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China reaches for a vibrant future as MIM takes off
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China reaches for a vibrant future as MIM takes off China may have been slow to embrace injection moulding technology, but MIM is now firmly established with strong commercial and academic roots. A team from Central South University in Changsha reviews the industry and looks at some of the work being done…
M
etal injection moulding (MIM) has attracted growing attention in the last 20 years because of its potential for the comparatively cheap manufacture of complex-shaped, high performance parts. MIM is characterised by large quantities of binder as the carrier of metal powders, the complex shapes gained through injection moulding and the high mechanical properties after sinter densification. Two US patents published in 1978 and 1980 can be said to represent the birth of MIM. In China, the earliest introduction paper about MIM was written by Shugong Sun in 1985. The first research paper of MIM was written by Zheng Li in 1992, and in 2001 the first Patent in China was published. It can be found from Figure 1 that only a few papers were published between 1985 and 1995, indicating the scarcity of the research and practice of MIM in China for the first 10 years. But from 1996 on, MIM research developed very fast and a large number of papers were published. The total number of published papers is 418, of which 366 are in Chinese and 52 are in English. The MIM process includes the mixing of powder and binder to prepare the feedstock, the injection moulding of the feedstock to gain green parts with desired shape, the debinding to remove the binder from green parts, and the sintering of debound parts to achieve high performance.
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The binder is a temporary vehicle for homogeneously packing the powder into the desired shape and holding the particles until the beginning of sintering. The binder usually consists of at least two components, a low molecular weight component with low melting point and a high molecular weight component with high melting point. Usually, a surface active agent is added to bridge between the binder and powder. There are three possible situations when the two binder components are mixed as shown in Figure 2: 1) Thermodynamically compatible: The
components become a true solution at the molecular level and can not be removed step by step; 2) Not compatible: The components cannot be mixed well and show phase separation; 3) Technically compatible: The components can be mixed well and the binder system looks like a true solution, but the components are not soluble at the molecular level. This means that each component can play its role and can be removed step by step during debinding. The technical compatibility is a necessity in
Figure 1. Statistics of MIM paper publication in China.
0026-0657/07 ©2007 Elsevier Ltd. All rights reserved.
MIM – where to find it in China The industrialisation of MIM in China began in the mid to late 1990s, some time later than other Asian countries such as Japan, Korea and Singapore. Many applications for early MIM products fell into those fields which did not have very strict requirements for the compacts’ dimensional tolerance and performance. Most of the products manufactured by Chinese companies and the moulds to make them have been transferred from foreign countries, mainly because only a few local industrial designers are familiar with MIM technology. In the mid 1990s there were only a few MIM companies in China. After 10 years’ development, there are now about 30 MIM companies possessing between 80 and 100 moulding machines and about 50 sintering furnaces. Unfortunately there is no accurate MIM sales volume data in China up as
yet. Most of the MIM companies are located in the coastal industrial area of China as shown. The main areas of activity are around the southern cities of Guangzhou and Shenzhen, the financial/industrial centre of Shaghai and Beijing and Jinan in the northern region. The map also pinpoints the location in Changsha of Central South University, the leading academic research institution in the country which is led by one of the world’s leading powder metallurgists, Professor Huang Bayun. Table 6 provides the major application fields and some typical parts in China. Despite the lack of accurate MIM sales volume data up to now, it is predictable that the Chinese MIM industry will have major opportunities and a bright future with the fast growing Chinese economy and the transfer of global manufacturing industries to China.
Beijing Ji’nan
Shanghai Changsha Guangzhou Shenzhen
Zhongshan
the MIM process, and this means a compatibility criterion is needed to judge the compatibility of binder components. For a mixing process of two components A and B: 1/2[A-A]+1/2[B-B] = [A-B] (1) The Gibbs free energy in the mixing process is: ∆G=∆H-T∆S (2) From the entropy and enthalpy expressions, an equation of the criterion of binder system compatibility can be derived:
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(3) Where ΦA and ΦB are the volume fraction of A and B, nA and nB the mole number of A and B; χ is the Flory-Huggins interaction parameter; VA and VB the volume of A and B per mole; V the volume of the mixture. The criterion of physical mixing process is different from the chemical reaction. From the mixing experiments of PEG/PMMA, PEG/PVA, PEG/EVA binder system, when
the number ∆G/T of a binder system is larger than 3.0, phase separation will occur and the binder system is not compatible; when 0<∆G/T<1.5, the binder system is technically compatible. Paraffin wax (PW) is a widely employed component in binder systems to provide viscosity. However, it has a phase transformation during cooling, resulting in larger shrinkage and residue stress. Using amorphous microcrystalline wax to replace part of the paraffin wax in PW-based binder systems is an effective way to solve the
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Figure 2. Schematic presentation of components’ compatibility. 9.0
30
8.0
▲
Tensile strength
▲
20
7.5
Shrinkage
▲
7.0
▲
15
6.5
▲
▲
Shrinkage %
Tensile strength/MPa
8.5
▲
25
6.0
10 5.5 5
5.0 0
5
10
15
20
25
30
Microcrystalline wax %
Figure 3. Effects of various MW additions on the tensile strength and shrinkage of binder.
MIM operations, but it has a relatively close melting point between PW and EVA, which is not good for shape retention during debinding because both the components melt during heating up and cannot hold the particles in their original position. The modification of bulk polymer by thermosetting polymer is one suggested way of improving strength during debinding. A new binder system, PW-EVA-solidified agent (epoxy and dicyandiamide), exhibits thermoplastic behaviour during the moulding process but solidifies during debinding and the strength of green parts increases to 19MPa as shown in Figure 5. A dimensional tolerance of ±0.02 mm is achieved through the WC-8Co and WC-5TiC-10 Co cemented carbide MIM process, which is better than those prepared using the usual wax-based binder. A surface-active agent is necessary to bridge between the binder and powder. The surface-active agent can reduce the wetting angles between metal powder and the viscosity of feedstock. It was found that a chemical bond exists on the surface of 17-4PH SS powder after mixing as shown in Figure 6. It helps a lot to enhance the interacting force between the binder and the powder as can be seen through Fourier transformation infrared spectroscopy (FTIR) analysis. Figure 7 is the schematic presentation of the chemical reactions. With the addition of the surfactant, the peak of C=O disappears and is replaced by a new peak COO-. Meanwhile, the peak of O-H becomes obvious, indicating that chemical adsorption occurs on the powder surface. Compared with physical adsorption, the chemical adsorption between the binder and powder can reduce the wetting angle and viscosity, which is beneficial for moulding. The selection and evaluation of the viscosity model is helpful to estimate the rheological properties of a feedstock. Most people adopt the powder law model: (4)
Figure 4. Fracture surfaces in tensile specimens of no MW addition and 5% MW addition.
