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Full density sintering of large powders
Full density sintering of Sintering to full density is the next major development for press and sinter powder metallurgy, providing a means to compete with castings and forgings. Here, Randall M. German of Penn State (University Par&, PA, USA) looks at strength evolution during sintering to understand the resistance to densification. Full density is possible, but requires microstructure manipulation during sintering to weaken large particles that have inherently low sintering stresses. Based on these princip/es, sintering cycles and alloys are emerging for full density sintering using large powders.
S
intering involves heating a compressed powder to a temperature where atomic motion is stimulated. In the standard theory of sintering, interparticle bonds grow through atomic motion promoted by high temperatures. Early atomic diffusion models for sintering were formulated by Kuczynski starting in 1949. A teaching from these models is that very long times are required to induce densification of large particles. For example, 127 pm copper spheres change from 60% to 67% density after 300 hours at 1020°C. Practical full density sintering processes must be thousands of times faster. Based on new results, we now realize one reason for slow sintering is the anomalous character of the compact strength during heating. Common bulk engineering materials weaken when heated. However, sintering powders are different, since the powder compact strengthens while the parent material is weakening. During 250 m
200
!i %
150
E 'j
100
s F
heating, inter-particle bonds grow between contacting particles that generate strength, even without densification. Thus, powders undergo strengthening for a good portion of the sintering cycle. It is this increasing strength that hinders sintering densiflcation. Consequently, if faster sintering is required, then softening is necessary to allow inherent sintering stresses to densify the compact. Large powders usually undergo little sintering densification. On the other hand, small powders, such as encountered in injection moulding, can be sintered to full density. The difference in behaviour depends on the sintering stress, atomic motion, and component strength - each of these will be taken up in sequence to establish the principles for full density sintering.
Sintering
Sintering is sensitive to the particle size, with small powders easily sintered to full density. The rate of sintering depends in part on the microstructure scale, as measured by particle size. Smaller powders have more surface energy that provides a larger driving force for sintering. A measure is the sintering stress, since the surface energy times the surface curvature (inverse of the particle size) gives units of stress. All powders have an inherent sintering stress that decreases as the particle size becomes larger. Most metals have surface energies between 1 and 2 J.me2. Since the microstructure scale depends on the particle size, estimates of the sintering stress are possible. For example, 10 pm stainless steel powder used in injection moulding reaches 6 MPa sintering stress during densification. For a larger powder, the sintering stress can be as small as 0.02 MPa. Thus, for metals the sintering stress probably ranges from 0.01 to 10 MPa. Note these are not high stresses, so most metals sinter bond (strengthen) but resist densification during sintering. The higher sintering stress is one reason small powders are easier to sinter dens&
Atomic 50 0 200
400
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800
1000
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Temperature, "C
_ FIGURE 1: The yield strength for bulk, annealed 316L versus test temperature. As with most engineering materials, thermal softening occurs with higher temperatures to a point where strength is lost at the melting range.
18 MPR July/August
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stress
motion
Atomic mobility is the other determinant of the sintering rate. It reflects the rate atoms vibrate and move to new positions. Atomic motion is very sensitive to temperature, increasing rapidly during heating, and is most rapid in the presence of a liquid phase. Improved sintering rates come with liquid phases because atomic motion in liquids is at least loo-times faster than in solids. Faster
see front matter 0 1999, Elsevier Science Ltd
All rights reserved
atomic motion is not enough to cause densification. For example, copper is used in many ferrous steels where it enhances sinter bonding, but has little benefit on densification. So, atomic motion by itself is not sufficient to induce full density. In alloys such as Fe-Cu-C, interparticle sinter bonds grow, the sintered compact is much stronger, but the porosity is unchanged by sintering. Thus, rapid atomic motion in a liquid phase is not sufficient to induce densification with a large particle size and low sintering stress.
