Properties of Anisotropic Nd(Fe,Co)B Type Sintered Magnets Produced by Powder Injection Moulding

Abstract

Anisotropic Nd(Fe,Co)B type sintered permanent magnets were fabricated by powder injection moulding (PIM) using paraffin wax as a binder. A conventional process was also used in order to compare the resultant properties with those of sintered magnets produced by PIM. Magnetic properties, microstructure, and constituents were investigated by dc fluxmeter, scanning electron microscopy, X-ray diffractometry, wavelength dispersive X-ray analysis, and ir absorption analysis. Effects of particle alignment of the sintered magnets on the magnetic properties and anisometric linear shrinkage ratios were studied. Particle alignment of sintered magnets produced by PIM was 4% lower than that of the conventional process. The PIM sintered magnet exhibited a maximum energy product of 232 kJ m^?3. The residual carbon after debinding affected the magnetic properties and sintering characteristics by contaminating the liquid phase during PIM sintering. PM/0782     

粉末注射成形蜡基粘结剂溶剂脱脂行为研究

以YG8硬质合金为实验对象,研究了蜡基粘结剂注射成形生坯的溶剂脱脂行为,考察了时间、温度、生坯形状、厚度、表面积、粉末装载率和液固比对脱脂速率的影响,分析讨论了各因素影响脱脂速率的原因.结果表明:溶剂脱脂速率随温度升高而升高,随时间的延长而降低;脱脂初期扩散是控制性环节,温度是影响反应速率的主要因素,脱脂后期溶解成为控制性环节,浓度差减为影响反应速率的主要因素;液固比越大,脱脂速率越快,粘结剂最终脱除率越高;生坯粉末装载率越高,脱脂速率越慢,粘结剂最终脱除率越低;生坯形状对脱脂速率影响表现为样品厚度和表面积的影响,回归分析表明,脱脂速率与生坯表面积成正比,与生坯厚度的平方成反比.     

In situ dimensional change,mass loss and mechanisms for solvent debinding of powder injection moulded components

Abstract

Dimensional change during solvent debinding is linked to defects such as cracking and slumping in powder injection moulded components. Owing to the delicate condition of the compact during debinding, considerable difficulty exists in determining the magnitude and cause of swelling or shrinkage. Previous studies of this dimensional change have used measurement techniques involving contact or force on the sample, which may alter the behaviour. This study observes in situ dimensional change during solvent debinding using a non-contact laser dilatometer. Dimensional change was investigated for iron and stainless steel powder injection moulded bars with wax-polymer binders. The effects of solvent temperature, paraffin wax content, paraffin wax density, particle size and solids loading were analysed. Comparisons between the dimensional change and mass loss observations during solvent debinding suggest a relationship between the two phenomena based on soluble polymer swelling.     

YG8硬质合金注射成形脱脂工艺的研究

采用传统蜡基和(油+蜡)基改进型两种粘结剂,对粒度为1.97μm的WC-8Co混合粉末注射成形,研究了两种喂料的热脱脂、溶剂脱脂+热脱脂工艺,确定了热脱脂、溶剂脱脂+热脱脂的优化工艺路线。从脱脂速度和脱脂时间、脱脂缺陷和脱脂工艺控制的难易程度等方面比较了两种粘结剂的热脱脂和溶剂脱脂效果。研究了热脱脂温度、时间、不同脱脂方式以及粘结剂组成对脱脂坯碳含量的影响。结果表明,以氢气为脱脂气氛,在450℃以下进行热脱脂可以将有机聚合物完全脱除而且不产生脱碳,溶剂脱脂后再进行热脱脂更有利于控碳和减少脱脂缺陷,而且大大缩短脱脂时间。(油+蜡)基改进型粘结剂在热脱脂和溶剂脱脂时具有更好的脱脂特性。     

Length change and deformation of powder injection-molded compacts during solvent debinding

Solvent debinding is one of the processes widely adopted by the powder injection molding industry. Despite the inherent advantages of short debinding cycles, the low temperature employed, and the low investment in processing equipment, dimensional control is still a challenge to the further promotion of this technology. The objective of this study was to investigate the causes of the tolerance-control problems by measuring the in-situ dimensional change and deformation behavior of powder injection-molded (PIM) specimens during debinding, using a self-designed laser dilatometer. Swelling and sagging were found when compacts were immersed in the solvent. Three major factors were found to be responsible for the expansion of the specimens: dissolution of soluble binder into the solvent, reaction between the insoluble binder and solvent, and thermal expansion due to the temperature rise from the solvent bath. The amounts of expansion and sagging were related to the thickness of the sample, the amount of the binder, and the temperature employed. These in-situ measurements on the dimensional change help explain how defects such as slumping, cracking, and distortion come about during debinding and provide some guidelines in selecting processing parameters and in designing binder compositions.     