problem. After blending the waxes the powder loading is increased and the feedstock becomes more homogeneous. It shows that the addition of between one and 30wt% causes an increase in tensile strength and a decrease in shrinkage of the binder as illustrated in Figure 3. Microcrystalline waxes have an amorphous structure with a higher
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melting point than paraffin wax which will nucleate first and form a lot of fine particles to be nucleating centres for the paraffin wax during cooling. This inhibits phase transformation and the microstructure is refined as shown in Figure 4. Paraffin wax/ethylene vinyl acetate (PW/ EVA) is a widely used binder system in
Where γ is the shear rate, T is the temperature, Ta, A are the constants, and n is the flow index. The influences of temperature and shear rate are considered in the model. This represents the essential influence of the two parameters: the model is simple, while the pressure influence on viscosity is ignored. The influence of pressure on the viscosity of the feedstock is considered in the Cross-Arrhenius model:
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(6)
16
Where η0 is the viscosity of zero shear rate, P is the pressure, T is the temperature, τ* is the shear stress; Tb, B and β are constants. Not only the viscosity, but also the sensitivity of viscosity to shear rate and temperature is important for the evaluation of the feedstock system. From the powder law model and Arrhenius relationship:
14
Strength/MPa
(5)
12 10 8 6 4 2
(7) Where E is active energy, R is the universal gas constant, T is the absolute temperature, m0 is a constant. Therefore, a general index STV can be derived:
(8)
where η is the viscosity, η0the referenced viscosity, γ is the shear rate, n is the sensitivity index. The general index STV increases with the decrease of viscosity, active energy and sensitivity index. It can be used to evaluate the integrated rheology properties of a feedstock. The objective of moulding is to attain the desired shape free of defects. To manage the moulding process successfully, moulding parameters must be well controlled simultaneously. Moulding temperature and pressure are the two most crucial parameters for success. Figure 8 shows the effect of moulding pressure on the percentage of distorted compacts during debinding. It can be found that the percentage of distorted compacts decreases with the increase of moulding pressure from 90 to 120 MPa. With the moulding pressure further increased to 150 MPa, the percentage of distorted compacts increases due to the occurrence of residual stresses. The effect of moulding temperature on the percentage of distorted compacts is shown in Figure 9, which shows the same trend as Figure 8. The higher the moulding temperature, the lower the distortion, up to the range of 135°C to 150°C, followed by an increase in distortion with further increases in moulding temperature, due to the occurrence of residual stresses.
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0
100
200
300
400
500
t/min Figure 5. The curve of strength versus debinding time.
Introducing electromagnetic dynamic vibration to the moulding process is an effective way of improving the homogeneity and packing density of as-moulded compacts. The screw is vibrated along the axis under the electromagnetic field so that vibration is introduced into moulding process. The influence of vibration frequency and amplitude on compacts’ density is shown in Figures 10 and 11. Co-ordination effect between the chains of polymer molecules is improved by the vibration. The flowability of the feedstock is improved while the viscosity is decreased in the vibration force field. Flow resistance is decreased and the density of the feedstock is improved. The influence of vibration frequency and amplitude on compact homogeneity is shown in Figures 12 and 13. The nonuniform binder distribution was found in compacts without vibration. The situation is improved after a certain degree of vibration, but the distribution of binder cannot be improved further when the vibration frequency and amplitude exceed the optimal value. However, it is still much better than the compacts without vibration. Thermal debinding is known as the most time consuming stage of the MIM process and the one where defects are most likely to be introduced. Debinding time is dependent on the slowest step (kinetic controlling step) in the four-step binder removal process: • Evaporation of low molecular weight components to generate gas molecules and solute into the liquid polymer; • The liquid diffusion of the gas molecules to the liquid-gas interface;
Figure 6. FTIR spectra for SA and (SA + 17-4PH SS powder) mixture.
Figure 7. Chemical reactions on the surface of 17-4PH SS powder.
• The permeation of gas molecules to the surface of the compact through opened pores; and • Removal by outer atmosphere. Liquid diffusion and gas permeation are regarded as the most likely rate control steps, as illustrated in Figure 14. The kinetic controlling step of the debinding process depends on the comparison of the diffusion flux (J) and the percolation flux (u) during the formation stage of connected pore channels. From the comparison of liquid flux and permeation flux:
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Figure 8. Effect of moulding pressure on the percentage of distorted compacts.
(9) Where J is the diffusion flux, Deff the diffusion efficiency coefficient of gas molecular in polymer melt, ∆C the concentration gradient, and L is the diffusion distance. The diffusion distance L is the distance that gas molecule diffuses through the polymer melt to the inner surface of the pore, ie the
Figure 9. Effect of moulding temperature on the percentage of distorted compacts.
binder–atmosphere interface. At the very beginning, because no pores form, L is the distance to the compact surface. As the connected pores depleting binder are continually formed and advance into the inside of compacts, the distance to the compact surface increases. The diffusion distance becomes the distance between nearby pores which are opening. The diffusion distance is several times the particle diameter now.
Figure 10. Density changes versus different frequency of vibration.
(10) Where u is percolation flux, B the permeability, G the vapour viscosity, P the gas pressure at the binder–vapour interface, P0 the external gas pressure on the compact surface, l is the distance from the binder–vapour interface to the compact surface, and S is the external compact surface area.
Figure 11 Density changes versus different amplitude of vibration.
Figure 12. The fracture morphology of compacts through different frequency vibration.
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Figure 13. The fracture morphology of compacts through different amplitude vibration.
