Solvent De-Binding of Bi-Material Green Component of Two-Component Powder Injection Moulded Stainless Steel and Zirconia

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 96 Solvent De-Binding of Bi-Material Green Component of Two-Component Powder Injection Moulded Stainless Steel and Zirconia Ukwueze, Bonaventure Emeka a, b, *, Abu Bakar Sulong a, *, Norhamidi Muhamad a, Zainuddin Sajuri a a) Department of Mechanical and Material Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 436 Bangi, Selangor, Malaysia. b) Department of Mechanical and Production Engineering, Institute of Management and Technology, P.M.B.179, Enugu, Nigeria. * Corresponding ukwbonaem@gmail.com Abstract-- Solvent de-binding is a critical step in implementation of a powder injection moulding process which may lead to defect formation, if neglected and cannot be reversed in subsequent stages. In this study, stainless steel (17-4PH) and zirconia (3YSZ) feedstocks were prepared based on two-binder system consisting of palm stearin (PS) and low-density polyethylene (LDPE). The rheological investigation of the feedstocks revealed that both of them possess characteristics suitable for injection moulding. The feedstocks were subsequently injected to form a bi-material component via two-component injection moulding process (2C- PIM). To predict the solvent optimum de-binding time, single and bi-material green components of SS17-4PH and 3YSZ were extracted in acetone. Next, the bi-materials were further pyrolyzed in argon environment and isothermally sintered for 2h at 13 o C in a vacuum atmosphere. SEM observations indicated no defects on the bi-materials. Index Term-- 2C-PIM: rheology: solvent de-binding: thermal de-binding: sintering 1. INTRODUCTION The need for a combination of dissimilar materials, such as bi-material of stainless steel/zirconia continues to rise due to the need for specific functions in advanced material structures and high-performance devices. Achieving a solid material joint through a shape forming process is particularly attractive since complex packaging and assembly operations can be reduced [1, 2]. These functions usually cannot be obtained using conventional materials or with other available metal casting processes. Thus, bi-material used in this study was fabricated by using a two-component powder injection moulding process (2C-PIM). This is quite a new manufacturing technique employed to join two dissimilar materials thereby producing a smooth gradient interface using the same injection moulding machine. Therefore, this technique offers an economical means of integrating dissimilar functions or properties in a component. The implementation commences with kneading of the metal and ceramic powders separately with multicomponent binders to form a homogeneous feedstock. The green bi-material part is produced in the mould by feeding each class of the granulated feedstock either concurrently or sequentially into the same moulding machine. The green interlocked part formed (bi-material) is ejected after cooling, and the de-binding process removes the polymers. The de-bound porous part is subsequently sintered to near theoretical density. The successful de-binding of a bi-material is often challenging. The processing variables such as heating and cooling due to the thermal mismatch of dissimilar materials can lead to localisation of the heat source. Consequently, this can increase the residual stresses due to the presence of a thermal gradient. Moreover, the powder characteristics differ such as the particle size, shape, surface area and porosity which can widen the difference in the de-binding rate of each part of the bi-material. Successful processing of bi-materials will require careful selection of parameters and controlled heating or cooling. Binder removal is known to be a delicate and critical stage in the powder injection moulding process, and often consuming much time. In the de-binding process, the ultimate goal is usually to remove binder materials in the shortest possible time with minimum impact on the compact [3]. It is therefore expedient that most of the binder is removed before and during sintering to avoid any defects such as cracking or distortion. Therefore, the selection of a suitable binder is a necessary prerequisite for the successful fabrication of complete PIM parts [4]. Usually, binder content comprises a surfactant or dispersant, flow modifier, major binder and a higher molecular polymer which provides sufficient green strength [5]. In this study, a two-component binder system comprising of palm stearin (PS) as a major binder, and lowdensity polyethene (LDPE) as minor or backbone was employed. The fatty acid in the palm stearin plays a significant role as a surface active agent [6]. Thus, palm stearin has a combined role of both surfactant and lubricant [7], and therefore, has been selected for this study. According to German and Bose [3], de-binding techniques can be classed as solvent de-binding (immersion extraction, supercritical, condensation and catalytic) and thermal de-binding (diffusion, permeation, wicking and oxidation). Furthermore, it has been demonstrated that considerable time can be saved by the combination of a solvent and thermal de-binding. Accordingly, the extraction of one component of the binder after solvent de-binding reveals a

