Abstract
To find a proper super alloy applied to gasoline engine turbochargers which demand a high operating temperature over 950°C, the IN713C super alloy was investigated in this study. The rheological properties of feedstock, density, microstructure and mechanical properties were measured. The proper powder loading was 61 vol.-%. Through the analysis of viscosity, 160°C was supposed to be more suitable for the injection. After debinding, most binder components were removed. A reasonable debinding process was formulated. Two pre-sintering temperature (850, 950°C) were investigated, then 850°C was considered to be more proper relatively. The samples were sintered at different temperatures for various time. The best properties of sintered parts were obtained when sintered at 1300°C for 3 h, the density of sintered parts was 7.83 g cm−3 while the hardness was 43.6 HRC and tensile strength was 1216.9 MPa.
Introduction
As an unusual group of metallic materials, Ni-based super alloys show an extraordinary combination of high-temperature strength, toughness and surface stability while being in corrosive or oxidative environment [1]. Owing to these superior properties, they are commonly used in the aerospace, automotive, medical, chemical and petrochemical industries. Inconel 713C (IN713C), one of the Ni-based super alloys, was developed by the International Nickel Company in the 1950s [2]. Although it is initially designed for vacuum melting and casting under the protection of inert gases, IN713C reaches the maximum effect of mechanical properties when the casts were manufactured by using vacuum casting techniques [3–5].
Investment casting is often used to process super alloys, but the cost is too high. An alternative process is metal injection moulding (MIM), which enables for complex geometry designs, minimum material loss, quick up-scale response times and significant cost savings on moderate or large production volumes. Current use of MIM super alloys is partially limited due to the unpromising mechanical properties and corrosion resistance data [6,7]. Some studies on injection moulding of nickel-based superalloys have been reported. Muhammad and Mohamad reported solvent debinding of Inconel 718 fabricated via MIM. They found that solvent temperature of 60°C and extraction time of 6 h were the most suitable conditions for solvent immersion technique to extract the PS binder completely from the binder mixture in the green moulded parts [8]. Meyer found sintering temperature of 1305°C shows the best results with DSC and dilatometry measurements. However, little research has been done on IN713 via MIM [9].
This article aims to develop a new process for preparing IN713C alloy by MIM method. Rheological properties of feedstock, densities and mechanical properties of the sintered samples were evaluated, through these works we hope to find a proper way to prepare high-performance nickel-based alloy using the MIM process.
Experimental
Materials
In the present study, gas atomised IN713C alloy powder (Osprey, UK) with tap density of 5.0 g cm−3 was used. Particle size distribution curves are presented in Figure 1. The chemical properties of the powder are listed in Table 1. The binder components were Paraffin wax (55–71 wt-%, SINOPEC, China), polypropylene (24–40 wt-%, LyondellBasell Corporation, Netherlands), Vegetable oil (2–8 wt-%, FUYU Reagents Corporation, China).
Cumulative particle size distributions for super alloy powders studied. Chemical composition of the powder (mass fraction, %).
The feedstock for injection moulding was prepared by mixing the powder and binder in a kneading machine (NH-2, JiuBao Vacuum Kneading Machine Building Co.,LTD, China) for 3 h. The mixing temperature was set at the range of 160–180°C, which was within the highest melting temperature (148°C) and the lowest degradation temperature of the binder system (220°C). After the mixing operation, the feedstock was cooled down, then it was manually granulated and injection moulded in the form of dumbbell samples. The feedstock was injection moulded by a HT90T injection moulding machine at 160–170°C.
Debinding
The debinding of green parts consists of two stages: solvent debinding and thermal debinding. The solvent debinding involved the immersion of the green component in dichloromethane at 35°C for 6 h to extract the solvent–soluble wax and oil. This step was completed in a self-made solvent debinding dish. The second step was thermal debinding. The solvent–debound samples were placed in an alumina crucible and then loaded into the debinding furnace (HSFVHDS4, ShaoShan HengSheng Machinery Industry Co., Ltd. China) and heated from room temperature to the designed temperature to remove the remaining binder.