Component strength The missing link in this problem of large particle sintering densification traces to the material strength at the sintering temperature. Wrought materials weaken at high temperatures, a feature known as thermal softening. Figure 1 plots the strength of annealed bulk austenitic stainless steel versus test temperature. Based on the extrapolation, at 1250°C it has a strength of 25 MPa. This strength is above the sintering stress even for PIM grades of powders according to the above estimations. Note that above 1300°C the strength falls below 10 MPa, and approaches stress. the range of the sintering Consequently, densification would be expected for a 10 pm stainless steel powder when sintered at temperatures above 13OO”C, in agreement with practice. Hence, the difficulty in sintering large particles to full density corn& from a low sintering stress and high in situ strength. A pressed compact starts with a typical green strength between 2 and 20 MPa. Unlike a bulk material that thermally softens, powders increase strength during heating due to sinter bonding. Unfortunately, until recently the in. situ strength evolution in sintering was unknown and only estimated. Accordingly, a special test apparatus was constructed for performing strength tests on compacts during sintering. Figure 2 shows a picture of the device, known as the Flaming Tensile Tester. The flaming aspect is because the furnace burns the reducing atmosphere around the upper ram. Early experiments determined that powder compacts were too weak for tensile testing so this version relies on a threepoint transverse rupture test. Recent studies have focused on polymer burnout events, which significantly weaken the compact. With respect to sintering, Figure 3 gives results from a study by Shoales and German for the in situ strength of die pressed bronze powder during heating at 10”C.min-l. Note the green strength is 10 MPa. Shown on this plot are the in situ and sintered strengths for compacts heated to various temperatures up to 850°C. The in situ strength was measured by fracturing compacts as they reached various test temperatures, while the sintered strengths were determined on parallel samples cooled from that point to room temperature. This latter curve is widely reported as part of sintering studies, showing more sintered strength with a higher temperature.
FlGURE 2: The Flaming Tensile Tester used to measure in situ strength during sintering for assessment of densification and thermal cycle effects on distortion. Gregory Shoales and Kimberly Comstock are shown operating the device. (Photo courtesy of Julian Thomas.) 750
m
500
$ r F g u-l
250
400
600
Temperature, ‘C
FIGURE 3: Strength versus sintering temperature for die compacted 26 pm bronze powder with a green density of 7.66 g.cmm3 heated at lO”C.min-‘. The green strength was 10 MPa, and by 400°C the in situ strength fell to 4 MPa, before sinter bonding caused the in situ strength to rise to a peak near 100 MPa at 600°C. At higher temperatures the strength declined, especially when a liquid phase formed. Also shown on this graph is the room temperature strength after heating to each temperature, giving evidence for the large thermal softening associated with the higher temperatures. For bronze powder, initial annealing occurs up to approximately 4OO”C, lowering the in. situ strength to 4 MPa. Beyond 400°C the strength climbs rapidly due to sinter bonding, and peaks at 600°C. As heating progressed, the compacts thermally soften but
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MPa. However, significant shrinkage is delayed to temperatures above 800°C where the in situ strength falls to ranges approaching the sintering stress.
Mechanical view
-1 1 0
200
400
600
800
25
From such measures, a new view of sintering densification is possible based on the in situ mechanical response of the compact. It involves the simultaneous consideration of the sintering stress and compact strength evolution. A solid deforms when the stress exceeds the strength. At high temperatures, slow densification occurs by creep. Ignoring creep gives a simple notion for full density sintering in short times based on the in situ stress state. The sintering stress arises from the curved surfaces. Accordingly, sintering densification is determined by the component strength. If the compact is weak (thermally softened) compared with the sintering stress, then it densifies. Unfortunately, large particles have a low sintering stress and resist densification. Consider the sintering of stainless steel. Figure 1 showed the bulk strength of 316 stainless steel extrapolated to the melting temperature. In a powder compact, the green strength is low. As sinter bonds grow during heating, the compact strengthens. Densification requires compact softening but, as evident in Figure 3, much of the sintering cycle acts to promote strengthening through the growth of sinter bonds. This is an inherent problem in the sintering densification of large powders - they strengthen at low temperatures and remain too strong to densify. Only near the melting temperature will the compact sufficiently weaken to densify in response to the low sintering stress. Even for an initially loose powder, such as those formed by injection moulding, considerable strengthening occurs during heating, but the sintering stress is high so high sintering temperatures allow sufficient compact softening to induce densification.