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 97 significant amount of porosity which facilitates the diffusion of degraded soluble binder to the surface. Moreover, the residual stresses on the specimen are equally relieved on account of the removal of the soluble binder [8]. Therefore, the time required for thermal de-binding of the insoluble binder component can be significantly reduced without affecting the integrity of the compact [9]. The solvent de-binding process results from the dissolution of binder in the solvent and the inter-diffusion between the soluble binder and the solvent. The solvent infiltrates the pores which are formed by the dissolved soluble binder through capillary action. This is then followed by diffusion of the soluble binder through the porous body to the surface. Several studies have proposed analytical models and provided theoretical frameworks for predicting solutions in solvent de-binding mechanisms. Lin and German [5] developed a process model for predicting molecule diffusion in extraction de-binding of injection moulded parts by the condensed solvent. In a separate study, a diffusion model was employed by Tsai and Chen [9] to describe the binder distributions in the green compact using effective diffusivity as a parameter in solvent de-binding of alumina green bodies. Moreover, (Nanjo et al. 23 cited in Zaky et al. 29) developed an equation for determining the diffusion coefficient of paraffin wax based binders [1]. These models are developed from Fick s law which governs the rate of diffusion of a solute in a solvent and relates the mass transfer rate as a function of the molar concentration gradient. Accordingly, these models describe the solution of one-dimensional mass flow by diffusion between a solvent and compact. The objective of this study, therefore, is to investigate the optimal solvent de-binding time for a green bi-material of SS and 3YSZ materials. The rheological studies was first performed to determine the range of injectability index of the feedstocks. The influence of other solvent de-binding parameters on weight loss of the bi-material was also investigated. The bi-material was subsequently thermally debound and sintered to near theoretical density. 2. EXPERIMENTAL PROCEDURE 2.1 Materials Sandvik Materials Technologies Ltd supplied the stainless steel (SS17-4PH) powder consisting of spherical particles of mean size 22 μm according to the manufacturer's specification. The yttria stabilised zirconia (3 mol% YSZ) powder with near-spherical shape, with an average size of <5 μm as reported by the manufacturer, was purchased from Nabond Technologies Co Ltd. The pycnometer densities of SS17-4PH and 3YSZ were determined using a helium gas pycnometer, measuring 7.78 g/cm 3 and 5.96 g/cm 3 respectively. The differential scanning calorimetry (DSC) was used to determine the melting point of the pure binders [11]. Table 1, lists the characteristics of the binder system employed in this study. Acetone (CH 3) 2O was used for solvent extraction of which, the boiling point of acetone is 56.2 o C Table I Characteristics of the binder system Binders Palm stearin Low-density polyethylene (LDPE) Composition (wt%) 6 4 Melting temp. ( o C) Density (g/cm 3 )

