Effect of carnauba wax as a part of feedstock on the mechanical behavior of a part made of 4605 low alloy steel powder using metal injection molding

Metal injection molding is a growing technology for producing complex metallic components. Preparation of feedstock for metal injection molding is a very crucial step during this process. This is because the deficiencies in quality of the feedstock once made, cannot be corrected by subsequent processing steps. One of the challenges in producing the feedstock is its formulation. In this study, the effect of the percentage of carnauba wax along with other binder constituents of paraffin wax, polypropylene and stearic acid is investigated on the density, strength, hardness and rheological behavior of a part, made of 4605 low alloy steel powder using metal injection molding process. For this reason, six binder systems including paraffin wax, polypropylene and stearic acid having different percentages of carnauba wax have been produced. After preparation of the samples, tensile testing, Vickers hardness test, density and rheological behavior of the samples have been measured and compared. The results show that carnauba wax as a part of feedstock, has a positive effect on the strength and density while negative effect on the hardness of the final sintered part. Also with the increase in the percentage of the carnauba wax inside the polymer binder, the viscosity of the feedstock was reduced significantly.     

新型POM/PE合金喂料的流变特性

本文以聚甲醛(POM)、低密度聚乙烯(LDPE)、乙烯-醋酸乙烯共聚物(EVA)、硬脂酸(SA)和石蜡(PW)为粘结剂体系,与17-4PH金属粉末制得新型POM/PE合金注射成型喂料,并测试其流变性能。喂料的粘度随着剪切速率及温度的变大而减小,喂料呈假塑性流体;非牛顿指数n随着温度的升高而增加。以新型POM/PE合金粘结剂与金属粉末制成的喂料表现出较好的流变性、较小的温度敏感性、较高的产品强度和较高的粉末装载量,适合用于金属粉末的注射成型。     

Metal injection molding of pure molybdenum

In this work the processing steps for producing molybdenum parts by means of metal injection molding technique, including milling, mixing, debinding and sintering, were investigated in detail. Different feedstocks were prepared from received and milled molybdenum powder and a thermoplastic binder based on paraffin wax. The feedstock including 50% by volume of milled molybdenum powder had better rheological property and was injected successfully at low pressure. The molded specimens were debinded by two different processes, direct thermal debinding and solvent–thermal debinding. The content of carbon and oxygen were respectively less than 0.060% and 0.0042% after the parts were sintered under hydrogen atmosphere. The parts sintered at 1850 °C for 120 min exhibited a highest density of 9.70 g/cm³,meanwhile the tensile strength and hardness was 229 MPa and 193 HV10, respectively.     

Experimental Study and Neural Network Modeling of the Stability of Powder Injection Molding Feedstocks

Due to their complex rheological behavior, feedstocks for powder injection molding (PIM) may exhibit non-homogeneous flow and separation. This can produce defects of green parts during mold filling, resulting in their cracking and warpage during debinding and sintering, and ultimately in poor physical and mechanical properties of the final part. An experimental rheological study has been performed to evaluate the influence of solids loading, shear rate, and powder particle size on feedstock stability. A micro-rheological explanation is given for the macroscopic effect of separation, and an instability index has been defined to describe quantitatively the threshold beyond which the variation of viscosity becomes unacceptable for PIM purposes. A neural network model has been developed for predicting the viscosity of feedstocks made from binary blends, when the powder characteristics, blend composition, and shear rate are known. The system enables determination of the process parameters for which powder-binder separation occurs in a given feedstock.     

金属注射成形蜡基粘结剂的研究

本文研究了综合蜡基和油基粘结剂优点的改进型蜡基粘结剂体系 ,其组成为PP PW 植物油 SA 改性剂。改进型蜡基粘结剂分阶段热解 ,各组元间有较强的相互作用 ,相容性较好。该粘结剂和金属粉末具有较好的混合均匀性 ,改性剂的加入提高了粉末装载量。喂料是一种假塑性流体 ,非牛顿指数n较大 ,粘流活化能ΔEη 较低 ,具有良好的注射填充性。改进型蜡基粘结剂既可热脱脂 ,又可溶剂脱脂 ,溶剂脱脂速度快、维形能力好。     

Experimental and Theoretical Investigation of Powder–Binder Mixing Mechanism for Metal Injection Molding

Metallic powder and binder mixing mechanism plays a vital role in the quality of molded parts in metal injection molding. The present study is intended for experimental and theoretical investigation of powder–binder mixing mechanism to investigate the functional correlation among mixing parameters and performance characteristics for different composition of feedstocks. Powder loading and shear rate are considered as input parameter. Fuzzy expert system is adopted to test the validity of the experimental results by analyzing different numerical error criteria using the viscosity as output with respect to input parameters. The mean relative error and correlation coefficient for type A and type B were found to be 6.09% and 8.51% (<10%) and 0.990 and 0.998, respectively. Hence the result indicates a reliable acceptability of the proposed amount of powder loading for feedstock preparation.     