ID
Densification of large powders
1000
,
Temperature, ‘C SGURE 4: Dilatometer measured sintering shrinkage versus sintering temperature for die compacted 26 pm bronze powder with a green density of 7.66 g.cmm3, heated at lO”C.min-‘. Note that most of the sintering process occurs with minor shrinkages, yet according to Figure 3, considerable strengthening occurs. Large shrinkage is delayed to a high temperature where the compact has undergone considerable thermal softening. (a)
100
20
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1350
b)
1450 1400 Test temperature, “C
1
20
400
100
450
500
550
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650
Test temperature, ‘C FIGURE 5: Plots showing the rapid loss of tensile strength and ductility for full density steel and aluminium alloys near the solidus temperature: (a) 0.2% C steel as reported by C.H. Yu et al., Mates Trans. Japan Inst. Metals, 1996, vol. 37, pp. 125 1- 1257, and (b) Al- 1% Si as reported by A. R. E. Singer and S.A. Cot&e/l, J. Inst. Metals, 1947, vol. 73, pp. 33-54. continue to have higher sintered strengths up to 800°C. For these same compacts, dimensional change shows a different view of sintering. Parallel dilatometer and differential thermal analysis tests show first melt formation at 861°C. Dilatometer measured shrinkage (in situ dimensional change) is plotted versus temperature in Figure 4 for a lO”C.min-’ heating rate. Shrinkage initiates near 6OO”C, the point where the first in situ softening occurs. At this point the sintered strength at room temperature is already more than 400
20 MPR July/August 1999
The in situ strength evolution findings show the importance of temperature in softening the sintering compact. The pathway shown here for bronze, where strength increases before densification, has been verified for several other materials. For example, conventional mixed water atomized iron and copper powders with graphite (2 wt% Cu and 0.8% C), compacted to 7.37 g.cmm3gives an in situ strength at 1100°C (after heating at 10”C.minl) near 56 MPa. Data on the high temperature strength of steel predicts 63 MPa at 1120°C for bulk steel, so the 56 MPa measured strength is reasonable. This strength is probably lOO-fold larger than the sintering stress for the typical large water atomized iron powder. Accordingly, only slow creep densification will occur in sintering. Thus, the fundamental problem in sintering large powders to full density is the in situ strength. As particle size increases, the sintering stress decreases, yet sinter bonding gives a
been applied to several systems. Unfortunately, details on these results are proprietary to the research sponsors, but the concepts are well developed using both mixed elemental and prealloyed large powders.
Dimensional control
Sintering shrinkage, % +
L
Distortion
- - Deflection
-v- Rate
FIGURE 6: Distortion as measured by dimensional change after sintering ‘distortion’, in situ video imaging of a cantilever section ‘deflection’ and time dependent creep ‘rate’ versus the sintering shrinkage for initially loose (5.2 g.cmm3)22 pm bronze powder (heating rate = lO”C.min-‘). The results illustrate densification occurs prior to distortion. 20 ,
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0
10
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30
40
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In situ strength, MPa FIGURE 7: A plot of the sintering shrinkage versus in situ strength for 22 pm bronze powder heated at 5”Cmin-‘. Here rapid densification corresponds to the strength falling below 10 MPa. high strength at the sintering temperature. In short, the sintered material is too strong at the sintering temperature to respond to the sintering stress and fails to densify. So, how can we sinter large powders to full density? The solution is to weaken (at the peak sintering temperature) the sinter bonds that form during heating. One route is via liquid phases that penetrate the grain boundaries. Bulk materials undergo significant weakening when a liquid penetrates the grain boundaries. Tensile tests on iron and aluminiurn alloys near the solidus temperature give strengths well below 1 MPa when a grain boundary liquid forms. Figure 5 shows two examples of such results taken from Yu et aZ., and Singer and Cottrell. Both alloys exhibit a sudden strength loss and ductility loss near the solidus. Sometimes this phenomenon is termed liquid metal embrittlement. In this manner we can rationalize the beneficial effects of boron on stainless steel sintering, where rapid densiflcation occurs with the formation of a grain boundary liquid. Likewise, supersolidus liquid phase sintering depends on the formation of grain boundary liquid films to promote rapid densification. Alloying additions that form liquid phases on the grain boundaries weaken the structure during sintering and promote densification, independent of the particle size. This principle has
22 MPR July/August 1999