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No: Feedstock preparation The SS-17PH feedstock was formulated with a solid loading of 68 vol% and the two-component binder system of 32 vol%, while the 3YSZ feedstock was based on a solid loading of 5 vol% with the binder system of 5 vol%. To ensure that the same de-binding procedure applies to the bi-material, a common binder was used for both materials. The powders were each mixed with the two-component binders in the same ratio of 6 vol% PS to 4 vol% LDPE. The PS has lower thermal stability than the LDPE. Thus, mixing was conducted at an elevated temperature, higher than the melting point of the higher molecular binder (LDPE), but lower than the decomposition temperature of PS. The binder components were initially fed into a twin blade mixer at a temperature of 15 C with a speed of 3 rpm, with small quantities of the powder successively added. The time-dependent torque increased with filling as the mixing of feedstock progressed until a maximum value was reached. Homogeneity of the mixture was attained when the torque was observed to be constant for 45 minutes. The dough upon cooling was then extracted from the mixer and crushed into granules. The strain rate of the feedstocks in response to the viscosity-profiles under different temperatures of 13 C, 15 o C and 18 C were examined using a capillary rheometer equipped with a die of L/D ratio of Injection moulding of feedstock The SS17-4PH and 3YSZ feedstock materials were simultaneously heated and pressurized and subsequently injected in sequence to form a green bi-material by manipulating a screw type injection moulding machine [12]. The interlocked assembly (bi-material) of the tensile test specimen was ejected from the mould upon cooling. The important moulding parameters that were used included: Injection pressure = 1 bar, Mould temperature (T6) = 3 o C, and Nozzle temperature (5) = 18 o C Solvent de-binding In order to predict the optimum de-binding time for the bi-material, individual parts were prepared and extracted in acetone for 1 h to study the de-binding effect on each constituent part of the bi-material. The bi-material was afterwards immersed in acetone for 8h. The initial weight and density of the green compacts were then measured. A 25-ml volume of the solvent was then added to completely cover the samples while placed on a porous substrate in a graduated glass container and immersed in a water bath. The graduated container was adequately covered at the top of the container to prevent any of the solvent escaping due to evaporation. The samples were then removed and dried every hour, for 1 h in an oven cabinet at 4 o C. The weight losses were determined at each stage of the process and the binder distribution and development of pores observed using SEM. The bi-material green parts were subsequently immersed in acetone at different extraction temperatures of 5 o C, 4 o C and 3 o C, respectively for 6 h to determine the influence of the de-binding parameters. The procedure above was replicated Thermal de-binding and Sintering The bi-materials were further subjected to thermal debinding under argon atmosphere in two stages to burn out the higher molecular binder (LDPE) leaving out a small quantity of the binder to preserve the geometry until sintering. Subsequently, the bi-materials were isothermally sintered for 2h at 13 o C under a vacuum environment. 3. RESULTS AND DISCUSSION 3.1 Rheology The rheological investigation of the feedstocks was undertaken to evaluate the stability of the feedstocks during the injection moulding process. The shear rate articulates the flow behaviour of the feedstock as a function of the viscosity given by Equation (1), [13]: K y n 1 (1) Where, is the viscosity related to the shear rate constant and n, is the flow behaviour index. y, K is a The flow behaviour index n is the material parameter which indicates how sensitive the feedstock is to the shear rate [14]. PIM feedstocks exhibit a non-newtonian shear thinning viscous [15] or pseudoplastic behaviour which means n < 1. However, when n > 1, the flow is dilatant, where powder-binder separation occurs at a high shear rate [16]. Therefore, it is recommended that the PIM values acceptable for the PIM process must not exceed 1 Pa.s within the shear range of 1 to 1, s -1. Figures 1(a) and (b) illustrate the plots of viscosity profiles against the shear rates within the temperature range of 13 o C to 18 o C for both feedstocks. The results indicate that viscosities of the flow will decrease with an increase in the shear rate. This hypothetically may be attributed to powder particle ordering and binder molecule orientation [17]. The flow behaviour index (n) is less than 1, ranging from.42 to.64 and.31 to.68 for 3YSZ and SS-4PH feedstock, respectively. Therefore, these feedstocks indicate pseudoplastic behaviour within an acceptable range. The smaller the value of n, the higher the response of viscosity to the shear rate [18]. The influence of temperature on feedstock viscosity was examined and is described, as given by Arrhenius Equation (2) thus: exp E RT (2) (2) Where, is the viscosity at the reference temperature, T is the temperature, R is the gas constant, and E is the activation energy. By plotting In(ƞ) against 1/T, E is determined from the slope of the linear fits at a shear rate of 1 s -1. Therefore, the activation energy for the feedstock of

4 Viscosity (Pa.s) Viscosity (Pa.s) International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 99 SS-4PH and 3YSZ are obtained as 15.6 KJ/mol and KJ/mol, respectively. The value of the activation energy indicates the temperature-sensitivity on feedstock viscosity. Moreover, the higher the value of E, the higher the feedstockdependence on temperature. Thus, both feedstocks show good rheological characteristics for powder injection moulding. (a) y = x-.578 y = x y = x deg.c 15 deg.c 18 deg.c 2, 4, 6, Shear rate (S -1 ) (b) y = 1165x y = 1465x y = x deg. C 15 deg.c 18 deg. C 5, 1, Shear rate (s -1 ) Fig. 1. Viscosity profiles versus shear rates for (a) 3YSZ and (b) SSI7-4PH feedstock 3.2. Determination of optimum de-binding time for the green bi-material. Figure 2(a) illustrates the effect of the de-binding time on weight loss for each green part of the bi-material of SS17-4PH and 3YSZ at a temperature of 5 o C. Thus, it can also be observed that the two green parts exhibit different de-binding rates. Although the SS-4PH part has a higher solid loading, the de-binding rate is faster than that of the 3YSZ part and may be associated with unique characteristics of fine powders. Also, it is understood that particle size varies inversely with the surface to volume ratio and therefore, fine powders present large surface areas which increase the interparticle friction resulting between the molecules of the compact. Accordingly, this may account for flow difficulties and the consequent slower debinding rate of the fine 3YSZ powder compared to the coarse SS-4PH. Notably, this affords credence to a study by Westcot