Table 1. Characteristics of the powders. Powder
D10(µm)
D50(µm)
D90(µm)
Powder shape
–150µm WA SS
22.30
65.71
160.96
Irregular
–75µm WA SS
8.51
26.69
63.47
Irregular
–38µm WA SS
7.21
19.25
40.34
Irregular
Table 2. Powder particle size and the mechanical properties. Particle size
Relative density
σb/MPa
σ0.2/MPa
δ/%
HRB
-150µm
93.3
307
122
30
17
-75µm
97.5
387
130
34.7
55
-38µm
98
506
193
54
61
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of debinding changes from the diffusion of gas molecules in the liquid melt to the transportation of gas molecules in opened connected pore channels: (11) Where φ is the powder loading, and τ is the tortuosity, ρ is the density of the evaporated or decomposed vapour of the
8.0
Binder remain ratio
If Ju, the transportation of gas molecules in opened connected pore channels is the kinetic controlling step. The kinetics of this stage are related to the compact thickness directly because the length of the pores through which the gas molecular needs to transport is decided by the thickness of the compacts. During the initial stage, the transport of gas molecules in open pore channels is very fast. This is not the kinetic control step because the distance from the binder-vapour interface to the compact surface is very short. The distance from the binder-vapour interface to the compact surface increases with the continuous removal of binder, until it reaches half the compact thickness. At the same time, the percolation flux u decreases continuously. This means there must exist a critical compact thickness HC, at which the kinetic control step
Figure 14. Schematic presentation of the debinding process: White areas: Liquid diffusion to the liquidgas interface; Black areas: Permeation to the compact surface.
Figure 15. Photograph of the annulus specimen (0.35mm, diffusion control).
+ 4um
+
+ 12um
0.8
+
0.6
+
+
0.4
0.2
0.0 0
20
40
60
80
100
Ime (mim) Figure 16. Binder retention ratio as a function of time for an annulus specimen (100°C).
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17
1.0
4µm 12µm
Binder remain ratio
0.8
0.6
0.4
0.2
0.0 0
20
40
80
60
100
120
140
160
180
Compacts prepared from fine powder should have a higher binder removal rate than those prepared from coarse powder, but where permeation is controlled fine powder compacts should have a lower diffusion rate because of the inverse flux
Time (min) Figure 17. Binder retention ratio as a function of time for cylinder specimen (100°C).
8.5
-75µm WA -38µm WA
Binder Weight Loss (%)
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 10
12
14
16
18
20
22
24
26
28
Thickness of /Green Compacts (mm) Figure 18. The influence of different powder size in binder weight loss for different thickness compacts.
binder, d is the powder particle size, Q is the active energy of diffusion, and E is the porosity. The critical thickness represents the kinetic control transformation
thickness in debinding. Therefore, if the compact thickness can be kept less than the critical thickness Hc, the kinetics depend on the diffusion of gas molecules into the polymer melt. If the compact thickness is larger
than the critical thickness, the kinetics depend on the permeation of gas molecules in open connected pore channels. For the compact with defined binder system and powder loading, the critical thickness has a definite value. Because the diffusion distance L is the distance that gas molecules diffuse through the polymer melt to the inner surface of the pore, which is several times the particle size, it means that the compacts prepared from fine powder have a shorter diffusion distance than those prepared from coarse powder. Compacts prepared from fine powder should have a higher binder removal rate than those prepared from coarse powder. However, where permeation is controlled, compacts prepared from fine powder should have a lower binder removal rate than those prepared by coarse powder because the permeation flux is inversely proportional to the particle size. Two kinds of specimens (0.35mm annulus; φ12×12mm cylinder) were prepared based on the calculation of critical thickness theory as shown in Figure 15. Each specimen was produced from two powders of different particle sizes (4µm and 12µm). They were under different rate control steps when debinding was carried out under design conditions.
Table 3. Chemical composition of 316L stainless steel powder. Element
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
O
Fe
Gas atomised
0.025
0.41
1.27
0.025
0.006
10.8
17.2
2.26
0.25
0.03
balance
Water atomised
0.025
0.83
0.78
0.015
0.010
12.59
16.56
2.13
0.04
0.241
balance
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The curves of the influence of particle size on binder retention at different rate control steps are shown in Figures 16 and 17. The 0.35 mm annulus compacts prepared from fine powder shows a lower binder retention ratio than the compacts prepared from coarse powder under the same conditions. In other words, compacts prepared from fine powder have a higher debinding rate than those prepared from coarse powder in a diffusion-controlled situation. The widely accepted concept that using coarse powder is always beneficial in terms of debinding is not true. MIM stainless steel process research is of long standing in China, including optimisation of moulding parameters, debinding behaviour, densification, dimension control and corrosion resistance. The available SS powders for MIM applications are produced by atomisation of a liquid melt. Water atomised 316L SS powders of –150µm, –75µm, –38µm were used in the investigation. Their characteristics are shown in Table 1. Coarse powder parts have a higher debinding rate than fine powder parts under the same conditions. It can be seen in Figure 18 that the binder weight loss of the compacts made by –75µm powder is higher than that of the compacts made by –38µm powder in the same debinding schedule, especially for the compacts with the thickness of 10 mm and 15mm. Coarse powder parts have a lower possibility of introducing defects than fine powder parts under the same conditions. The photographs of the as-debound parts with the thickness of 15mm are shown in Figure 19. It can be seen that the appearance of the compact made by –75µm powder is good while defects are observed in the compact made by –38µm powder in the same debinding schedule. Table 2 provides the mechanical properties of compacts with different sized powders. MIM parts with satisfactory mechanical properties can be obtained by –75µm
Figure 19. The as-debound compacts with the thickness of 15mm (a) –75µm powder compact (good) (b) –38µm powder compact(distorted).
Figure 20. SEM of 316L stainless steel powders.
Figure 21. Mechanical properties of sintered stainless steels.
Table 4. Mechanical properties of as sintered and heat-treated Fe2Ni alloy. Alloy
As-sintered
Heat-treated
Relative density /%
Tensile strength /MPa
Elongation Hardness /% /HRB
Impact work /J•cm-3
Tensile strength /MPa
Elongation /%
Hardness /HRC
Impact work /J•cm-3
Fe-2Ni
95.3
562
12.0
44.6
7.60
1046
5.56
33.7
8.58
Fe-2Ni-0.5Cr
94.7
754
9.2
62.6
5.85
1240
4.90
35.8
4.80
Fe-2Ni-1Cr
94.2
941
7.7
69.0
4.44
1061
3.18
40.6
3.40
Fe-2Ni-3Cr
91.5
975
4.0
73.3
3.05
1060
2.70
41.0
2.00
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Investigations into improving properties of low alloy steel have shown that chemical composition modification is more effective than process optimisation and heat treatment.
Figure 22a. Scanning election micrographs for sintered samples (3000X).