Sintering
After the thermal debinding, the parts were continuously heated to 600°C at 2.5°C min−1 and holding the temperature for 0.5 h. Next, the temperature was raised to 850 or 950°C to finish the pre-sintering in debinding furnace. Finally, The pre-sintered parts were heated from the room temperature to designed temperature in the vacuum sintering furnace (VGS-446, Bei zhen Vacuum Technology Co., Ltd. China). The heating rate was 5°C min−1. The atmosphere of pre-sintering and sintering is argon. The Differential Scanning Calorimetry (DSC) curve can be used to determine the approximate range of suitable sintering temperature. The solidus temperature of the alloy can be corresponding to the temperature at the intersection point between the baseline of the DSC curve and the first peak tangent. According to the report, the liquidus temperature of IN713C is 1288 ± 60°C [10]. In this study, the temperature of solidus phase was about 1280°C as shown in Figure 2. To obtain a high density of sintered parts, the super solidus liquid phase sintering (SLPS) should be used, which meant the sintering temperature should be above 1280°C. So 1280, 1300 and 1320°C were chosen as the sintering temperatures.
DSC curve of alloy IN713.
The density of the sintered samples was measured by the Archimedes water displacement method. Rheological properties were measured by MLW-400B capillary rheometer manufactured by Changchun Intelligent Instrument and Equipment Co., Ltd. The cross-sectional micrograph of the sample was observed using the Nova Nano SEM 230 scanning electron microscope. Thermal degradation characteristics were measured by STA449 F3 synchronous thermal analyzer manufactured by NETZSCH Company of Germany. The content of interstitial elements was determined by FlashSmart element analyzer produced by german ThermoFisher. The porosity structure and quantity of the sintered samples were observed using the MeF3A Optical microscopy produced by REICHERT. Mechanical properties were tested by the American Instron3369 electronic universal testing machine.
Results and discussion
Determination of powder loading
The powder loading of feedstock is a critical factor which influences the injection process and the properties of sintered parts. Phase separation may happen during injection if the powder loading is too low. The feedstock may have a high viscosity and is difficult to be moulded when the powder loading is too high. Ideally, the powder loading at critical state can be described by the following equation [11]:
is the powder loading at critical state,
is the tap density of powder,
is the theoretical density of powder.
The
SEM micrographs of cross-sectional fractured surface of green parts.
and
in this study are 5.0 and 7.91 g cm−3, respectively. The
is 0.63 by calculation. Cao et al. [12] found that the optimum powder loading is 2% lower than powder loading at critical state. So the powder loading in this study was set at 61 vol.-%. Figure 3 shows the fracture morphology of green parts. The powders were packing uniformly without powder agglomeration, and different particles contacted each other. It indicated that the powder loading was suitable.
Viscosity is a basic performance index of a feedstock. The lower the viscosity, and the better the fluidness. Viscosity of feedstock is required to be less than 1000 Pa·s for shearing rate range of 100–1000 s−1 at the injection temperature [13]. In this study, green parts with no obvious defect can be obtained after injection at the range of 160 and 170°C, so the viscosity tests were carried out at these two temperatures which provides the best moulding of the IN713C. As is shown in Figure 4(a), the viscosity decreased with increasing the shear rate whether at 160 or 170°C due to the molecular chains of the binder component are disentangled and oriented at a high shear rate. It also shows that the viscosity decreased with the temperature elevation. This was due to the increase of internal energy of molecules resulting in intensification of molecular motion.
(a)Rheological characteristics of feedstock at different temperature; (b) the relationship of log viscosity versus log shear rate.
MIM feedstock usually exhibits a pseudo-plastic behaviour. For pseudo plastic fluid, it can be expressed by the following equation:
is the shear rate, n is the flow behaviour index and K is a constant. The value of n is generally less than 1 for a feedstock, and it is also supposed to approach 0.2 but greater than 0.2 [14]. If n > 1, it indicates a dilatant material on which occasion the metal powder and binder would separate at a high shear rate. The mathematical significance of n can be more clearly obtained from the logarithmic form of the below power-law:
It is shown in Figure 4(b) that n is 0.28 at 160°C while 0.43 at 170°C. It turned out that 160°C was more suitable for injection of IN713C feedstock comparing to 170°C.
Figure 5(a) shows that the residual amount of binder in the green part decreases by increasing the extraction time from 1 to 6 h in dichloromethane solvent. The weight loss percentage of the samples increased with time. The debinding rate was fast in the first two hours and then slower. After 6 h extraction in the solvent, the weight loss percentage of all samples was above 4.2%. The oil and wax in the green parts were removed and the internal pores have been formed as shown in Figure 5(b), so the subsequent thermal debinding could be carried out.
(a)Weight loss percentage of green parts as a function of extraction time. (b) SEM micrographs of cross-sectional fractured surface of dichloromethane-treated green parts for 6 h.