5 Weight loss % Weight loss International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 1 et al., on solvent de-binding of the average sizes of iron powder of 4 µ and 316 L of 12 µ. They suggested that the compact with the smaller particle size had more interparticle contact and smaller pores which resulted in lower permeability [19]. Thus, this condition has the potential to induce a thermal gradient at the bonding area. Therefore, too higher temperatures and extended de-binding times are not advisable, given that these could activate undesirable defects at the bonding zone. In order to determine the optimum de-binding time for the bi-material, the susceptibility to shape distortion or other defects must be considered. Figure 2(b) displays the de-binding curve for the bi-material (SS17-4PH/3YSZ) at an immersion temperature of 5 o C. The SEM image of the solvent debound bi-material indicates that most of the extractable portion of the binder was removed after 6 h to 8 h. This corresponds to a weight loss of 88 % to 92 % of soluble binder for the bi-material (3YSZ/SS-4PH). These values are slightly higher than 87.5 % % for the individual part of 3YSZ and is lower than 92 % - 95 % for the SS-4PH part. Consequently, no defect was observed on bi-material after 8 h. The SEM image after 6 h indicates sufficient pore structure which is adequate for subsequent thermal de-binding. Even though there were no visible defects after 8 h; less processing time of 6 h was considered optimum for better bond strength and economy of production. Figure 3(a - f) illustrates the SEM images of the green and debound individual and bi-material of SS-4PH and the 3YSZ parts after 6 h. 1 8 (a) 1 8 Optimum Time (b) 6 4 SS Zr 6 4 SS/Zr Time (hrs) Time (hrs) Fig. 2. Weight loss % of (a) the individual part, and (b) the bi-material part of SS17-4PH and 3YSZ However, from green state to 5h after solvent de-binding, there is varying proportions of loose layers of binders in the moulded compacts observed using SEM. Thus, if the pores created by these extractable phase layers are not sufficiently open, they may lead to defects which may not be apparent but could manifest in subsequent processing stages. Furthermore, it can be seen from Figure 2(b), that weight loss of the binder increases with increasing extraction time. In the beginning, the weight loss of binder is fast which is indicated by the high slope. Also, this can be attributed to deposited binder layers at the surface due to displacement by more densely packed metal and ceramic particles during the injection moulding process (see Figure 3(b, c)). Moreover, the bimaterial parts are in direct contact with the solvent, and consequently, the binders dissolve rapidly. The solvent then diffuses into the bi-material, dissolves and extracts polymer molecules. The reduced slope illustrates slower de-binding rates as time progressed. The dissolved polymer is transported through the tortuous porous structure to the surface, assisted by capillary forces and concentration gradients of the dissolved binder. At the later stages, the de-binding rates become slower, and the weight loss rate is almost constant.

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 11 Fig. 3(a-c). SEM image of green 3YSZ, SS17-4PH and bi-material of 3YSZ/SS-4PH, (d-f) SEM image after 6 h debinding at 5 o C 3.3. The effect of de-binding temperature and time on the weight loss of bi-material. Next, the bi-material component was extracted in acetone at a temperature of 5 o C, 4 o C and 3 o C for 6 h, as shown in Figure 4. The amount of binder extracted increases with increasing de-binding temperature and time. An extraction temperature of 5 o C indicated a high weight loss percentage after 6 h without distortion or other defects occurring on the bimaterial. The de-binding rates at 4 o C and 3 o C were relatively slower and more uniform as indicated by the equal slopes. It was further observed that the extraction temperature at 5 o C provided better shape retention in comparison to 4 o C. At the extraction temperature of 3 o C, swelling was noticed on the 3YSZ part of the bi-material. Thus, swelling occurs when the rate of soluble binder dissolution is restricted by solvent/binder interface and low solvent temperature [19].

7 Weight loss % International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No: deg.c 4 deg.c 3 deg.c Time (mins) Fig. 4. Influence of temperature and time on mass loss of the bi-material 3.4. Thermal de-binding The thermogravimetric analysis (TGA) curves are indicative of the temperature dependence of the binder system. The thermal cycle utilized was in two stages and established based on the thermogravimetric study of the binder system discussed elsewhere (11). The first dwell was at a temperature of 32 o C which corresponds to the degradation of the lower molecular binder (palm stearin). The next dwell was at a temperature of 55 o C to ensure the complete removal of the remaining binder. This temperature represents the degradation temperature of the low-density polyethylene. A low heating rate was employed was employed to minimize the incidences of stress concentration across the interface of the joint Sintering The density of the sintered bi-material obtained was 97% of the theoretical value. To investigate of the integrity, the bimaterial was sectioned, ground, polished and observed for flaws, cracks and other defects using scanning electron microscopy (SEM). The SEM micrograph shows only presence of pores in black contrast. Figure 5 depicts the SEM image of the polished surface of the bi-material. Thus, no defects or cracks across the interface were observed. The crack phenomenon and other defects are common in processing of bimaterials due to differences in material properties and can only be controlled through proper and careful combination of processing parameters.

