Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing

Lack of assurance of quality with additively manufactured (AM) parts is a key technological barrier that prevents manufacturers from adopting AM technologies, especially for high-value applications where component failure cannot be tolerated. Developments in process control have allowed significant enhancement of AM techniques and marked improvements in surface roughness and material properties, along with a reduction in inter-build variation and the occurrence of embedded material discontinuities. As a result, the exploitation of AM processes continues to accelerate. Unlike established subtractive processes, where in-process monitoring is now commonplace, factory-ready AM processes have not yet incorporated monitoring technologies that allow discontinuities to be detected in process. Researchers have investigated new forms of instrumentation and adaptive approaches which, when integrated, will allow further enhancement to the assurance that can be offered when producing AM components. The state-of-the-art with respect to inspection methodologies compatible with AM processes is explored here. Their suitability for the inspection and identification of typical material discontinuities and failure modes is discussed with the intention of identifying new avenues for research and proposing approaches to integration into future generations of AM systems

Microstructural Characterization of LENS\u3csup\u3eTM\u3c/sup\u3e Ti-6Al-4V: Investigating the Effects of Process Variables Across Multiple Deposit Geometries

Laser based additive manufacturing of Ti-6Al-4V components is under consideration for aerospace applications. The mechanical properties of the finished components depend on their microstructure. Process mapping compares process variables such as heat source power, heat source travel speed, material feed rate, part preheat temperature and feature geometry to process outcomes such as microstructure, melt pool geometry and residual stresses. In this work, the microstructure of two-dimensional pads, multilayer pads, thin walls, and structural components at the steady state location was observed. A method for measuring β grain widths that allows for the calculation of standard deviations, confidence intervals, and variances in grain size was developed. This represents an improvement over the commonly used line-intercept method. The method was used to compare variability of β grain widths across different part geometries. It was found that thin wall parts have the highest β width variability and that the width of the β grains varies more towards the top of multi-layered samples than towards the bottom. Experimental results for α and β grain size across multiple deposit geometries are presented that offer new insight into the effect of process variables on microstructure. β grain widths are also compared for different deposit geometries with the same power, velocity, and feed rate. Single layer pad geometries were found to have the smallest β grain widths, multi-layer pads had larger β grain widths, and thin wall samples had the largest β grain widths. Trends in α width with Vickers hardness were also considered in the context of thermal gradient measurements. Hardness maps were created for the structural component samples. Optical microscopy was used to observe a layering effect in structural component samples. It was found that light and dark bands had different Vickers microhardness values

An Experimental Investigation of Residual Stress Development during Selective Laser Melting of Ti-6Al-4V

Selective laser melting (SLM) is an additive manufacturing (AM) process that gives rise to large thermal gradients and rapid cooling rates that lead to the development of undesirable residual stress and distortion. In this work, a number of different techniques (i.e., x-ray-diffraction, hole-drilling, layer-removal, and contour) were utilized to establish the effect of process parameters on residual stress development during SLM of Ti-6Al-4V. The measurements indicated that higher laser power, slower scan speed, smaller stripe width, reduced substrate overhang, and reduced build plan area each reduce the level of residual stress. In addition, the correlation between microstructure, crystallographic texture, and residual stress were investigated using electron backscatter diffraction (EBSD) and backscatter electron (BSE) imaging. The experimental results from this work provide a quantitative foundation for future simulations of residual stress evolution during SLM and provide an informed understanding of residual stress development that can be used for process planning and improvement

Shape complexity and process energy consumption in electron beam melting: a case of something for nothing in additive manufacturing?

Additive manufacturing (AM) technology is capable of building up component geometry in a layer-by-layer process, entirely without tools, molds, or dies. One advantage of the approach is that it is capable of efficiently creating complex product geometry. Using experimental data collected during the manufacture of a titanium test part on a variant of AM technology, electron beam melting (EBM), this research studies the effect of a variation in product shape complexity on process energy consumption. This is done by applying a computationally quantifiable convexity-based characteristic associated with shape complexity to the test part and correlating this quantity with per-layer process energy consumption on the EBM system. Only a weak correlation is found between the complexity metric and energy consumption (ρ = .35), suggesting that process energy consumption is indeed not driven by shape complexity. This result is discussed in the context of the energy consumption of computer-controlled machining technology, which forms an important substitute to EBM. This article further discusses the impact of available additional shape complexity at the manufacturing process level on the incentives toward minimization of energy inputs, additional benefits arising later within the product’s life cycle, and its implications for value creation possibilities

