Effects of powder characteristics and chemical composition on the properties of 25Cr7Ni stainless steel fabricated by laser-powder bed fusion and evaluation of process simulation

The 25Cr7Ni stainless steel alloy system is gaining increasing interest in the oil and gas industry because of its combination of high strength and corrosion resistance properties. However, very few studies on the effects of starting powder attributes and chemical composition on the as-printed properties of 25Cr7Ni stainless steel fabricated through laser-powder bed fusion (L-PBF) exist in the literature. This study examined the influence of powder attributes and chemical composition on the samples from gas atomized and water atomized 25Cr7Ni stainless steel powders, fabricated through L-PBF, on their as-printed microstructure and properties. The mechanical properties that were examined included ultimate tensile strength (UTS), elongation (%), and hardness. The corrosion behavior was also studied using linear sweep voltammetry in 3.5 wt.% NaCl solution. The evolved phases were characterized using optical and scanning electron microscopy, as well as through X-ray diffraction. The gas atomized powders, with their spherical and uniform morphology, yielded as-printed parts of higher relative densities when compared to water atomized powders, with irregular morphology due to better powder bed compaction. The higher densification obtained in the L-PBF samples from gas atomized powders translated into the highest UTS, hardness, and yield strength among the L-PBF samples from water atomized powders and wrought–annealed 25Cr7Ni stainless steel. The presence of higher amounts of N and Mn in the chemical composition of the gas atomized powders over water atomized powders promoted the presence of retained austenite in the corresponding L-PBF samples. Higher amounts of Mo, combined with austenite content, yielded a higher corrosion resistance in the L-PBF samples from the gas atomized powder than in the L-PBF samples from the water atomized powders. The latter part of the work is focused on the evaluation of simulation parameters for analyzing the fabrication procedure for the L-PBF process using Simufact software. For a given set of process parameters, Simufact provides the distortion and internal stresses developed in the printed parts as output. The present study sought to evaluate the process simulation by comparing the experimental observations in terms of the part distortion achieved in a stainless steel cube fabricated through L-PBF with Simufact process simulation obtained using the same set of process parameters

MASTER DECOMPOSITION CURVE ANALYSIS OF ETHYLENE VINYL ACETATE PYROLYSIS: INFLUENCE OF METAL POWDERS

Polymer burnout (pyrolysis or delubrication) is a crucial step in sintering die compacted powders. To systematically analyse and design the thermal delubrication step, the master decomposition curve (MDC) has been formulated based on the intrinsic kinetics of polymer pyrolysis. The Kissinger method was used to estimate the activation energy from thermogravimetric analysis (TGA) experiments. The activation energy of poly(ethylene-co-vinyl acetate) (EVA) was determined and an MDC analysis was performed to map the weight loss of the polymer as a function of time and temperature. The developed MDC was used to investigate the effects of powder chemistry, powder shape, and particle size of 316L stainless steel on the decomposition behaviour of EVA. The activation energies for decomposition of EVA decreased in the presence of gas and water atomised 316L stainless steel powders, indicative of a catalytic effect. This effect was more pronounced for the first decomposition step suggesting the possible role of a carboxylate ion - metal transition state complex that promoted decomposition. In addition, the gas atomised 316L stainless steel had a greater effect on lowering the activation energy for decomposition compared to water atomised 316L stainless steel, emphasising the influence of powder surface chemistries. Based on the MDC analysis, the required hold time can be predicted for a given temperature and target binder weight loss. This reduces the experimentation required to optimise the delubrication cycle. Furthermore, when extrapolating to very small particle sizes, this approach is of particular interest for predicting the behaviour of nano-particulate materials.X116sciescopu

IN SITU OBSERVATION OF SHAPE LOSS DURING POLYMER BURNOUT IN PM PROCESSING

Shape loss or distortion of powder metal (PM) components can occur during polymer burnout. A study has been carried out to understand the evolution of shape loss during polymer burnout utilizing thin beams compacted from admixed powders of 316L stainless steel and 1 w/o ethylene vinyl acetate (EVA). The effect of particle shape on shape loss was studied by using gas-and water-atomized 316L stainless steel powders. The results showed that shape loss occurs primarily due to viscous creep during softening of the polymer. The shape loss was also found to depend on the degradation behavior of the polymer. Recovery of shape loss was observed on burnout in the case of polymers degraded by formation of double-bond product. The recovery of shape loss was found to be absent in injection molded samples in which the burnout of the polymers occurred without the formation of a double-bond product. The midpoint deflection of the beam under the influence of gravity was monitored in situ to quantify shape loss during polymer softening and burnout. Viscosity evolution during polymer burnout was estimated from the rate of midpoint deflection and was found to increase on heating from room temperature to 250 degrees C.X112sciescopu

Powder injection molding process design for UAV engine components using nanoscale silicon nitride powders

The feasibility of powder injection molding to fabricate silicon nitride engine components was evaluated in the presented study. Experiments were carried out on a feedstock consisting of nanoscale silicon nitride powders mixed with magnesia, yttria and a paraffin wax polypropylene binder system. The measured rheological and thermal properties of feedstock were used to simulate the flow of material during injection molding of a combustion engine for an unmanned aerial vehicle (UAV). Simulations based on the Box-Behnken design were used to identify the critical parameters affecting the injection molding process. The simulation results identified melt temperature as the dominant factor affecting the injection pressure, clamp force, shear stress, sink mark depth, temperature at flow front and volumetric shrinkage. Injection time was found to be the dominant factor affecting the bulk temperature and time at the end of the packing. The optimal injection molding parameters were further estimated using a non-linear programming (NLP) model. It is expected that the engineering community can use the simulation techniques discussed in the study to identify optimum processing conditions for fabricating the complex engine parts, thereby avoiding iterative expensive and time-consuming trials. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.X1189sciescopu