Figure 22b. Scanning election micrographs for sintered samples (3000X).
and –38µm WA powder, which exceed MPIF standard of sintered SS parts. With the application of inert gas and a moderate cooling rate, gas atomisation can produce powder with low oxygen content and spherical particle shape. In contrast with gas atomisation, water atomisation is carried out under higher liquid pressure. Because of the liquid atomising agent and the rapid cooling rate, it produces the powder with higher oxygen content and less spherical particle shape. However, water atomised SS powder with almost completely spherical particle shape is also available. The effect of the atomisation medium on the sintering of austenitic SS can be studied individually by eliminating the influence of particle shape and particle size using the very close particle size and powder morphology which was ignored by previous studies. The powder morphologies of the gas atomised and water atomised 316L stainless steel powders are shown in Figure 20. The
densities and mechanical properties of the compacts are given in Figure 21. Gas atomised SS compacts have a relatively higher density and tensile strength, while water atomised SS compacts have a lower density and higher elongation. It is clear that the mean particle size of the gas atomised and water atomised powders was very close, and the particle size distribution of them is almost the same. It can also be seen that there was no obvious difference of particle shape. As a result, the powder morphology and particle size should not be the key factor affecting the densification and microstructure of these two kinds of sintered SS compacts. It was found from Table 3 that the oxygen content and silicon content of water atomised powder were much higher than that of gas atomised powder. Figure 22 shows the fractographs of the sintered compacts. It is clear in Figure 22 (b) that there are certain second-phase particles dispersed in the sintered compacts produced by water atomised powder Energy dispersive X-ray analysis (EDAX)
suggests that the second-phase particles should be the silicon oxide. On the other hand, there is no second-phase particle in Figure 22 (a). It corresponds with the fact that the silicon content is negligible in the sintered compacts produced from gas atomised powder. Powder chemistry determines the difference of the sintering densification processes and influences the microstructure, mechanical properties and corrosion resistance of sintered compacts. The silicon oxides enriching particle surfaces and pore surfaces are nearly impossible to remove. The compacts cannot attain full densification due to the obstacle effect caused by the silicon oxide diffusing towards grain boundary and deteriorating the bonding between particles during sintering. Consequently, the grain of sintered water atomised SS is harder to grow, the grain size is smaller than that of gas atomised SS and the density of water atomised SS is slightly lower. Most of the early applications of MIM in China were based on Fe2Ni low alloy steel, including business equipment, firearms, household, personal care and microelectronics where a key requirement was
Table 5. Mechanical properties of as sintered and heat-treated Fe2Ni alloy with Mo additions. Alloy
As-sintered
Heat-treated
Relative density /%
Tensile strength /MPa
Elongation /%
Hardness /HRB
Tensile strength /MPa
Elongation /%
Hardness /HRB
Fe-2Ni+0.3C
97.2
321.0
22.9
32
406.9
24
66
Fe-2Ni-1Mo+0.3C
96.8
375.6
13.3
53
505.6
9.1
88
Fe-2Ni-2Mo+0.3C
95.6
401.4
11.0
59
580.1
8.4
92.5
Fe-2Ni-5Mo+0.3C
95.0
549.2
6.3
88
690.4
5.0
115
Fe-2Ni-1Mo+0.5C
96.0
423.2
8.7
79
1089.2
4.1
107.5
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mechanical properties. Process optimisation investigations to improve the properties of Fe2Ni low alloy steel have shown however, that chemical composition modification is more effective than process optimisation and heat treatment. The team at Central South University looked at the influence of chromium (Cr) and molybdenum (Mo) additions on properties of Fe2Ni low alloy steel. Table 4 provides the mechanical properties of as sintered and heat-treated Fe2Ni low alloy with different amounts of Cr addition. With the increase of Cr addition, the tensile strength and hardness increase while the density, elongation and the impact work of the sintered and heat-treated FeNi-Cr alloy decrease. The reason for the change of the properties can be found in the microstructure morphology shown in Figure 23. The microstructure of Fe2Ni consists of pearlite and ferrite as shown in Figure 24 (a), and it can be found in Figures 23 (b), (c) and (d) that the amount of ferrite increases and the microstructure is refined with the increase of Cr addition. The fracture changes from a dimple fracture to the cleavage fracture shown in Figure 25. Fe-2Ni-0.5Cr alloy has the best set of mechanical properties after heat treatment.
Titanium moulding improves Most titanium alloys manufactured in China are made by the lower cost hydridedehydride process (HDH) instead of using part or all of the expensive gas atomised powder. HDH Ti powder is less suitable for injection moulded high-performance products for two reasons: The first is the irregular shape and unfavourable rheological properties. The second is the susceptibility of titanium to carbon, nitrogen and oxygen. HDH titanium powder can easily be polluted during production. An interesting way to shorten the procedure and avoid the process contamination of HDH powder is using TiH2 powder directly. How to investigate a route to carry out the dehydrogenation in debinding is the problem that needs to be considered. For the dehydrogenation of TiH2 powder: (12)
(13)
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Figure 23. Microstructure morphology of the Fe2Ni with different amount of Cr additions. (a)
(b)
(d)
(c)
Figure 24. SEM fracture morphology of the Fe2Ni with different amount of Cr additions.
Figure 25. Microstructure morphology of the Fe2Ni with different amounts of Mo addition.
Table 6. Summary of current applications for MIM in China. Application field
Component
Medical apparatus and instruments
Orthodontic brackets, Surgical apparatus parts
Firearms
Barrel sleeve, Ratchet, Bayonet base,
Automobiles
Air bag striker, Transmission gear, Safety lock, Port fire
Watch
Watch ring, Watch button, Watch band, Watch Case
Electronics
Capsulation parts, Mobile phone parts, Weight for micro-motor
For isothermal processes, the relationship between ∆rGm, T and PH2 can be represents as:
(14) The partial pressure needed for dehydrogenation process according to the equation is that: When T=200°C, ∆rGm<0,PH2<1.04×10-4Pa; and When T=300°C, ∆rGm <0,PH2 <0.072 Pa It requires a vacuum of 10-4 Pa to carry out the dehydrogenation at 200°C which is very difficult to achieve. However 10-2 Pa can be achieved at 300°C, which means that
the new route has the possibility of being realised through the modification of holding temperature and atmosphere during debinding. Combined with the debinding schedule of wax and polymer according to the DTA and TGA curves, the holding temperature needed for debinding and dehydrogenation can be confirmed. The temperature range of dehydrogenation of TiH2 powder is within that of debinding. For the necessity of dehydrogenation and binder removal, schedule has to be specified. Given a satisfactory dehydrogenationdebinding schedule, the compact shows an acceptable microstructure and mechanical properties (σb=770MPa, δ=4.3%).