Hot micro-embossing: effect of pressure on 316L metal parts

Abstract

The use of replicative processes has become strategic and critical in industry to produce precise, microscopically detailed metallic parts and devices via low cost manufacturing routes. Metal powder hot embossing is an emerging process that brings some advantages associated with the reduction of production costs relative to powder injection moulding (PIM). The technology involves four distinct steps: preparation of the selected feedstock material (powder and binder); hot embossing; debinding; and sintering. The effect of continuous pressure during the hot embossing step as a means of replicating microdetails in 316L stainless steel parts is examined. Dimensional accuracy, microstructure and mechanical properties of the parts produced were evaluated. For the configuration tested, the most promising results were achieved when processing at 180°C for 30 min at a pressure of 14 MPa.     

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 residual carbon on the sintering process of M2 high speed steel parts obtained by a modified metal injection molding process

In this article, we present the evolution of the microstructure during sintering of M2 high speed steel (HSS) parts obtained by a modified powder injection molding (PIM) process, which uses a new binder system based on a thermosetting resin. The most important characteristics of this process is that molding is carried out at room temperature by pouring the slurry (resin and tool steel previously mixed) directly into the mold. The mold is then heated to the curing temperature of the resin. The best mixture of polymer and steel powders was 60 pct volume of metal powder. The resin was removed by thermal debinding. The sintering process was carried out under vacuum atmosphere. We tested different debinding temperatures in order to retain residual carbon in the samples coming from the thermal degradation of the polymer. The best results were obtained at low debinding temperature (300 °C). In this case, residual carbon had a beneficial effect, extending the sintering temperature range by 100 deg, making it possible to reach very high density at temperatures as low as 1100 °C. The mechanism of this densification seems to be via supersolidus liquid phase (SPLS). The microstructural study of sintered parts revealed a homogeneous distribution of carbides that change their morphology with increasing temperature. Besides spherical M₆C carbides, which appear in all the temperature ranges studied, a new rodlike M₂C carbide appears.     

Optimisation of eco-friendly binary binder system for powder injection moulding

Abstract

In recent years, many efforts have been made to obtain more environmentally acceptable powder injection moulding processes. In this sense, the purpose of this study is to optimise an eco-binder based on polyethylene glycol (PEG) as a water soluble component and cellulose acetate butyrate (CAB) as a natural backbone polymer derived from cellulose for powder injection moulding of zirconium silicate powders until a solvent debinding stage. Four different feedstocks have been investigated. As well as, a volume fraction of PEG and CAB 70/30 (vol.-%) and a solid loading of 57·5 (vol.-%) were maintained, molecular weights of polymers were combined in order to minimize distortion during binder solvent extraction. Water solvent debinding was carried out at three temperatures stepwise during 5 h. As a result, efficient removal of the PEG as well as free defects samples were obtained after solvent debinding for binder systems based on low molecular weight of PEG.     

Comparative study of pore structure evolution during solvent and thermal debinding of powder injection molded parts

The solvent debinding process has been widely accepted in the powder injection molding (PIM) industry due to its short debinding cycle. In the current study, specimens were immersed in a heptane bath for different lengths of time, and the pore structure evolvement in the compact was analyzed. Mercury porosimetry analyses and scanning electron micrographs showed that the binder extraction started from the surface and progressed toward the center of the compacts. As the debinding contin-ued, the pores grew and were widely distributed in size. This pore structure evolvement was different from that of straight thermal debinding in which the pore size distribution was quite narrow and the mean pore diameter shifted toward smaller sizes as debinding time increased. After the soluble binders were extracted, parts were subjected to a subsequent thermal debinding during which these pores served as conduits for decomposed gas to escape. Concurrently, the remaining binder became fluidlike and was redistributed within the compact due to capillarity. This pore structure, as observed from the mercury intrusion curves, showed a sharp increase in the pore volume at the 0.8-μm size, followed by a series of fine pores, which is different from the pore structure of straight thermal debinding. The difference in the pore structure evolvement between solvent and thermal debinding and its effect on the debinding rate are discussed.     

脱脂工艺对注射成形钼合金碳含量的影响

采用粉末注射成形技术制备钼合金坯体,研究溶剂脱脂和热脱脂工艺对注射坯中碳含量的影响.结果表明,溶剂脱脂率随温度升高而增大,随坯体厚度增加而减小;热脱脂坯的碳含量和热脱脂的升温速率、坯体厚度和保温工艺密切相关.随升温速率加快,或坯体尺寸增大,热脱脂后坯体的碳含量增多;在400～550℃保温1h,碳含量急剧降低;采用多步保温能较大程度地降低热脱脂坯的碳含量,最低降至0.059％.脱脂坯中一部分热分解碳以游离态形式存在,另一部分热分解碳和钼颗粒发生反应生成钼的碳化物.     