Laser diode area melting for high speed additive manufacturing of metallic components

Additive manufacturing processes have been developed to a stage where they can now be routinely used to manufacture net-shape high-value components. Selective Laser Melting (SLM) comprises of either a single or multiple deflected high energy fibre laser source(s) to raster scan, melt and fuse layers of metallic powdered feedstock. However this deflected laser raster scanning methodology is high cost, energy inefficient and encounters significant limitations on output productivity due to the rate of feedstock melting. This work details the development of a new additive manufacturing process known as Diode Area Melting (DAM). This process utilises customised architectural arrays of low power laser diode emitters for high speed parallel processing of metallic feedstock. Individually addressable diode emitters are used to selectively melt feedstock from a pre-laid powder bed. The laser diodes operate at shorter laser wavelengths (808 nm) than conventional SLM fibre lasers (1064 nm) theoretically enabling more efficient energy absorption for specific materials. The melting capabilities of the DAM process were tested for low melting point eutectic BiZn2.7 elemental powders and higher temperature pre-alloyed 17-4 stainless steel powder. The process was shown to be capable of fabricating controllable geometric features with evidence of complete melting and fusion between multiple powder layers

Laser powder fusion of H13 tool steel using pulsed Nd:YAG laser

Laser Powder Fusion has been used to explore the possibility of fabricating 3-dimensional parts from Computer Aided Design (CAD) data, using the underlying philosophy of 'Rapid Prototyping' or 'Freeform fabrication' techniques. Such techniques have emerged in the past decade or so, as a revolutionary way of designing and realising products primarily for form, fit and functionality. Applications have evolved in the areas of automotive, aerospace, medicine to name a few. The Laser Powder Fusion technique is characterised by the use of a heat source, like a laser or electron beam used for melting or fusing powder material like metal, polymers etc to the required geometry, by tracing and filling in (or rastering) 2-dimensional contours, line by line, layer upon layer, to obtain the 3D geometry. The motivation for the present work comes from the use of this technique for fabricating components with enhanced functionality, for applications in production and prototype tooling, engine parts etc. [Continues.

Measurement and Simulation of Low Carbon Steel Alloy Deposit Temperature in plasma Arc Welding Additive Manufacturing

Additive manufacturing has the potential to produce near-net shape parts directly from weld metal. Prior work has proved that it is possible to directly manufacture components with complex geometric features and with good productivity. However, under high productivity conditions, deposit temperature increases to a level that it is no longer possible to develop appropriate deposit microstructure and therefore mechanical properties. In this study, Plasma Arc welding was used to produce experimental deposits of 1018 low carbon steel under various conditions. An analytical heat flow model was developed to study the influence of interlayer wait time on deposit temperature and therefore grain size and hardness. The results of the model indicated that as wall height increased, the rate of deposit heat removal by conduction to the substrate decreased leading to a higher preheat temperature after a fixed interlayer wait time causing grain size to increase as wall height increased. However, the model results also show that as wall height increased, the deposit surface area from which heat energy is lost via convection and radiation increased. The model also demonstrated that the use of a means of forced convection to rapidly remove heat from the deposit could be an effective way to boost productivity and maintain smaller grain size and therefore higher hardness and strength in the deposit

Analytical process model for wire + arc additive manufacturing

An analytical process model for predicting the layer height and wall width from the process parameters was developed for wire + arc additive manufacture of Ti-6Al-4V, which includes inter-pass temperature and material properties. Capillarity theory predicted that cylindrical deposits were produced where the wall width was less than 12 mm (radius <6 mm) due to the large value of the surface tension. Power was predicted with an accuracy of ±20% for a wide range of conditions for pulsed TIG and plasma deposition. Interesting differences in the power requirements were observed where a surface depression was produced with the plasma process due to differences in melting efficiency and/or convection effects. Finally, it was estimated the impact of controlling the workpiece temperature on the accuracy of the deposit geometry