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China reaches for a vibrant future as MIM takes off China may have been slow to embrace injection moulding technology, but MIM is now firmly established with strong commercial and academic roots. A team from Central South University in Changsha reviews the industry and looks at some of the work being done…
M
etal injection moulding (MIM) has attracted growing attention in the last 20 years because of its potential for the comparatively cheap manufacture of complex-shaped, high performance parts. MIM is characterised by large quantities of binder as the carrier of metal powders, the complex shapes gained through injection moulding and the high mechanical properties after sinter densification. Two US patents published in 1978 and 1980 can be said to represent the birth of MIM. In China, the earliest introduction paper about MIM was written by Shugong Sun in 1985. The first research paper of MIM was written by Zheng Li in 1992, and in 2001 the first Patent in China was published. It can be found from Figure 1 that only a few papers were published between 1985 and 1995, indicating the scarcity of the research and practice of MIM in China for the first 10 years. But from 1996 on, MIM research developed very fast and a large number of papers were published. The total number of published papers is 418, of which 366 are in Chinese and 52 are in English. The MIM process includes the mixing of powder and binder to prepare the feedstock, the injection moulding of the feedstock to gain green parts with desired shape, the debinding to remove the binder from green parts, and the sintering of debound parts to achieve high performance.
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The binder is a temporary vehicle for homogeneously packing the powder into the desired shape and holding the particles until the beginning of sintering. The binder usually consists of at least two components, a low molecular weight component with low melting point and a high molecular weight component with high melting point. Usually, a surface active agent is added to bridge between the binder and powder. There are three possible situations when the two binder components are mixed as shown in Figure 2: 1) Thermodynamically compatible: The
components become a true solution at the molecular level and can not be removed step by step; 2) Not compatible: The components cannot be mixed well and show phase separation; 3) Technically compatible: The components can be mixed well and the binder system looks like a true solution, but the components are not soluble at the molecular level. This means that each component can play its role and can be removed step by step during debinding. The technical compatibility is a necessity in
Figure 1. Statistics of MIM paper publication in China.
0026-0657/07 ©2007 Elsevier Ltd. All rights reserved.
MIM – where to find it in China The industrialisation of MIM in China began in the mid to late 1990s, some time later than other Asian countries such as Japan, Korea and Singapore. Many applications for early MIM products fell into those fields which did not have very strict requirements for the compacts’ dimensional tolerance and performance. Most of the products manufactured by Chinese companies and the moulds to make them have been transferred from foreign countries, mainly because only a few local industrial designers are familiar with MIM technology. In the mid 1990s there were only a few MIM companies in China. After 10 years’ development, there are now about 30 MIM companies possessing between 80 and 100 moulding machines and about 50 sintering furnaces. Unfortunately there is no accurate MIM sales volume data in China up as
yet. Most of the MIM companies are located in the coastal industrial area of China as shown. The main areas of activity are around the southern cities of Guangzhou and Shenzhen, the financial/industrial centre of Shaghai and Beijing and Jinan in the northern region. The map also pinpoints the location in Changsha of Central South University, the leading academic research institution in the country which is led by one of the world’s leading powder metallurgists, Professor Huang Bayun. Table 6 provides the major application fields and some typical parts in China. Despite the lack of accurate MIM sales volume data up to now, it is predictable that the Chinese MIM industry will have major opportunities and a bright future with the fast growing Chinese economy and the transfer of global manufacturing industries to China.
Beijing Ji’nan
Shanghai Changsha Guangzhou Shenzhen
Zhongshan
the MIM process, and this means a compatibility criterion is needed to judge the compatibility of binder components. For a mixing process of two components A and B: 1/2[A-A]+1/2[B-B] = [A-B] (1) The Gibbs free energy in the mixing process is: ∆G=∆H-T∆S (2) From the entropy and enthalpy expressions, an equation of the criterion of binder system compatibility can be derived:
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(3) Where ΦA and ΦB are the volume fraction of A and B, nA and nB the mole number of A and B; χ is the Flory-Huggins interaction parameter; VA and VB the volume of A and B per mole; V the volume of the mixture. The criterion of physical mixing process is different from the chemical reaction. From the mixing experiments of PEG/PMMA, PEG/PVA, PEG/EVA binder system, when
the number ∆G/T of a binder system is larger than 3.0, phase separation will occur and the binder system is not compatible; when 0<∆G/T<1.5, the binder system is technically compatible. Paraffin wax (PW) is a widely employed component in binder systems to provide viscosity. However, it has a phase transformation during cooling, resulting in larger shrinkage and residue stress. Using amorphous microcrystalline wax to replace part of the paraffin wax in PW-based binder systems is an effective way to solve the
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Figure 2. Schematic presentation of components’ compatibility. 9.0
30
8.0
▲
Tensile strength
▲
20
7.5
Shrinkage
▲
7.0
▲
15
6.5
▲
▲
Shrinkage %
Tensile strength/MPa
8.5
▲
25
6.0
10 5.5 5
5.0 0
5
10
15
20
25
30
Microcrystalline wax %
Figure 3. Effects of various MW additions on the tensile strength and shrinkage of binder.
MIM operations, but it has a relatively close melting point between PW and EVA, which is not good for shape retention during debinding because both the components melt during heating up and cannot hold the particles in their original position. The modification of bulk polymer by thermosetting polymer is one suggested way of improving strength during debinding. A new binder system, PW-EVA-solidified agent (epoxy and dicyandiamide), exhibits thermoplastic behaviour during the moulding process but solidifies during debinding and the strength of green parts increases to 19MPa as shown in Figure 5. A dimensional tolerance of ±0.02 mm is achieved through the WC-8Co and WC-5TiC-10 Co cemented carbide MIM process, which is better than those prepared using the usual wax-based binder. A surface-active agent is necessary to bridge between the binder and powder. The surface-active agent can reduce the wetting angles between metal powder and the viscosity of feedstock. It was found that a chemical bond exists on the surface of 17-4PH SS powder after mixing as shown in Figure 6. It helps a lot to enhance the interacting force between the binder and the powder as can be seen through Fourier transformation infrared spectroscopy (FTIR) analysis. Figure 7 is the schematic presentation of the chemical reactions. With the addition of the surfactant, the peak of C=O disappears and is replaced by a new peak COO-. Meanwhile, the peak of O-H becomes obvious, indicating that chemical adsorption occurs on the powder surface. Compared with physical adsorption, the chemical adsorption between the binder and powder can reduce the wetting angle and viscosity, which is beneficial for moulding. The selection and evaluation of the viscosity model is helpful to estimate the rheological properties of a feedstock. Most people adopt the powder law model: (4)
Figure 4. Fracture surfaces in tensile specimens of no MW addition and 5% MW addition.