The process of thermal debinding was determined by TGA of the feedstock. As it is shown in Figure 6(a), the degradation of the feedstock started at about 220°C and the degradation completes at about 490°C. The total weight loss percentage of the feedstock was 7.5 wt-% which was close to theoretical weight percentage of binder. There were two stages in the process of thermal decomposition. The first weight loss stage in the temperature range of 220–400°C was due to the degradation of wax and oil in the binder system. The second weight loss stage above 400°C was reflected to the degradation of the minor binder component PP. The thermal debinding process was shown as Figure 6(b). The thermal debinding process consisted of three stages. First, the samples were heated from room temperature to 200°C at a rate of 3°C min−1 and holding the temperature for 1.5 h to soften the binder. Then the temperature was raised to 400°C at a rate of 2°C min−1 and holding the temperature for 2 h to remove the residual oil and wax.
(a)TGA curves for feedstock. (b)The thermal debinding process.
Properties of IN713C MIM final sintered samples at 1300°C for 2 h and carbon oxygen test data after thermal debinding.
Effects of sintering temperature on properties of IN713c alloy
The sintering temperatures were 1280, 1300 and 1320°C. However, when sintered at 1320°C, sintering deformation occurred and carbides appeared on the surface of the sample [15]. Therefore, 1310°C was chosen to replace 1320°C. The densities and mechanical properties of MIM IN713C sintered parts at different temperatures are summarised in Table 3. The best sintering density and properties were achieved at 1300°C. The optical micrographs of samples sintering at different temperatures and holding times are shown in Figure 7.
Optical micrographs of samples sintering at (a)1280°C, 2 h; (b) 1300°C, 2 h; (c) 1310°C, 2 h; (d)1300°C, 1 h; (e)1300°C, 3 h. Properties of IN713C MIM samples after sintered at different temperatures.
At 1280°C, the porosity was the highest and many irregular pores can be seen as in Figure 7(a). When the alloy was sintered at 1280°C, some low-melting point components, especially the MC carbide from niobium, titanium, molybdenum based on nickel, iron and chromium, exhibited full melting or partial melting with the production of limited liquid phase and utmost solid phase. When sintered at 1300°C, amounts of liquid phase increased, which reached the condition of super-solidus liquid phase sintering. In such circumstances, particles rearrange and pores were eliminated in the liquid phase, resulting in the highest density and the lowest pore size.
When sintered at 1310°C, the migration velocity of the grain boundary faster than the contracting speed of the pores due to the stronger driving force of the grain boundary. The pores stay inside, the porosity increase and the density decrease. It shows that the SLPS mechanism exhibits the best effects when sintering at 1300°C. The research published by Hu etc. about Alloy 718 sintering also shows similar [16]. It explains that a mesh niobium-rich texture is formed in the liquid phase which would strongly hinder the flow of the liquid phase and the rearrangement of particles.
Properties of IN713C MIM samples after sintering at different holding time.
Properties of IN713C MIM samples after sintering at different holding time.
The effects of different holding times at the same optimised sintering temperature of 1300°C are illustrated in Figure 7(b,d,e). A maximum density of 99.0% was achieved when the holding time was 3 h, whereas a density of 98.1% was achieved with a holding time of 1 h. Similar observations for Ni-based super alloys (Astroloy and Rene 95) have been reported by Jeandin et al. [18].
The sintering kinetics were consistent with the three established super-solidus liquid phase sintering stages, where liquid flow provided a densification–induced viscous flow, contributing to a re-precipitation-after-dissolution mechanism caused by further densification.
In summary, the sintered samples’ densities in this study indicate that high densities can be obtained by using pre-alloyed powders combined with SLPS at an optimised sintering temperature for an appropriate holding time of 3 h.
A series of experiments have been carried out to determine the optimised process parameters for IN713C super alloy materials by the MIM method. The proper powder loading is 61 vol.-%. Through the viscosity analysis, 160°C is more suitable for the injection of IN713C. After debinding, most binder can be removed. The pre-sintering temperature has a vital effect on the properties of the sintered samples. The better density and properties could be gained when pre-sintering at 850°C. The relative density of sintered samples depends on the sintering temperature and holding time. The highest density is obtained when sintering at 1300°C for 3 h and the density reaches 7.83 g cm−1 with a relative density of 99.0%. The hardness and the tensile strength are strongly related to the density of the alloys. The maximum hardness obtained is 43.6 HRC, while the maximum tensile strength is 1216.9 MPa.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.
Notes on contributors