8 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 13 Fig.5 SEM image of polished surface of the bi-material 4. CONCLUSION In this study, solvent de-binding of the green bimaterial component has been investigated. The rheological investigation of the feedstocks revealed significant pseudoplastic behaviour for injection moulding. A de-binding time of 6 h at 5 o C in acetone was considered as optimum for solvent de-binding of the bi-material without incurring any defects, particularly at the bonded area. The SEM micrograph indicates a sufficient network of pores developed after 6 h. It was observed that during solvent de-binding, weight loss increases with increase in time and temperature. Furthermore, the sintered bi-material presented a density of 97% of the theoretical value and SEM observation of the polished surface revealed no defects. ACKNOWLEDGEMENT The authors would like to thank the Ministry of Higher Education Malaysia and the Universiti Kebangsaan Malaysia for their financial support under the grant TRGS/2/214/UKM/2/4/1 and DIP/217/1. REFERENCES [1.] Imgrund, P., et al., Manufacturing of multi-functional micro parts by two-component metal injection moulding. The International Journal of Advanced Manufacturing Technology, (1): p [2.] Imgrund, P., A. Rota, and L. Kramer. Processing and Properties of Bi-Material Parts by Micro Metal Injection Molding. in First International Conference on Multi-Material Micro Manufacture, Karlsruhe, Germany, W. Menz and S. Dimov, eds., Elsevier Science, Oxford, UK. 25. [3.] R, M., German; A. Bose ;, Injection Molding of Metals and Ceramics. MPIF Princeton Nj [4.] Hsu, K.-C., C. Lin, and G. Lo, The effect of wax composition on the injection molding of carbonyl iron powder with LDPE. Canadian metallurgical quarterly, (2): p [5.] Lin, S. and R. German, Extraction debinding of injection molded parts by condensed solvent. Atlanta, [6.] Nor, M., et al. Characterisation of titanium alloy feedstock for metal injection moulding using palm stearin binder system. in Advanced Materials Research Trans Tech Publ. [7]. Arifin, A., et al., Palm stearin as alternative binder for MIM: A review. Journal of Ocean, Mechanical and Aerospace-Science and Engineering-, : p [8.] Thian, E., et al., Effects of debinding parameters on powder injection molded Ti-6Al-4V/HA composite parts. Advanced Powder Technology, (3): p [9]. Tsai, D.-S. and W.-W. Chen, Solvent debinding kinetics of alumina green bodies by powder injection molding. Ceramics international, (4): p [1]. Zaky, M., Effect of solvent debinding variables on the shape maintenance of green molded bodies. Journal of materials science, (1): p [11]. Ukwueze, B.E., A.B. Sulong, N. Muhammad, Z. Sajuri, The Characterization and Rheological Investigation of Materials for Powder Injection Moulding Juournal of Mechanical Engineering, 217. S1 3(2): p [12]. Ukwueze B. E, A.B. Sulong, N. Muhammad, Z. Sajuri, Two Component Injection Moulding of Bi-material of Stainless Steel and Yttria Stabilized Zirconia Green Part. Jurnal Kejuruteraan, (1): p

9 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:3 14 [13]. Chen, X., et al., Determination of phenomenological constants of shear-induced particle migration model. Computational materials science, 24. 3(3-4): p [14]. Ahn, S., et al., Effect of powders and binders on material properties and molding parameters in iron and stainless steel powder injection molding process. Powder Technology, (2): p [15]. Kwon, T. and S. Ahn, Slip characterization of powder/binder mixtures and its significance in the filling process analysis of powder injection molding. Powder technology, (1): p [16]. Huang, B., S. Liang, and X. Qu, The rheology of metal injection molding. Journal of Materials Processing Technology, (1): p [17]. Li, Y., L. Li, and K. Khalil, Effect of powder loading on metal injection molding stainless steels. Journal of Materials Processing Technology, (2): p [18]. Amin, M., et al., Rheological investigation of MIM feedstocks prepared with different particle sizes [19]. Westcot, E., C. Binet Andrandall, and M. German, In situ dimensional change, mass loss and mechanisms for solvent debinding of powder injection moulded components. Powder metallurgy, (1): p