Partially Water-Soluble Binder Formulation for Injection Molding Submicrometer Zirconia

A binder system containing 60% by volume of a water-soluble major constituent was formulated and tested for powder injection molding ceramic materials. The main components of the binder phase were a low molecular weight polyethylene glycol, an oxidized high density polyethylene, polyvinyl butyral and stearic acid. Ceramic powder-binder mixes containing these binder components and up to 45% by volume of a submicrometer stabilized zirconia powder were prepared and characterized. The apparent viscosity of these mixes at a temperature of 190°C and a shear rate of 100 s^-1 was determined to be 700–800 Pa·s. Binder removal from parts produced by injection molding these feedstocks was accomplished using a two-stage process. The water-soluble constituent, polyethylene glycol, was removed by dissolution in water at 50°C. Tests showed that approximately 90% of the polyethylene glycol could be removed from a 2-mm thick part during a 2 h immersion in water. The remaining binder constituents were removed using a thermal treatment to 500°C at a heating rate of 100°C/h. Thermogravimetric analysis revealed a good separation of the decomposition regions for the various components when heated in an argon atmosphere. A final sintering step in air at 1500°C produced parts having a density above 99%.     

Effect of solvent debinding variables on the shape maintenance of green molded bodies

In order to study the effect of solvent extraction debinding variables on the shape maintenance of green molded bodies, powder compression molded bodies (the feed) were debound in various organic solvents such as n-hexane, n-heptane and isooctane at various extraction temperatures ranging from 30 to 60°C. The solvent to feed ratio of dilution was studied in the range of 7:1 to 15:1 by weight. Finally the effect of extraction time on the amount of binder extracted and diffusion coefficient of paraffin wax was studied in the range of 1 to 5 h. Thermo gravimetric analysis (TGA) was used to confirm the extraction of the major binder and stearic acid constituents completely from the binder mixture. Scanning electron microscopy (SEM) was used to see the pores in the green bodies created after solvent extraction process which allowing the degraded products (minor binder) to diffuse to the surface easily.     

高氮奥氏体不锈钢金属注射成形喂料的流变行为研究

采用金属注射成形制备高氮奥氏体不锈钢,研究了石蜡(PW)、高密度聚乙烯(HDPE)、聚丙烯(PP)以及硬脂酸等几种聚合物组成的注射成形喂料的黏度、剪切速率以及温度对黏度的影响,比较了聚合物组元对喂料流变行为的影响。研究结果表明,黏结剂配方为65%PW/30%HDPE/5%SA(质量分数,下同)所组成喂料具有较好的综合流变性能,更适合作为高氮奥氏体不锈钢粉末注射成形的有机载体。     

Powder injection moulding of premixed ferritic and austenitic stainless steel powders

In this experimental work, powder injection moulding (PIM) of premixed 316L and 430L gas-atomized powders was developed to obtain duplex stainless steels. A multicomponent binder constituted of high density polyethylene (HDPE) and paraffin wax (PW) in a volume ratio 50/50 was selected for the process. Feedstocks with powder loadings of 50, 65, 68 and 70 vol.% were prepared. Mixing experiments were carried out at 170 °C according to differential scanning calorimetry (DSC) results. The rheological characterization of feedstocks allowed establishing different rheological parameters as power flow index (n) and activation energy (E_a) in order to know their suitability for injection moulding. Critical powder volume concentration (CPVC) was determined by means of oil absorption method and a rheological model. The feedstock was injected at 170 °C and three-point bending and tensile parts were obtained. Thermogravimetrical analysis (TGA) of binder allowed us to design the thermal debinding cycle and sintering was carried out in low vacuum at different temperatures. Finally, mechanical properties such as hardness and tensile strength were evaluated.     