粉末注射成形钛合金的脱脂和烧结性能

应用粉末注射成形技术制备出高精度、高性能的异形钛合金零器件。通过多粒度粉末搭配,采用聚甲醛为主组元的多组元粘结剂,制备出高装载量催化脱脂型钛合金喂料,再经真空烧结获得制品。研究了催化脱脂工艺的影响因素以及喂料配比对烧结性能的影响。结果表明:当大（D₅₀=25.28 μm）、中（D₅₀=16.75 μm）、小（D₅₀=12.66 μm）颗粒按质量比17:6:2搭配时,钛合金混合粉末相对振实密度较大,为55%。喂料较佳的催化脱脂工艺为:脱脂温度120 ℃,N₂通入速率120 cm3·min?1,HNO₃气体通入速率1.5 cm3·min?1,脱脂时间6 h,粘结剂脱除率85%。采用全流程控制杂质含量技术,粉末注射成形钛合金制品的烧结性能可以达到相对密度95.9%,拉伸强度933 MPa,抗弯强度1282 MPa,延伸率7.5%,其中C质量分数为0.10%,O质量分数为0.21%。     

Solvent debinding behavior of powder injection molded components prepared from powders with different particle sizes

It is generally accepted that the solvent debinding rate of powder injection molded (PIM) parts can be improved when coarse powder is used due to the larger pore size present in the compact. However, little hard experimental evidence on this has been reported. In this study, the as-received gas-atomized stainless steel powder was classified into four different particle sizes. Little difference in the debinding rate was found among these four groups. Similar results were also obtained using classified fine carbonyl iron powder and coarse water atomized iron powders. The diameter of the pore channel that was developed in the compact, while increasing as the particle size increased for both iron and stainless steel powders, did not affect the debinding rate. A comparison between the sizes of the pores and diffusing molecules suggests that the pores are significantly large for the small diffusing molecules. The calculation of the diffusion path or torturosity also indicates that the particle size does not affect the diffusion length and thus not the debinding rate.     

Dilatometric analysis of thermal debinding of injection moulded iron compacts

Abstract

The less than desired tolerance control of powder injection moulded compacts is a result of inconsistent dimensional changes in the compacts accumulated during moulding, debinding, and sintering. This study investigated the in situ length changes and their causes during thermal debinding on compacts which have been solvent debound. The dilatometric analysis showed that the specimen shrank in the early stage between 250 and 370°C, not because of sintering, but through the loss of N, C, and O in the carbonyl iron powder. At temperatures between 370 and 450°C, the specimen expanded owing to the carburisation of the iron powder. The length change was also influenced by the heating rate, debinding atmosphere, and the amount of the backbone binder. These dilatometric results are helpful in establishing the guidelines in designing binder compositions and debinding schedules.     

Water debinding behaviour of water soluble Ti-MIM feedstock

Abstract

A water soluble polyethylene glycol (PEG) based binder system was used to formulate three feedstocks containing prealloyed Ti–6Al–4V, NiTi and blended elemental Ni/Ti powders respectively. The selection of these metal powders was to investigate the effect of powder characteristics such as particle size and particle shape. Various solid loadings were investigated in these feedstocks. The water debinding behaviours were systematically studied in terms of debinding temperature, time, sample thickness, powder characteristics and solid loading. Corresponding debinding rate was calculated from the measurements using a shrinking core reaction kinetic model.     

粉末注射成形粘结剂及脱脂技术研究进展

综述了粉末注射成形中粘结剂配方及其脱脂技术研究进展,并比较了现行工业上常用的三大脱脂方法,讨论了粉末注射成形脱脂工艺的发展趋势。     

Simulation of polymer removal from a powder injection molding compact by thermal debinding

Powder injection molding (PIM) is an important net-shape manufacturing process. Thermal debinding is a common methodology for the final removal of residual polymer from a PIM compact prior to sintering. This process is an intricate combination of evaporation, liquid and gas migration, pyrolysis of polymer, and heat transfer in porous media. A better understanding of thermal debinding could lead to optimization of the process to prevent the formation of defects. Simulation of the process based on an integrated mathematical model for mass and heat transfer in porous media is proposed. The mechanisms of mass transport, i.e., liquid flow, gas flow, vapor diffusion, and convection, as well as the phase transitions of polymer, and their interactions, are included in the model. The macroscopic partial differential equations are formulated by volume averaging of the microscopic conservation laws. The basic equations consist of mass conservation and energy conservation and are solved numerically. Polymer residue, pressure, and temperature distributions are predicted. The importance of the various mass transfer mechanisms is evaluated. The effects of key mass transfer parameters on thermal debinding are discussed. It is revealed from the results that the assumed binder front, which is supposed to recede into the powder compact as removal progresses, does not exist. The mass flux of polymer liquid is of the same order of the mass flux of polymer vapor in the gas phase, and the polymer vapor diffusion in the liquid phase is negligible.     

Sm(Co,Cu,Fe,Zr)z永磁体注射成形坯溶剂脱脂工艺的研究

本文对Sm(Co0.721Cu0.08Fe0.174Zr0.025)7.5注射成形坯进行了溶剂脱脂实验,通过溶剂、温度、装载量和生坯厚度对脱脂速率的影响,分析了注射坯的溶剂脱脂工艺过程.结果表明在溶剂中添加植物油能有效地抑制脱脂过程中注射坯开裂,并且粘结剂的脱除率可达70%以上;溶剂脱脂的初期主要受扩散反应控制、随着脱脂温度升高和装载量降低,脱脂速率增加,脱脂的后期溶解成为脱脂的主要控制性环节,温度和装载量对脱脂速率的影响与起始阶段相反;注射坯越厚,粘结剂的脱除率越低.