A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties

Consolidation phenomena in laser and powder-bed based layered manufacturing

Selective Laser Melting (SLM) is an additive manufacturing (AM) technique which has been heavily investigated for the processing of Ti-6Al-4V (Ti64) which is used in the biomedical, aerospace and other industries. To date the SLM processing of this material has been inhibited by the requirement of post processes due to three primary challenges of martensitic microstructures, undesired porosity and residual stresses which are present in the as-built state. This work identifies the state of the art in process optimisation which is being used to confront these challenges in the as-built state with a view to removing the reliance on post processing. Regarding process optimisation, maximising part density is the primary goal due to the negative influence of pores on fracture and fatigue properties. To accomplish this, a high energy input is required which results in high cooling rates during processing. It is these cooling rates which are instrumental in the microstructural evolution and residual stress production. Accordingly novel methods have been proposed which aim to maintain the necessary high level of energy input but control the cooling rates to tailor the microstructure and reduce residual stresses. Research gaps have been identified pertaining to all three of these challenges when considering mechanical properties of as-built components. Thus in its current state post processes remain critical, however promising techniques in early stage development provide encouragement going forward.

Optimisation of process parameters to address fundamental challenges during selective laser melting of Ti-6Al-4V: A review

The powder bed fusion additive manufacturing process enables fabrication of metal parts with complex geometry and elaborate internal features, the simplification of the assembly process, and the reduction of development time; however, its tremendous potential for widespread application in industry is hampered by the lack of consistent quality. This limits its ability as a viable manufacturing process particularly in the aerospace and medical industries where high quality and repeatability are critical. A variety of defects, which may be initiated during powder bed fusion additive manufacturing, compromise the repeatability, precision, and resulting mechanical properties of the final part. One approach that has been more recently proposed to try to control the process by detecting, avoiding, and/or eliminating defects is online monitoring. In order to support the design and implementation of effective monitoring and control strategies, this paper identifies, analyzes, and classifies the common defects and their contributing parameters reported in the literature, and defines the relationship between the two. Next, both defects and contributing parameters are categorized under an umbrella of manufacturing features for monitoring and control purposes. The quintuple set of manufacturing features presented here is meant to be employed for online monitoring and control in order to ultimately achieve a defect-free part. This categorization is established based on three criteria: (1) covering all the defects generated during the process, (2) including the essential contributing parameters for the majority of defects, and (3) the defects need to be detectable by existing monitoring approaches as well as controllable through standard process parameters. Finally, the monitoring of signatures instead of actual defects is presented as an alternative approach to controlling the process “indirectly.”

Common defects and contributing parameters in powder bed fusion AM process and their classification for online monitoring and control: a review

Purpose – The purpose of this paper is to study the effects of particle size distribution, component ratio, particle packing arrangement, and chemical constitution on the laser sintering behaviour of blended hypoeutectic Al‐Si powders.Design/methodology/approach – A range of bimodal and trimodal powder blends were created through mixing Al‐12Si and pure aluminium powder. The powder blends were then processed using selective laser sintering to investigate the effect of alloy composition, powder particle size and bed density on densification and microstructural evolution.Findings – For all of the powder blends the sintered density increases with the specific laser energy input until a saturation level is reached. Beyond this saturation level no further increase in sintered density is obtained for an increase in specific laser energy input. However, the peak density achieved for a given blend varied significantly with the chemical constitution of the alloy, peaking at approximately 9 wt% Si. The tap density ...

/pdf/laser-sintering-of-blended-al-si-powders-11o76l2x49.pdf

Laser sintering of blended Al‐Si powders

The research presented in this thesis was part of a larger collaborated project (LastForm Programme) to research engineering solutions for the rapid manufacture of large scale (0.5m – 5.0m in length) low, medium and high temperature tooling (from room temperature to 1000C) for use in the automotive and aerospace industry. All research was conducted using small scale investigations but with a final discussion including implications of the work in future large scale planning. 
The aim of the work presented in this thesis was to develop current understanding about the sintering and melting behaviour of metal powders by Selective Laser Sintering (SLS). The powder used in the research was an argon atomised austenitic stainless steel of type 314s HC. The powder was supplied in four batches, each differentiated by particle size distribution; -300+150m, -150+75m, -75+38m and -38m. The characteristics of each powder, in particular flow properties, differed considerably allowing powder handling and powder flow during melting to also be explored in this work. Three different environmental conditions were also investigated to asses the role of atmospheric and residual (powder) oxygen: (1) air atmosphere (control), (2) argon atmosphere and (3) argon atmosphere with argon percolation through the powder layer. In this, the design of an environmental control chamber and its integration into a research SLS machine was central to the work. 
Experimental studies of the selective laser sintering/melting process on room temperature stainless steel 314s powder beds has been successfully carried out. The methodology was progressive; from tracks to layers to multiple layers. Single tracks were produced by melting the powder by varying laser power and scan speed. Results from experiments have been used to construct a series of process maps. Each map successfully charts the heating and melting behaviour of the irradiated powder. Behaviours can now be predicted with reasonable accuracy over a dense power and speed range, including laser powers up to 200W and scan speeds up to 50mm/s. The experiments also allowed melt pool geometries to be investigated. Three types of melt cross-section were categorised; flattened, rounded and bell shape. Flat tracks generally occurred at low speed (0.5mm/s) but also occurred up to 4mm/s at lower power (77W). Rounded tracks occurred between 1mm/s and 4mm/s and had a much larger area than expected. In the rounded track regime tracks sink well into the powder bed. Powder to either side of a track collapses into it, leaving a trench surrounding the track. The admission of extra powder is thought to be one cause of increased mass. However, a remaining question that still needs answering is what causes the change from a flattened to a rounded track. 
Values of laser absorptivity were also estimated from track mass per unit length data and from melting boundaries displayed within the process maps. The results showed that absorptivity changed considerably depending on the powder, process conditions and atmospheric conditions. Within an argon atmosphere an „effective‟ absorptivity from mass data was estimated to range from 0.1 to 0.65, the lower value at low speed scanning (0.5mm/s) and the higher value from high speed scanning (>4mm/s). These values were much higher than expected for a CO2 laser. 
Melt pool balling was found to be a big problem, limiting the process speed at which continuous tracks could be successfully constructed (<12mm/s). Comparisons between a mathematical model developed in this work and experimental results suggested that balling within an air environment occurred when the ratio of melt pool length to width reached a critical value close to . Balling within an argon atmosphere was more difficult to model due to higher viscous melts caused by the take up of surrounding powder. 
Melted single layers were produced by varying laser power, scan speed, scan length and scan spacing or melt track overlap. Scan length proved to be a significant factor affecting layer warping and surface cracking. Provided the scan length remained below 15mm, layer warping could be largely avoided. 
Multiple layer blocks were produced by melting layers, one on top of the other. They were constructed over a range of conditions by varying laser power, scan speed, scan spacing and layer thickness. Layer thickness was a crucial parameter in controlling the interfacial bond between layers, but the spreading mechanism proved to be the overriding factor affecting layer thickness and therefore the quality and density of the blocks.

Selective Laser Sintering of a Stainless Steel Powder

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Selective Laser Sintering of a Stainless Steel Powder