Sintering parameters and mechanical properties of injection moulded aluminium powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded aluminium powder. Gas atomised aluminium powder was injection moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for feedstock. Binder debinding was performed in solvent and thermal method. After debinding, the samples were sintered at different temperatures and times in high purity N₂. Metallographic studies were conducted to determine the extent of densification and the corresponding microstructural changes. The results show that gas atomised aluminium powder could be sintered to a maximum 96·2% of theoretical density. Maximum density, tensile strength and hardness were obtained when sintered at 650°C for 60 min.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

Variability of powder characteristics and their effect on dimensional variability of powder injection moulded components

Abstract

In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations in powder characteristics such as surface area, tap and apparent density, and chemistry have little effect on dimensional stability.     

Mechanical properties of metal injection moulded 316L stainless steel using both prealloy and master alloy techniques

Abstract

Stainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of ? 16 μm and ? 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed ? 16 μm and the ? 22 μm powders. Also, the viscosity of the mixtures was higher for the ? 16 μm material and the master alloy mixtures than for the –22 μm gas atomised prealloyed powders.     

Injection moulding of 316L stainless steels reinforced with nanosize alumina particles

Abstract

This study aims to compare the effect of Al₂O₃ nanoparticle additions on the densification and mechanical properties of the injection moulded 316L stainless steels. The 316L stainless steel and Al₂O₃ nanoparticles were dry mixed and moulded using a wax based binder. The critical powder loading for injection moulding were 60 vol.-% for all samples. Debinding process was performed in solvent using thermal method. After the debinding process, the samples were sintered at 1405°C for 60 and 120 min under vacuum. Metallographic examination was conducted to determine the extend of densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all the samples after wear testing. All the powders, fracture surfaces of moulded and sintered samples were examined using scanning electron microscope. The sintered density of straight as well as Al₂O₃ nanoparticles reinforced injection moulded 316L stainless steels increases with the increase in sintering time. The additions of Al₂O₃ nanoparticles improve the hardness and wear resistance with the increase of sintering time.     

Effect of nickel boride and boron additions on sintering characteristics of injection moulded 316L powder using water soluble binder system

Abstract

A new metal injection moulding system for 316L stainless steels has successfully been derived and tested. A mixture of small water atomised powder (average size 15 µm), larger gas atomised powder (average size 75 µm), and sintering additives has been coupled with a new water soluble binder system for economical powder injection moulding. The details for each process step and the effect of sintering additives are described. The binder system consists of poly (2-ethyl-2-oxazoline) as the leachable polymer, polyethylene as the backbone, and stearic acid as a surfactant and plasticiser. This binder system provides satisfactory mixture stability, excellent mouldability, and reasonably fast water leaching and thermal debinding rates. The optimum powder/binder compositions were determined using torque and capillary rheometry. Densification was by persistent liquid phase sintering through additives, such as nickel boride and boron. This 316L powder system was sintered to 7·9 g cm ^-3 (98·75% of theoretical) at 1285°C using nickel boride addition and at 1245°using boron addition. nickel boride additions are particularly effective at increasing the tensile strength and ductility. In contrast, the boron additions only increase the tensile strength and decrease ductility. Based on microstructure evaluations, this effect is traced to a continuous boride phase on the grain boundaries of the boron doped samples and a discontinuous boride phase on the grain boundaries of the nickel boride samples.     

Keeping to form in the powder business

This work evaluated the influence of gas and water-atomised powders on form retention and mechanical properties of PIM 316L stainless steel components. Two different particle morphologies were used for powder mixtures varying the proportions of spherical and irregular powder -- gas and water atomised powders respectively — varying the content from zero through 25 per cent, 50 per cent, 75 per cent and 100 per cent by mass. The mixtures of gas and water-atomised powders with higher solids loading showed lack of homogeneity and inadequate moulding characteristics. The form retention of samples moulded with only water-atomised powder showed only small shrinkage anisotropy and distortion. The gas-atomised powder PIM samples also demonstrated minor anisotropy, but they showed more slump during chemical and thermal binder removal. Samples with the two powders presented a higher degree of geometric anisotropy. Best results for densification and mechanical properties were obtained by components using water-atomised powder showing 95 per cent of theoretical density and resistance up to 500 MPa.     

Modified metal injection moulding process of 316L stainless steel powders using thermosetting binder

Abstract

A modified metal injection moulding (MIM) process of 316L stainless steels powders using an acrylic thermosetting resin has been developed. Gas and water atomised 316L powders were used. In order to optimise the mixing and moulding steps, different volume fractions of the two components were investigated. Mixing of metal powder and binder was carried out at room temperature and immediately moulding was performed by pouring the slurry in the moulds. It was then heated at 90°C to permit the polymerisation and cross linking of the resin. Different heating cycles, rates, and atmospheres were studied by means of thermogravimetrical analysis. The data obtained were used to establish the best debinding cycle. The debound samples were sintered at different temperatures and high densities (98% of theoretical) were obtained. Materials in as moulded (green part), debound (brown part), and sintered conditions were examined by means of SEM.     