Comparative study of nanoparticle effects on feedstock behavior for injection molding

Rheological properties of bimodal feedstocks containing nano- and micro-particles were studied. To examine nanoparticle effects, five feedstocks were prepared with the nanoparticle contents from 0 to 75 vol. %. Each feedstock was formulated with the optimal amounts of the powders, determined from the critical solids loadings of the powders. Viscosity measurement of the feedstocks was performed using a capillary rheometer. From the result, the flow behavior index and flow activation energy were obtained as representative rheological parameters. The moldability index for each feedstock was also calculated and compared. Some bimodal feedstocks showed lower viscosity than microfeedstock due to the roller bearing effect of the nanoparticles. The rheological parameters indicated the bimodal feedstocks were more desirable for mold filling except for 75% nano-bimodal feedstock. The results revealed 25% nano-bimodal feedstock showed the highest moldability index and was the most desirable feedstock to prepare a green body via nano/micro-bimodal powder injection molding.     

Water-soluble binder with high flexural modulus for powder injection molding

A new binder system, containing ternary polymer blends of poly(methyl methacrylate) (PMMA), cellulose acetate butyrate (CAB) and polyethylene glycol (PEG), was introduced for the powder injection molding of stainless steel powders. The weight fraction of PEG in the binder was maintained to be 0.65. When the weight fraction of PMMA in the binder was 0.1–0.25 (thus the weight fraction of CAB was 0.1–0.25), the feedstocks with 56 vol% of steel powders were successfully injection molded without showing any jetting. Furthermore, the feedstocks have much higher flexural modulus compared with those prepared by a binder consisting only 35/65 (wt/wt) CAB/PEG blend. Three binary blends, PMMA/CAB, PMMA/PEG, and CAB/PEG, were completely miscible at an injection molding temperature of 130^°C. The extraction process employed in this study becomes environmentally favorable due to the major component (PEG) of the binder which is easily extracted by a solvent of water or ethanol. Also, the shape maintenance during the solvent extraction was excellent, and final sintered parts had excellent dimensional stability. Due to high flexural modulus of feedstocks, this binder system can be employed for preparing large sized injection-mold articles.     

Revealing the formation mechanism of granules by drying simulation of slurry droplet

The formation mechanism of granules in spray drying process was investigated by DEM-CIP method simulation coupled with a new binder segregation model. To confirm the validity of proposed binder segregation model, experiment on drying of aqueous slurry containing fine particles and binder was performed. The experimental concentration distribution of binder in a dried powder bed agreed well with the simulated one and this result shows that the proposed segregation model is valid to represent segregation phenomenon in the dried granule. Spherical hollow granules were formed with increasing of binder concentration, and the granules were depressed in higher concentration of binder. When the binder concentration in the surface region of a slurry droplet increased during drying, a crust of granule was formed because particle migration was hindered by high fluid viscosity. For the rigid crust layer, granules were hollow. When the crust layer was formed early in drying and its thickness was thin, granule surface partially collapsed inward, whereas the granule resulted in depression granules.     

Thermal, Solvent, and Vacuum Debinding Mechanisms of PIM Compacts

The mechanisms of thermal, solvent, and vacuum debinding processes for powder injection molded (PIM) compacts were investigated. Mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) observations on PIM specimens showed that fine interconnected pores were developed in the early stages of debinding in all three processes. These pores were formed as a result of the decomposition of low temperature binders such as paraffin wax during thermal debinding or arise from the extraction of soluble binder components in solvent debinding. In the later stages of thermal and vacuum debinding, decomposed gases could be trapped in the center region and build up a pressure causing cracking or blistering defects. The solvent debinding process could alleviate these problems, but the penetration of solvent into the binder could still cause cracking or distortion due to swelling of the binder.

It was found that the pore structure evolution was influenced by the heating rate, temperature, pore size, and the amount of existing porosity. From the observed microstructure and mercury porosimetry data, debinding mechanisms were derived and the defects which were frequently seen during debinding were explained with these mechanisms.     

Structure and thermal conductivity of powder injection molded AlN ceramic

AlN powders doped with Y₂O₃ (5 wt.%) were compacted by employing powder injection molding (PIM) technique. The binder consisted of paraffin wax (PW, 60 wt.%), polypropylene (PP, 35 wt.%) and stearic acid (SA, 5 wt.%). The feedstock was prepared with a solid loading of 62 vol.%. The binder was removed through debinding process in two steps, solvent debinding followed by thermal debinding. At last, the debound samples were sintered in flowing nitrogen gas at atmospheric pressure. The result reveals that thermal debinding atmosphere has significant effect on the thermal conductivity and structure of AlN ceramics. The thermal conductivity of injection molded AlN ceramics thermal debound in flowing nitrogen gas is 231 W m^?1 K^?1.     