problem. After blending the waxes the powder loading is increased and the feedstock becomes more homogeneous. It shows that the addition of between one and 30wt% causes an increase in tensile strength and a decrease in shrinkage of the binder as illustrated in Figure 3. Microcrystalline waxes have an amorphous structure with a higher
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melting point than paraffin wax which will nucleate first and form a lot of fine particles to be nucleating centres for the paraffin wax during cooling. This inhibits phase transformation and the microstructure is refined as shown in Figure 4. Paraffin wax/ethylene vinyl acetate (PW/ EVA) is a widely used binder system in
Where γ is the shear rate, T is the temperature, Ta, A are the constants, and n is the flow index. The influences of temperature and shear rate are considered in the model. This represents the essential influence of the two parameters: the model is simple, while the pressure influence on viscosity is ignored. The influence of pressure on the viscosity of the feedstock is considered in the Cross-Arrhenius model:
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(6)
16
Where η0 is the viscosity of zero shear rate, P is the pressure, T is the temperature, τ* is the shear stress; Tb, B and β are constants. Not only the viscosity, but also the sensitivity of viscosity to shear rate and temperature is important for the evaluation of the feedstock system. From the powder law model and Arrhenius relationship:
14
Strength/MPa
(5)
12 10 8 6 4 2
(7) Where E is active energy, R is the universal gas constant, T is the absolute temperature, m0 is a constant. Therefore, a general index STV can be derived:
(8)
where η is the viscosity, η0the referenced viscosity, γ is the shear rate, n is the sensitivity index. The general index STV increases with the decrease of viscosity, active energy and sensitivity index. It can be used to evaluate the integrated rheology properties of a feedstock. The objective of moulding is to attain the desired shape free of defects. To manage the moulding process successfully, moulding parameters must be well controlled simultaneously. Moulding temperature and pressure are the two most crucial parameters for success. Figure 8 shows the effect of moulding pressure on the percentage of distorted compacts during debinding. It can be found that the percentage of distorted compacts decreases with the increase of moulding pressure from 90 to 120 MPa. With the moulding pressure further increased to 150 MPa, the percentage of distorted compacts increases due to the occurrence of residual stresses. The effect of moulding temperature on the percentage of distorted compacts is shown in Figure 9, which shows the same trend as Figure 8. The higher the moulding temperature, the lower the distortion, up to the range of 135°C to 150°C, followed by an increase in distortion with further increases in moulding temperature, due to the occurrence of residual stresses.
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0
100
200
300
400
500
t/min Figure 5. The curve of strength versus debinding time.
Introducing electromagnetic dynamic vibration to the moulding process is an effective way of improving the homogeneity and packing density of as-moulded compacts. The screw is vibrated along the axis under the electromagnetic field so that vibration is introduced into moulding process. The influence of vibration frequency and amplitude on compacts’ density is shown in Figures 10 and 11. Co-ordination effect between the chains of polymer molecules is improved by the vibration. The flowability of the feedstock is improved while the viscosity is decreased in the vibration force field. Flow resistance is decreased and the density of the feedstock is improved. The influence of vibration frequency and amplitude on compact homogeneity is shown in Figures 12 and 13. The nonuniform binder distribution was found in compacts without vibration. The situation is improved after a certain degree of vibration, but the distribution of binder cannot be improved further when the vibration frequency and amplitude exceed the optimal value. However, it is still much better than the compacts without vibration. Thermal debinding is known as the most time consuming stage of the MIM process and the one where defects are most likely to be introduced. Debinding time is dependent on the slowest step (kinetic controlling step) in the four-step binder removal process: • Evaporation of low molecular weight components to generate gas molecules and solute into the liquid polymer; • The liquid diffusion of the gas molecules to the liquid-gas interface;
Figure 6. FTIR spectra for SA and (SA + 17-4PH SS powder) mixture.
Figure 7. Chemical reactions on the surface of 17-4PH SS powder.
• The permeation of gas molecules to the surface of the compact through opened pores; and • Removal by outer atmosphere. Liquid diffusion and gas permeation are regarded as the most likely rate control steps, as illustrated in Figure 14. The kinetic controlling step of the debinding process depends on the comparison of the diffusion flux (J) and the percolation flux (u) during the formation stage of connected pore channels. From the comparison of liquid flux and permeation flux:
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Figure 8. Effect of moulding pressure on the percentage of distorted compacts.
(9) Where J is the diffusion flux, Deff the diffusion efficiency coefficient of gas molecular in polymer melt, ∆C the concentration gradient, and L is the diffusion distance. The diffusion distance L is the distance that gas molecule diffuses through the polymer melt to the inner surface of the pore, ie the
Figure 9. Effect of moulding temperature on the percentage of distorted compacts.
binder–atmosphere interface. At the very beginning, because no pores form, L is the distance to the compact surface. As the connected pores depleting binder are continually formed and advance into the inside of compacts, the distance to the compact surface increases. The diffusion distance becomes the distance between nearby pores which are opening. The diffusion distance is several times the particle diameter now.
Figure 10. Density changes versus different frequency of vibration.
(10) Where u is percolation flux, B the permeability, G the vapour viscosity, P the gas pressure at the binder–vapour interface, P0 the external gas pressure on the compact surface, l is the distance from the binder–vapour interface to the compact surface, and S is the external compact surface area.
Figure 11 Density changes versus different amplitude of vibration.
Figure 12. The fracture morphology of compacts through different frequency vibration.
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Figure 13. The fracture morphology of compacts through different amplitude vibration.