Development of Duplex Stainless Steels by Field-Assisted Hot Pressing: Influence of the Particle Size and Morphology of the Powders on the Final Mechanical Properties

The feasibility of processing duplex stainless steels with promising properties using a powder metallurgical route, including the consolidation by field-assisted hot pressing, is assessed in this investigation. The influence of the particle size and morphology of the raw austenitic and ferritic powders on the final microstructure and properties is also evaluated for an austenitic content of 60 wt pct. In addition, the properties of a new microconstituent generated between the initial constituents are analyzed. The maximum sintered density (98.9 pct) and the best mechanical behavior, in terms of elastic modulus, nanohardness, yield strength, ultimate tensile strength, and ductility, are reached by the duplex stainless steel processed with austenitic and ferritic gas atomized stainless steel powders.

     

Microstructure and mechanical properties of microwave sintered austenitic stainless steel

The objective of this work is focused on understanding the effect of microwave heating on sintering of 316L powders. The stainless steel samples were prepared from prealloyed powders of 316L. The powder samples were compacted at a pressure of 560 MPa and sintered at 1300°C in a microwave furnace of 2.4 GHz and 2KW capacity in nitrogen atmosphere. The sintering time was varied from 10 to 20 minutes in order to study the effect of sintering time on sintering behavior and mechanical properties of the sintered samples. The sintered samples were subjected to optical metallography, hardness testing, tensile testing and fractogrphy. The average density of sintered stainless steels was 92% of the theoretical density, approximately 18% increment from green density for 20 min. sintering time. Microstructural analysis showed the regularly distributed porosity with very small grains. The hardness value was in the range of 365VHN to 396VHN and tensile strength, in the range of 255MPa to 580 MPa. Fractographs for these steels revealed mixed mode of fracture.     

Effect of Cu3P addition on sintering behaviour of elemental powders in the composition of 465 stainless steel

Abstract

The addition of Cu₃P for developing the high strength 465 maraging stainless steel from elemental powders was studied. The sintering parameters investigated were sintering temperature, sintering time and wt-%Cu₃P. In vacuum sintering, effective sintering took place between 1300 and 1350°C. The maximum sintered density of 7·44 g cm^?3 was achieved at 1350°C for 60 min with 4–6 wt-%Cu₃P. More than 6 wt-%Cu₃P content and temperature >1350°C caused slumping of the specimens. The sintered specimens were heat treated and a maximum ultimate tensile strength (UTS) of 767 MPa was achieved with 4 wt-%Cu₃P content. The maximum hardness of 45·5 HRC was achieved in heat treated condition with 4 wt-%Cu₃P content. Above 4 wt-%Cu₃P content increase in density was observed whereas the response to heat treatment decreased. Fracture morphologies of the sintered specimens were also reported. A comparison of sintering behaviour and mechanical properties of elemental powders with prealloyed powders was also given in the present study.     

Influences of sintering atmospheres on densification process of injection moulded gas atomised 316L stainless steel

Abstract

Densification of metal injection moulded 316L stainless steel is one of the hottest research fields in powder metallurgy in recent years. In the present work, various sintering atmospheres, i.e. N₂, N₂ + H₂, Ar and Ar + H₂, were adopted to investigate their influences on the sintered density, pore size, pore shape, grain size and mechanical properties. Relative densities of 98% of the theoretical density (7·84 g cm^-3 and 7·85 g cm^-3), pore sizes about 2~3 μm and mean grain sizes about 50 μm could be obtained after the specimens were sintered in Ar and Ar + H₂ atmospheres. Optimised mechanical properties could be acquired in Ar + H₂ atmosphere, i.e. σ _b = 630 MPa, σ _0·2 = 80 MPa, δ = 52%, HRB = 71. When sintered in N₂ and N₂ + H₂, the specimens have lower densities than those sintered in Ar and Ar + H₂. An ultimate tensile strength of 765 MPa and a hardness of 82 HRB could be obtained respectively in N₂ + H₂ and N₂, atmospheres. The dimensional tolerance of the sintered part is less than ±0·5% with the sintered dimension ranged from 2·64 to 43·07 mm.     

粉末形状与松装密度对不锈钢烧结多孔材料制备工艺及其性能的影响

通过选用气雾化及水雾化两种工艺方法制备的不锈钢粉末来制取粉末烧结多孔材料。探讨了粉末形状及松装密度对不锈钢粉末烧结多孔材料制造工艺中的成形压力和烧结温度等工艺参数的影响;研究了原料粉末松装密度对不锈钢粉末烧结多孔材料的透气性、拉伸强度的影响。结果表明:成形压力、烧结温度和制品的透气性受粉末松装密度影响显著。粒度范围为0.18～0.90mm时,气雾化粉末的成形压力比水雾化粉末要高近1倍;当粉末的粒度相同时,采用松装密度大的球形粉末所需的烧结温度比松装密度小的不规则粉末的高60～70℃;粒度为0.45～0.60mm时,选用松装密度为4.13 g/cm3粉末所制备的多孔制品的透气性为3.16×10-10m2,而选用松装密度为2.67 g/cm3的粉末所制备的多孔制品的透气性仅为8.8×10-11m2。不锈钢多孔材料的强度受原料粉末的松装密度影响显著;粒度相同,制备工艺相同时,采用较低松装密度的粉末的制品,能够得到较高的强度。     

