revealed lower deterioration of hardness over ageing times as compared to the datasheets for conventional Al alloys and Scalmalloy\uae, thus suggesting that longer ageing times are needed

Linking in situ melt pool monitoring to melt pool size distributions and internal flaws in laser powder bed fusion

In situ monitoring of the melt pools in laser powder bed fusion (LPBF) has enabled the elucidation of process phenomena. There has been an increasing interest in also using melt pool monitoring to identify process anomalies and control the quality of the manufactured parts. However, a better understanding of the variability of melt pools and the relation to the incidence of internal flaws are necessary to achieve this goal. This study aims to link distributions of melt pool dimensions to internal flaws and signal characteristics obtained from melt pool monitoring. A process mapping approach is employed in the manufacturing of Hastelloy X, comprising a vast portion of the process space. Ex situ measurements of melt pool dimensions and analysis of internal flaws are correlated to the signal obtained through in situ melt pool monitoring in the visible and near-infrared spectra. It is found that the variability in melt pool dimensions is related to the presence of internal flaws, but scatter in melt pool dimensions is not detectable by the monitoring system employed in this study. The signal intensities are proportional to melt pool dimensions, and the signal is increasingly dynamic following process conditions that increase the generation of spatter

Advancing novel Al-Mn-Cr-Zr based family of alloys tailored for powder bed fusion-laser beam process

Additive manufacturing coupled with modern computational tools have enabled novel alloy design possibilities to create materials for the future. One such example is the Al-Mn-Cr-Zr based family of alloys tailored for powder bed fusion-laser beam process. This alloy system has previously been shown to produce crack free samples with good precipitation hardening response and strong thermal stability up to 523\ua0K 2500\ua0h. The current study investigates modifications made to enhance the mechanical response of the alloys. It was done by creating three novel alloy variants with higher Zr and addition of Mg. Interestingly, increasing Zr independently triggered grain refinement while addition of Mg independently causes significant increase in as-printed hardness albeit causing solidification cracking. Desirable properties were achieved when both higher Zr and addition of Mg was done at the same time. As-printed hardness enhanced by 30 % from previously known as-printed hardness of 102 HV to 132 HV in one of the variants. Upon direct ageing, peak hardness of 172 HV is observed thus suggesting retention of precipitation hardening

Surface chemical analysis of copper powder used in additive manufacturing

Additive manufacturing (AM) has during years gained significant interest owing to its endless component design possibilities. One of the most popular AM techniques is laser powder bed fusion (LPBF), which selectively melts metal powder layer-by-layer in a chamber with protective argon atmosphere. This technique is attractive for realizing Cu-based products in which the high electrical conductivity of Cu is combined with component design possibilities. The successful use of Cu powder not only poses challenges owing to the high reflectivity and thermal conductivity of Cu but also involves the important concern of controlling the powder surface chemistry since the powder surface constitutes the main source of oxygen. It is of crucial importance to control the oxygen level in order to maintain good electrical conductivity and brazing ability of the AM-fabricated Cu-part. In LPBF, fine spherical powder with size of 10-60 mu m is used, providing significant specific surface area, and this powder is also usually recycled several times, and hence, the role of powder surface chemistry is evident. Two kinds of copper powder with purities 99.70 and 99.95 wt% were analysed in both virgin and in used conditions after numerous printing cycles using LPBF. The powder was analysed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A clear difference between the two powder grades in terms of surface chemistry was observed. The oxide thickness and bulk oxygen content increased for both powder grades after recycling. The surface oxides under different conditions are identified and the effect of powder purity on the oxide formed is discussed

A neural network for identification and classification of systematic internal flaws in laser powder bed fusion

Quality control of mechanical components is crucial to ensure their expected performance and prevent their failure. For components manufactured additively, quality control performed in-process is particularly interesting, as the sequential deposition and remelting of layers represent a possibility to mitigate existing flaws. The first step towards closed-loop control is to ensure that the monitoring setup and the data analytics approach can flag and discriminate flaws. This study aims to assess the potential of a layerwise monitoring system associated with a supervised machine learning approach to identify and classify internal flaws in laser powder bed fusion of Hastelloy X. For that, systematically generated internal flaws were mapped ex-situ in 72 distinct process conditions. The outputs of the near-infrared long-exposure acquisition system were labeled according to the ex-situ characterization and used to train a fully convolutional neural network. The network was then used to classify previously unseen monitoring images into three classes, according to the predominant flaw type expected, lack of fusion, keyhole porosity, or residual porosity. Accuracy, precision and recall over 96% are obtained, indicating that the monitoring system combined with this supervised machine learning approach successfully identifies and classifies internal flaws

Influence of Coolant Flow Rate on Tool Life and Wear Development in Cryogenic and Wet Milling of Ti-6Al-4V

The use of cryogenic coolants has emerged as a way to improve productivity in machining Ti-alloys. In this study, liquid carbon dioxide is used as coolant in face milling of Ti-6Al-4 V with PVD coated inserts. The influence of coolant flow rate on tool life is studied by means of controlled experiments. Tool life is shown to improve with higher flow rates of coolant, the effect being stronger in cryogenic compared to wet milling due to the fact that the cryogenic coolant delays the wear development. The tool life is determined by notch wear irrespective of coolant nature in titanium milling. Different analyses were used to understand the mechanism behind the delay of notch wear development when using carbon dioxide coolant