Table 1. Characteristics of the powders. Powder
D10(µm)
D50(µm)
D90(µm)
Powder shape
–150µm WA SS
22.30
65.71
160.96
Irregular
–75µm WA SS
8.51
26.69
63.47
Irregular
–38µm WA SS
7.21
19.25
40.34
Irregular
Table 2. Powder particle size and the mechanical properties. Particle size
Relative density
σb/MPa
σ0.2/MPa
δ/%
HRB
-150µm
93.3
307
122
30
17
-75µm
97.5
387
130
34.7
55
-38µm
98
506
193
54
61
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of debinding changes from the diffusion of gas molecules in the liquid melt to the transportation of gas molecules in opened connected pore channels: (11) Where φ is the powder loading, and τ is the tortuosity, ρ is the density of the evaporated or decomposed vapour of the
8.0
Binder remain ratio
If J
Figure 14. Schematic presentation of the debinding process: White areas: Liquid diffusion to the liquidgas interface; Black areas: Permeation to the compact surface.
Figure 15. Photograph of the annulus specimen (0.35mm, diffusion control).
+ 4um
+
+ 12um
0.8
+
0.6
+
+
0.4
0.2
0.0 0
20
40
60
80
100
Ime (mim) Figure 16. Binder retention ratio as a function of time for an annulus specimen (100°C).
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1.0
4µm 12µm
Binder remain ratio
0.8
0.6
0.4
0.2
0.0 0
20
40
80
60
100
120
140
160
180
Compacts prepared from fine powder should have a higher binder removal rate than those prepared from coarse powder, but where permeation is controlled fine powder compacts should have a lower diffusion rate because of the inverse flux
Time (min) Figure 17. Binder retention ratio as a function of time for cylinder specimen (100°C).
8.5
-75µm WA -38µm WA
Binder Weight Loss (%)
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 10
12
14
16
18
20
22
24
26
28
Thickness of /Green Compacts (mm) Figure 18. The influence of different powder size in binder weight loss for different thickness compacts.
binder, d is the powder particle size, Q is the active energy of diffusion, and E is the porosity. The critical thickness represents the kinetic control transformation
thickness in debinding. Therefore, if the compact thickness can be kept less than the critical thickness Hc, the kinetics depend on the diffusion of gas molecules into the polymer melt. If the compact thickness is larger
than the critical thickness, the kinetics depend on the permeation of gas molecules in open connected pore channels. For the compact with defined binder system and powder loading, the critical thickness has a definite value. Because the diffusion distance L is the distance that gas molecules diffuse through the polymer melt to the inner surface of the pore, which is several times the particle size, it means that the compacts prepared from fine powder have a shorter diffusion distance than those prepared from coarse powder. Compacts prepared from fine powder should have a higher binder removal rate than those prepared from coarse powder. However, where permeation is controlled, compacts prepared from fine powder should have a lower binder removal rate than those prepared by coarse powder because the permeation flux is inversely proportional to the particle size. Two kinds of specimens (0.35mm annulus; φ12×12mm cylinder) were prepared based on the calculation of critical thickness theory as shown in Figure 15. Each specimen was produced from two powders of different particle sizes (4µm and 12µm). They were under different rate control steps when debinding was carried out under design conditions.
Table 3. Chemical composition of 316L stainless steel powder. Element
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
O
Fe
Gas atomised
0.025
0.41
1.27
0.025
0.006
10.8
17.2
2.26
0.25
0.03
balance
Water atomised
0.025
0.83
0.78
0.015
0.010
12.59
16.56
2.13
0.04
0.241
balance
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The curves of the influence of particle size on binder retention at different rate control steps are shown in Figures 16 and 17. The 0.35 mm annulus compacts prepared from fine powder shows a lower binder retention ratio than the compacts prepared from coarse powder under the same conditions. In other words, compacts prepared from fine powder have a higher debinding rate than those prepared from coarse powder in a diffusion-controlled situation. The widely accepted concept that using coarse powder is always beneficial in terms of debinding is not true. MIM stainless steel process research is of long standing in China, including optimisation of moulding parameters, debinding behaviour, densification, dimension control and corrosion resistance. The available SS powders for MIM applications are produced by atomisation of a liquid melt. Water atomised 316L SS powders of –150µm, –75µm, –38µm were used in the investigation. Their characteristics are shown in Table 1. Coarse powder parts have a higher debinding rate than fine powder parts under the same conditions. It can be seen in Figure 18 that the binder weight loss of the compacts made by –75µm powder is higher than that of the compacts made by –38µm powder in the same debinding schedule, especially for the compacts with the thickness of 10 mm and 15mm. Coarse powder parts have a lower possibility of introducing defects than fine powder parts under the same conditions. The photographs of the as-debound parts with the thickness of 15mm are shown in Figure 19. It can be seen that the appearance of the compact made by –75µm powder is good while defects are observed in the compact made by –38µm powder in the same debinding schedule. Table 2 provides the mechanical properties of compacts with different sized powders. MIM parts with satisfactory mechanical properties can be obtained by –75µm
Figure 19. The as-debound compacts with the thickness of 15mm (a) –75µm powder compact (good) (b) –38µm powder compact(distorted).
Figure 20. SEM of 316L stainless steel powders.
Figure 21. Mechanical properties of sintered stainless steels.
Table 4. Mechanical properties of as sintered and heat-treated Fe2Ni alloy. Alloy
As-sintered
Heat-treated
Relative density /%
Tensile strength /MPa
Elongation Hardness /% /HRB
Impact work /J•cm-3
Tensile strength /MPa
Elongation /%
Hardness /HRC
Impact work /J•cm-3
Fe-2Ni
95.3
562
12.0
44.6
7.60
1046
5.56
33.7
8.58
Fe-2Ni-0.5Cr
94.7
754
9.2
62.6
5.85
1240
4.90
35.8
4.80
Fe-2Ni-1Cr
94.2
941
7.7
69.0
4.44
1061
3.18
40.6
3.40
Fe-2Ni-3Cr
91.5
975
4.0
73.3
3.05
1060
2.70
41.0
2.00
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Investigations into improving properties of low alloy steel have shown that chemical composition modification is more effective than process optimisation and heat treatment.
Figure 22a. Scanning election micrographs for sintered samples (3000X).