Review: Thermal Debinding Process in Particulate Materials Processing

Developing a rapid and efficient method for removing polymers (termed binders) from a shaped powder component, know as a green body, is important to forming defect-free metal, ceramic, and cermet structures. The rapid growth in powder injection molding to form complex shapes at high precision in large quantities has increased the need for faster, cleaner, and cheaper polymer removal processes. Binder removal using controlled heating of the component in gaseous atmosphere is the most popular method. This thermal debinding or burnout process is a delicate process, since it is easy to crack, blister, slump, or otherwise damage the component with an improperly designed cycle. To avoid these issues, often long heating cycles are used to remove the binder, but with a loss of productivity. Considerable progress has been made over the past several decades in understanding various phenomena during polymer burnout, resulting in substantial reduction in the thermal debinding time. This article provides an overview of the research carried out on thermal debinding process (primarily from powder injection molded samples) with major emphasis on progress reported over the last fifteen years. This review article proposes a model to predict the formation of defects during all stages of thermal debinding and suggests future research direction in the field.X112218sciescopu

The effects of nanoparticle addition on binder removal from injection molded aluminum nitride

The effects of nanoparticle addition on the multi-step debinding of injection molded aluminum nitride (AlN) samples were studied. Experiments varying the solvent debinding conditions (time, temperature and aspect ratio) were performed on monomodal, microscale (mu) and bimodal, micro-nanoscale (mu-n) AlN samples. Variations in the solvent debinding kinetics as a result of the reduced particle size and increased powder content were examined. The bimodal mu-n AlN samples showed a slower solvent extraction of binder components compared to monomodal mu-AlN samples. The activation energy for solvent extraction estimated from diffusion coefficients (Arrhenius equation) was in close agreement with the value estimated by the master debinding curve (MDC) method. An activation value around 50 kJ/mol was estimated by both the methods for mu and mu-n AlN samples. The thermal debinding behavior of dewaxed samples was also studied and the trends correlated with the solvent debinding behavior. (c) 2012 Elsevier Ltd. All rights reserved.X1189sciescopu

Taguchi analysis on the effect of process parameters on densification during spark plasma sintering of HfB2-20SiC

Field assisted sintering (FAST) has emerged as a useful technique to densify ultra high temperature ceramics like HfB2-20SiC to a high density at relatively low temperatures and shorter times. The effect of various process variables on the densification during spark plasma sintering of HfB2-20SiC was studied using Taguchi analysis. The statistical analysis identified sintering temperature as the most significant parameter affecting the densification of HfB2-20SiC material. A density of 99% was achieved on sintering at 2373 K for 8 min at 30 kN pressure and heating rate of 100 K/min. (C) 2011 Elsevier Ltd. All rights reserved.X111313sciescopu

The effects of nanoparticle addition on SiC and AlN powder-polymer mixtures: Packing and flow behavior

The development of methods to increase sintered density and improve dimensional tolerances is a crucial issue in powder metallurgy and ceramic processing. Increasing the packing density of starting powders is one effective route to achieve high sintered density and dimensional precision. The current paper presents an in-depth study on the effect of nanoparticle addition on the powder content of SiC and AIN powder-polymer mixtures. In particular, bimodal mixtures of nanoscale and sub-micrometer particles were found to have significantly increased powder volume fraction (solids loading) in the mixtures for injection molding. This observation to increasing packing density by using nanoparticles is surprising and novel since nanoparticles are known to inherently exhibit poor packing behavior. Additionally, for a given volume fraction of powder, the bimodal mu-n suspensions had a lower viscosity at any shear rate compared to the monomodal mu-suspensions. The ability to lower the suspension viscosity by adding nanoparticles to micron-sized particles has important implications for processing of particulate suspensions by powder injection molding (PIM), extrusion, slip casting and tape casting. Samples made from bimodal powders exhibited slower polymer removal during debinding and higher densification with lower shrinkage on sintering compared to the corresponding samples made from monomodal powder mixtures. (C) 2012 Elsevier Ltd. All rights reserved.X111820sciescopu

In Situ Characterization of Strength and Distortion During Powder Metal Processing

Thermogravimetric analysis and dilatometry are two common in situ measurement techniques used in powder processing to obtain information regarding thermal debinding and sintering. However, these two techniques provide negligible information regarding critical phenomena such as distortion or cracking. This paper discusses newer characterization techniques used to measure in situ strength evolution to help understand defect generation during the thermal debinding process. The information obtained from these in situ measurement techniques assists in intelligent design of optimal thermal cycles and in selection of binders targeted at manufacturing dense sintered components with minimum defects and distortion. The paper discusses examples of applications developed based on the information provided by in situ characterization of strength and distortion.X111sciescopu

Development of master sintering curve for field-assisted sintering of HfB2-20SiC

Field assisted sintering (FAST) or spark plasma sintering (SPS) has emerged as a promising technology for densification of ultra high temperature ceramics like HfB2-20SiC at relatively low temperatures and shorter times. In the present study the concepts of master sintering curve (MSC) was applied to model the densification behavior of HfB92-20SiC during FAST process. An activation energy of 300 kJ/mol was estimated for field assisted sintering of HfB92-20SiC. The densification curves at various heating rates merged for activation energy of 300 kJ/mol confirming the applicability of MSC concepts to FAST process. The developed master sintering curves can be used to design sintering cycles and predict the densification of HfB92-20SiC during FAST process. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.X1188sciescopu