Shear creep response of an airport asphalt mastic

Two runways were resurfaced with 50–60 mm of typical airport asphalt at the same airport. One runway surface performed well while the other exhibited a lack of resistance to cyclic shear stress under heavy aircraft braking. Both runways had the same hydrated lime filler and coarse aggregate source. The fine aggregate (dust) used to manufacture the two runway surfaces was obtained from two different basalt quarries. The dust associated with the poorly performing asphalt contained a potentially detrimental clay mineral (Hisingerite). It was subsequently determined that the crude oil used to manufacture the feedstock for the acid-modified binder also changed at the transition from one runway to the other. The changes in dust and binder were confounded. A combination of viscosity testing and performance-based multiple stress creep recovery (MSCR) testing determined that the two runway binders responded significantly differently to shear stress and aged differently with sample storage time. The differences were magnified at higher temperatures. Further, mastic samples were manufactured from binder associated with both feedstocks, in combination with dust from both quarries. MSCR testing of mastic indicated that the dust containing significant Hisingerite had no adverse impact on the mastic response to shear stress. The change in binder feedstock was concluded to be the root cause of the lack of resistance to cyclic shear stress observed in one runway surface. This occurred despite all batches of binder meeting the viscosity-based Australian specification for paving grade bitumen. The specification has no mechanism to prevent similar changes in bitumen feedstock affecting airport bitumen performance in the future. Incorporating performance-based testing in the Australian bitumen specification is recommended.     

The influence of powder loading and binder additive on the properties of alumina injection-moulding blends

The effects of powder loading and binder additive on the rheology, moulding, and sintered properties of alumina injection-moulding blends were studied. The melt viscosity increased with the powder loading, which enhanced the defect concentration of the moulded parts when the powder loading was higher than 0.54 volume fraction. This transitional powder loading was much smaller than the critical powder loading of 0.65 derived by fitting the rheological data with a mathematical model. The low molecular weight acidic binder additives used in this study effectively reduced the melt viscosity. However, additives that yielded a large quantity of burn-out residue deteriorated the sintered properties. A high powder-loading mixture having a low melt viscosity and good powder dispersion was achieved by eliminating powder agglomerates before mixing, and by using an appropriate binder additive.     

Effects of debinding parameters on powder injection molded Ti-6Al-4V/HA composite parts

Titanium alloy-hydroxyapatite(Ti-6Al-4V/HA) composite powders produced by a ceramic slurry were mixed with a multi-component binder system in the powder injection molding process. The binder system comprises mainly of natural wax, fatty acid wax, stearic acid, poly-oxialkyen-etherand olefln-hydrocarbons. After molding, the binder system was removed by conventional thermal debinding. The effects of heating rate and gas flow rate on the quality of the debound part in terms of defects, mass of binder removed and residual carbon level were discussed. A slow heating rate (20° C/h) at the beginning stage of the debinding step and a higher gas flow rate (250 cm³ /min) were required for the production of defect-freeparts with minimal carbon level.     

金属注射成形喂料的流变学性能评价

从理论上对金属注射成形喂料的流变学性能进行分析发现,评价ＭＩＭ喂流变不遥主要指标是喂料的粘度及粘度对应变和温度的敏感性,这些指标可统一用ＭＭ喂料综合流学因子进行评价,运用该评价因子研究几种不同粘结剂组成的Ｆｅ－Ｎｉ及Ｗ－Ｎｉ－Ｆｅ合金喂料的流变学性能。     

硬脂酸表面改性对PLZT压电陶瓷粉末注射成型性能的影响

以硬脂酸为粉末改性剂, 聚乙二醇/聚乙烯醇缩丁醛/聚甲醛(PEG/PVB/POM)为粘结剂体系制备锆钛酸铅镧(PLZT)粉末注射成形喂料, 并通过先水脱脂后烧结的工艺制备了PLZT压电陶瓷。研究了硬脂酸用量对粉末特性、喂料黏度、水脱脂率以及坯体强度的影响, 并对烧结后陶瓷的微观形貌与电性能进行对比与分析。结果表明：硬脂酸通过湿法改性成功包覆于PLZT粉体表面, 硬脂酸改性打破了粉末间的团聚, 且当硬脂酸包覆量为2wt%时, 喂料具有较低的剪切黏度及较高的坯体弯曲强度。但过量改性反而使得喂料黏度上升, 坯体弯曲强度下降。改性后的粉体在坯体内分散均匀, 烧结后的陶瓷晶粒生长完善, 具有更大的烧结致密度。与未改性的PLZT陶瓷相比, 在2 kV/mm极化电压下, 2wt%硬脂酸改性的PLZT的压电常数d₃₃由638 pC/N提高为682 pC/N。