Figure 22b. Scanning election micrographs for sintered samples (3000X).
and –38µm WA powder, which exceed MPIF standard of sintered SS parts. With the application of inert gas and a moderate cooling rate, gas atomisation can produce powder with low oxygen content and spherical particle shape. In contrast with gas atomisation, water atomisation is carried out under higher liquid pressure. Because of the liquid atomising agent and the rapid cooling rate, it produces the powder with higher oxygen content and less spherical particle shape. However, water atomised SS powder with almost completely spherical particle shape is also available. The effect of the atomisation medium on the sintering of austenitic SS can be studied individually by eliminating the influence of particle shape and particle size using the very close particle size and powder morphology which was ignored by previous studies. The powder morphologies of the gas atomised and water atomised 316L stainless steel powders are shown in Figure 20. The
densities and mechanical properties of the compacts are given in Figure 21. Gas atomised SS compacts have a relatively higher density and tensile strength, while water atomised SS compacts have a lower density and higher elongation. It is clear that the mean particle size of the gas atomised and water atomised powders was very close, and the particle size distribution of them is almost the same. It can also be seen that there was no obvious difference of particle shape. As a result, the powder morphology and particle size should not be the key factor affecting the densification and microstructure of these two kinds of sintered SS compacts. It was found from Table 3 that the oxygen content and silicon content of water atomised powder were much higher than that of gas atomised powder. Figure 22 shows the fractographs of the sintered compacts. It is clear in Figure 22 (b) that there are certain second-phase particles dispersed in the sintered compacts produced by water atomised powder Energy dispersive X-ray analysis (EDAX)
suggests that the second-phase particles should be the silicon oxide. On the other hand, there is no second-phase particle in Figure 22 (a). It corresponds with the fact that the silicon content is negligible in the sintered compacts produced from gas atomised powder. Powder chemistry determines the difference of the sintering densification processes and influences the microstructure, mechanical properties and corrosion resistance of sintered compacts. The silicon oxides enriching particle surfaces and pore surfaces are nearly impossible to remove. The compacts cannot attain full densification due to the obstacle effect caused by the silicon oxide diffusing towards grain boundary and deteriorating the bonding between particles during sintering. Consequently, the grain of sintered water atomised SS is harder to grow, the grain size is smaller than that of gas atomised SS and the density of water atomised SS is slightly lower. Most of the early applications of MIM in China were based on Fe2Ni low alloy steel, including business equipment, firearms, household, personal care and microelectronics where a key requirement was
Table 5. Mechanical properties of as sintered and heat-treated Fe2Ni alloy with Mo additions. Alloy
As-sintered
Heat-treated
Relative density /%
Tensile strength /MPa
Elongation /%
Hardness /HRB
Tensile strength /MPa
Elongation /%
Hardness /HRB
Fe-2Ni+0.3C
97.2
321.0
22.9
32
406.9
24
66
Fe-2Ni-1Mo+0.3C
96.8
375.6
13.3
53
505.6
9.1
88
Fe-2Ni-2Mo+0.3C
95.6
401.4
11.0
59
580.1
8.4
92.5
Fe-2Ni-5Mo+0.3C
95.0
549.2
6.3
88
690.4
5.0
115
Fe-2Ni-1Mo+0.5C
96.0
423.2
8.7
79
1089.2
4.1
107.5
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mechanical properties. Process optimisation investigations to improve the properties of Fe2Ni low alloy steel have shown however, that chemical composition modification is more effective than process optimisation and heat treatment. The team at Central South University looked at the influence of chromium (Cr) and molybdenum (Mo) additions on properties of Fe2Ni low alloy steel. Table 4 provides the mechanical properties of as sintered and heat-treated Fe2Ni low alloy with different amounts of Cr addition. With the increase of Cr addition, the tensile strength and hardness increase while the density, elongation and the impact work of the sintered and heat-treated FeNi-Cr alloy decrease. The reason for the change of the properties can be found in the microstructure morphology shown in Figure 23. The microstructure of Fe2Ni consists of pearlite and ferrite as shown in Figure 24 (a), and it can be found in Figures 23 (b), (c) and (d) that the amount of ferrite increases and the microstructure is refined with the increase of Cr addition. The fracture changes from a dimple fracture to the cleavage fracture shown in Figure 25. Fe-2Ni-0.5Cr alloy has the best set of mechanical properties after heat treatment.
Titanium moulding improves Most titanium alloys manufactured in China are made by the lower cost hydridedehydride process (HDH) instead of using part or all of the expensive gas atomised powder. HDH Ti powder is less suitable for injection moulded high-performance products for two reasons: The first is the irregular shape and unfavourable rheological properties. The second is the susceptibility of titanium to carbon, nitrogen and oxygen. HDH titanium powder can easily be polluted during production. An interesting way to shorten the procedure and avoid the process contamination of HDH powder is using TiH2 powder directly. How to investigate a route to carry out the dehydrogenation in debinding is the problem that needs to be considered. For the dehydrogenation of TiH2 powder: (12)
(13)
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Figure 23. Microstructure morphology of the Fe2Ni with different amount of Cr additions. (a)
(b)
(d)
(c)
Figure 24. SEM fracture morphology of the Fe2Ni with different amount of Cr additions.
Figure 25. Microstructure morphology of the Fe2Ni with different amounts of Mo addition.
Table 6. Summary of current applications for MIM in China. Application field
Component
Medical apparatus and instruments
Orthodontic brackets, Surgical apparatus parts
Firearms
Barrel sleeve, Ratchet, Bayonet base,
Automobiles
Air bag striker, Transmission gear, Safety lock, Port fire
Watch
Watch ring, Watch button, Watch band, Watch Case
Electronics
Capsulation parts, Mobile phone parts, Weight for micro-motor
For isothermal processes, the relationship between ∆rGm, T and PH2 can be represents as:
(14) The partial pressure needed for dehydrogenation process according to the equation is that: When T=200°C, ∆rGm<0,PH2<1.04×10-4Pa; and When T=300°C, ∆rGm <0,PH2 <0.072 Pa It requires a vacuum of 10-4 Pa to carry out the dehydrogenation at 200°C which is very difficult to achieve. However 10-2 Pa can be achieved at 300°C, which means that
the new route has the possibility of being realised through the modification of holding temperature and atmosphere during debinding. Combined with the debinding schedule of wax and polymer according to the DTA and TGA curves, the holding temperature needed for debinding and dehydrogenation can be confirmed. The temperature range of dehydrogenation of TiH2 powder is within that of debinding. For the necessity of dehydrogenation and binder removal, schedule has to be specified. Given a satisfactory dehydrogenationdebinding schedule, the compact shows an acceptable microstructure and mechanical properties (σb=770MPa, δ=4.3%).
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