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.     

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.     

Analyses of powder segregation in MIM

The effects of main process parameters on final components made by metal injection moulding have been studied through experiments correlated with accurate modelling and numerical simulations. A mould specially designed for experiments was made in our laboratory for components with different processing conditions, then the components were debound and sintered. Mechanical properties were measured by tensile and bending tests and correlated with processing conditions. The prediction of segregation during injection moulding was based on a bi-phasic model with a newly developed algorithm. The results from simulation were compared with experimental results and the agreement demonstrates that proposed methods are well adapted for mould design and determination of injection moulding parameters.     

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.     

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.     

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.     

316L不锈钢粉末微注射成形的烧结过程有限元模拟研究

金属粉末微注射成形技术(Micro MIM)在大规模生产微型产品领域有其特有的优势。将316L不锈钢细颗粒粉末进行混料、脱粘和烧结后制备了微型工件,对烧结过程进行数学建模得出注射料的本构方程,并对剪切黏稠度、体黏稠度、烧结应力等参数进行标定,实现了对316L不锈钢粉末烧结过程中的收缩率、相对密度等的有限元模拟,模拟结果与实验数据吻合良好。     

Processing of Mn–Zn ferrites using mould casting with acrylic thermosetting binder

Abstract

In this paper, a simple manufacturing process for Mn–Zn ferrite powder is described, which can be considered as a modified powder injection moulding process. This method uses acrylic thermosetting resin as the binder. The moulding is carried out at room temperature by directly pouring the slurry (resin and ferrite) in the mould. The mixture is heated at the curing temperature (70°C) of resin to permit polymerisation and cross linking of the polymer. In order to optimise the moulding step, different volume fractions of powder with resin were mixed. The optimal powder load was 50 vol.-%. The best thermal debinding cycle was determined by means of thermo-gravimetric analysis. Sintering was performed according to oxygen partial pressure equilibrium curves at 1330°C for 3 h. Magnetic properties were compared with those obtained by uniaxial compacted parts.     

Experimental investigation and numerical simulation analysis of sintered micro-fluidic devices

ABSTRACT

This study investigates the use of numerical simulations to describe the solid-state diffusion of a sintering stage during a metal injection moulding process for micro-fluidic components with 316L stainless steel powders. Finite element (FE) analysis based on a thermo-elasto-viscoplastic model was conducted to describe the densification process of a stainless steel porous component during solid-state sintering. The numerical analyses, which were performed on a 3D micro-structured component with various powder volume loadings to take into account the thermal debinding effect to propose a full debinding sintering simulation, demonstrated that the FE simulation results are in agreement with the experimental ones.     

Integrated filling,packing and cooling CAE analysis of powder injection moulding parts

Abstract

A coupled numerical analysis of the filling, packing and cooling stages of powder injection moulding (PIM) has been implemented. Finite element method/finite difference method methodologies were used in the filling and packing stages while Boundary Element Method (BEM) was used for the cooling stage. Using these methodologies, a numerical simulation program for the injection moulding process of PIM parts, PIMSolver was developed by taking into account the peculiar rheological behaviour of powder–binder mixtures. Specifically, the apparent slip phenomena at the mould wall and the yield stress were incorporated into the above analysis. The coupled analysis among the filling, packing and cooling stages was performed because the viscosity and slip phenomena of powder–binder mixture highly depend on temperature. In order to evaluate the significance of the coupled analysis and slip phenomena, several PIM experiments were performed using 316L stainless steel powders dispersed in a paraffin wax–polypropylene binder system. Using the examples of a U-shaped test specimen and an electronic package part, the importance of coupled numerical analysis for PIM parts and the significance of slip dependency of temperature during the coupled analysis were demonstrated.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

气雾化压力对3D打印用316L不锈钢粉末性能的影响

采用真空熔炼气雾化工艺制备3D打印用316L不锈钢粉末,通过调整雾化参数,研究了不同雾化压力对粉末化学成分、粒度分布、球形度、表面形貌、流动性及松装密度等特性的影响。结果表明:在保温温度(1560±20)℃、保温时间20 min、漏包温度(1050±30)℃、高纯氮气雾化及雾化压力3.0 MPa工艺参数下,制备得到的粉末性能可达到氧含量(质量分数)0.08%、中位径31.39μm、球形度0.75、流动性21.56 g/(50 s)及松装密度3.88 g/cm~3,基本满足不同金属3D打印技术对粉末材料性能的要求。     

Interface formation and diffusion of alloying elements during cosintering of MIM 316L/17-4PH stainless steel parts: experiments and simulation

Abstract

Interface investigations of cosintered parts produced via two component metal injection moulding (2CMIM) require a high demand of experiments. To reduce the experimental time and effort, the commercial simulation software DICTRA–Thermo-Calc was used to predict the interface formation. In this work, the material combination 316L/17-4PH is examined at different sintering temperatures: 1270 and 1360°C, and three holding times: 1, 2 and 4 h at 1360°C. Micrographs, energy dispersive X-ray/scanning electron microscopy linescans and simulations showed the evolution of the interfaces for different conditions. The broadest diffusion layer was obtained at 1360°C for 4 h. At an elevated temperature, the interdiffusion provoked the formation of a rich ferrite region close to the interface in the 17-4PH, which was increased for longer holding times. The simulation was able to predict the same tendencies obtained by experimental results.     

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Abstract

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

Effect of heating mode and electrochemical response on austenitic and ferritic stainless steels

Abstract

Powder metallurgical (P/M) processing has the main advantage of making near net shape products. Nowadays, in automobile industries, stainless steels have become the most promising material owing to their good corrosion resistance. In the current study, 316L and 434L stainless steel powders were sintered using microwave and conventional methods through powder metallurgy route. The effects of sintering modes on the microstructure, mechanical properties and corrosion responses of 316L and 434L stainless steel composites are investigated in detail. The results showed that the sample prepared through microwave sintering route exhibited significantly superior densification, higher hardness and better corrosion resistance as compared to the conventionally processed counterpart. On the whole, 316L composites showed better corrosion resistance than 434L stainless steels.     

Improvements in microstructure homogenization and mechanical properties of diffusion-alloyed steel compact by the addition of Cr-containing powders

The use of diffusion-alloyed powders for fabricating powder metal parts, despite alleviating the segregation problem of the alloying elements while retaining good compressibility, still cannot attain homogeneous microstructure in as-sintered products. The presence of soft Ni-rich areas and pores causes poor mechanical properties compared to those of wrought steel counterparts. This study investigated the effects of adding 0.5 wt pct Cr, which was introduced in the 316L stainless steel powder form, on the microstructure and mechanical properties of diffusion-alloyed Fe-4Ni-1.5Cu-0.5Mo-0.5C (Metal Powder Industries Federation (MPIF) FD-0405) steels. The results show that weak Ni-rich areas were present in the Cr-free specimen when sintered at 1120 °C and 1250 °C. These areas were lean in carbon because of the strong repelling effect between Ni and C. With the addition of 316L powders, the Cr was uniformly distributed and helped eliminate the soft Ni-rich areas, particularly in specimens sintered at 1250 °C. The distribution of carbon also improved. With a more uniform distribution of Ni and C, and more homogeneous microstructure, which consisted mainly of bainite and martensite, the mechanical properties of the Fe-4Ni-1.5Cu-0.5Mo-0.5C diffusion alloy steels were improved significantly.     

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.     

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.