Influence of injection moulding and sintering parameters on properties of 316L MIM compact

Abstract

This study elucidates the effects of key injection moulding and sintering factors on the dimensions and mechanical properties of 316L stainless steel metal injection moulded compact. Sintered parts of optimal quality can be produced by properly setting the process parameters. Taguchi method and principal component analysis are performed initially to elucidate and optimise the key control factors that affect the qualities of metal injection moulded compact. Next, a feasible process window is tested by observing the powder and binder distribution of green parts, for various control factors of injection moulding. Experimental findings show that, first, a proper injection speed facilitates mould filling during injection moulding and so improving the quality of sintered parts; second, temperature critically determines the rate of dimensional shrinkage, density and hardness of sintered parts; Third, optimal parameters setting can efficiently improve the quality of 316L metal injection moulded compact.     

Structure property correlations in hot isostatically pressed AISI-304 stainless steel

Abstract

Stainless steel components are usually fabricated by conventional manufacturing methods. This route becomes uneconomical for highly intricate shapes. Therefore, processing of AISI-304 stainless steel has been carried out by powder metallurgy and hot isostatic pressing (HIP) route. The microstructure and mechanical properties of the hipped steel produced from inert gas atomised powder were studied. The steel has shown a homogeneous and fine grained microstructure containing annealing twins and the absence of undesirable prior particle boundaries. This has led to higher yield strength, ultimate tensile strength and the ductility than that of the conventionally processed steel. The tensile deformation behaviour of hipped steel was found to be similar to that of the wrought material. Impact strength and fracture toughness of the hipped steel were superior to that of the wrought material; hence the hipped steel could be recommended for manufacture of components for critical applications.     

PM processing and characterisation of Ti–7Fe low cost titanium alloys

Abstract

This work studies a set of low cost beta alloys with the composition Ti–7Fe, processed by conventional powder metallurgy (PM). The materials were prepared by conventional blending of elemental Ti hydride–dehydride powder with three different Fe powder additions: water atomised Fe, Fe carbonyl and master alloy Fe–25Ti. The optimal sintering behaviour and the best mechanical properties were attained with the use of Fe carbonyl powder, which reached a sintered density of up to 93% of the theoretical density, with UTS values of 800 MPa in the ‘as sintered’ condition. Coarse water atomised powder particles promoted reactive sintering, and coarse porosity was found due to the coalescence of Kirkendall porosity and by the pores generated during the exothermic reaction between Ti and Fe. The addition of Fe–25Ti produced brittle materials, as its low purity (91·5%) was found to be unsuitable for formulating Ti alloys.     

Master decomposition curve analysis of ethylene vinyl acetate pyrolysis: influence of metal powders

Abstract

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.     

Comparison of mechanical properties of Fe-1.75Ni-0.5Mo-1.5Cu-0.4C steels made from PIM and press and sinter processes

Abstract

Powder injection moulding (PIM) is a relatively new process and only a few alloy standards have been recognised. To further promote the application of this technology, new alloys with competitive mechanical properties need to be developed. Fe-1.75Ni-0.5Mo1.5Cu-xC is one of the compositions widely used in the conventional powder metallurgy industry in making press and sinter parts. To benefit from the excellent mechanical properties and the accumulated knowledge of this alloy system, the same composition was employed in this study to make powder injection moulded compacts with the expectation that evenbetter mechanical properties would be attained. The results obtained on the compacts sintered at 1200°C for 1h showed a tensile strength, hardness, and elongation of 685MPa, 91 HRB, and 7 5% respectively. With heat treatment, the tensile strength and hardness increased to 1530MPa and 52 HRC, respectively. However, the elongation decreased to less than 1 0%. These properties are better than those of the press and sinter counterparts owing to higher sintered density, finer grain size, and more homogeneous microstructure.     

Production of titanium grade 4 components by powder injection moulding of titanium hydride

Abstract

Recent developments are presented on powder injection moulding of titanium from metal hydride powders and binders composed of polyethylene, paraffin wax and stearic acid. The feasibility of using this route to process fit for purpose, complex parts is assessed. Titanium hydride offers a low cost solution compared with pure titanium powders. Feedstocks for powder injection moulding were prepared in a sigma mixer. Tensile test specimens and demonstration parts were injection moulded. Solvent debinding in heptane was followed by thermal debinding and dehydrogenation under argon. Titanium parts were sintered at 1200°C under argon. Sintered parts exhibit a linear shrinkage of about 20%, good shape preservation and reproducibility. The yield strength (519 MPa), ultimate tensile strength (666 MPa), elongation to fracture (15%) and interstitial content measured by quantitative analysis meet the requirements for titanium grade 4.     

Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m^?2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.