One major difficulty with attaining a low in situ strength for sintering densiflcation is concomitant distortion. To investigate this problem, experiments were conducted with loose 22 pm bronze powder where the initial strength was as low as possible. Above 400°C strength evolution followed the same pattern as observed with the die compacted bronze, but the strength was not measurable up to 400°C. Beyond that temperature, the lower density loose powder exhibited a lower strength - for example, it was 51 MPa at 800°C (versus 100 MPa for the die compacted bronze). Curiously, this powder did not distort once sinter bonding created in situ strength. Most important, even with a lower strength it did not distort until temperatures above 900°C. This is most evident in Figure 6, which shows densification occurs before significant distortion. These data are from the thesis of La1 using loose 22 pm bronze powder heated at 10”C.min-l. Shrinkage was measured using dilatometry. Distortion after sintering was quantified using the difference in point-to-point (anisotropic) final dimensions. Deflection and its rate were measured in situ via video imaging. All three measures show resistance to distortion up to near full density, in spite of starting with loose powder. The in situ strength evolution followed a temperature dependence similar to that shown in Figure 3. Figure 7 plots sintering shrinkage versus in situ strength for this 22 ,um bronze powder heated at 5”C.min-l, with notations of the fractional density (starting from 58% loose powder density). Significant densification occurs as the compact thermally softens. It is characteristic of most systems that densification occurs before distortion (beyond distortion from non-uniform heating or green density gradients). For the loose powder experiments, the sintering stress was calculated for each point where the in situ strength was measured, leading to the plot shown in Figure 8. Two critical regions are marked, one where densification occurred and the second where distortion became evident. Most of the sintering shrinkage occurred before the first detectable evidence of distortion. Distortion occurs when the compact strength falls below 1 MPa (below 150 psi). Note that strength measures were conducted up to 95O”C, where the compact distorted under its own weight with an in situ strength of 0.002 MPa. Other than green density and heat transfer problems, it is excessive compact weakening at high temperatures that causes a loss of dimensional uniformity. The sintering stress is hydrostatic and only tends to spheroidize a compact, but gravity is not isotropic. The larger the gravitational stress, the larger the expected distortion. Such a finding was verified using a variety of height and diameter compacts. The
distortion data rationalize when viewed on a height to diameter ratio basis, as shown in Figure 9 where the compacts were intentionally heated to induce distortion. A simple consequence is to keep the height small to minimize gravitational stresses. The key question is how to soften large powders with a grain boundary liquid while not lowering the strength to a point where gravity causes distortion? The response to this is to control temperature and compact strength or to keep the component squat to reduce the height. Consequently, new thermal cycles become evident for the optimization of dimensional control in full density, large powder structures. These findings point to some exciting new opportunities for powder metallurgy. This fundamental research on strength evolution during sintering and gravity effects on distortion has isolated several new alloys and sintering cycles that allow full density sintering with large powders.
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0.001 0.001
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Sinteringstress,MPa
FIGURE 8: The sintering pathway shown in terms of the calculated sintering stress and in situ sintering strength for the same 22 pm bronze powder heated at 5”Cmin-‘. Rapid densification without distortion occurs over a narrow range of stress and strength conditions.
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Summary Basic stress-strength mechanical ideas are applied to the sintering of large powders. Unlike bulk materials that thermally soften with heating, the pathway for powder compacts is more complicated. A combination of annealing, sinter bonding, and thermal softening determines the resistance to densification for both large and small powders. Small powders have higher sintering stresses and prove easier to densify when the in situ strength falls at high temperatures. On the other hand, large powders have lower sintering stresses and the in situ strength is too large to allow much densification with the same sintering cycles. Liquid phases that penetrate grain boundaries provide a means to lower strength during sintering. With large powders the sintering stress is low, so only a low in situ strength will allow densification. From these principles, several alloys have been designed for full density processing. Further, thermal cycles emerge to manipulate strength for improved dimensional control. New commercial products are emerging from this new understanding of sintering densiflcation.
This paper resulted from collaborations with members of the Sintering Study Group at Penn State. Valuable contributions were made by Dr Anand La1 (now with Motorola), Dr Gregory Shoales (now with the US Air Force Academy), Dr Jason Liu and Dr Ronald Iacocca (both with the P/M Lab at Penn State), and Dr Anish Upadhyaya (now with Widia Carbide). Funding for application-specific research was from many sponsors, while fundamental research support was provided by NASA and the National Science Foundation.
Background references Theory and Practice,
John Wiley and Sons, (1996). (2) A. Lal, Mechanisms and Mechanics Loss
During
Supersolidus
J
0.5
Liquid
of Shape Phase
10
1.5
2.0
Aspect ratio, hwghtidiameter
FIGURE 9: Results from initially loose 22 pm bronze powder compacts with varying height to diameter ratios, sintered at 975°C where measurable distortion occurs (heating rate = S”C.min-’ sintering time = 30 min). A case is easily made for short, squat compacts to reduce distortion in sintering. Ph.D. Thesis, Engineering Science and Mechanics Department, The Pennsylvania State University, (1999). (3) A. Lal, J. Liu, G.A. Shoales, and R.M. German, Sintering,
(4)
(5)
Acknowledgements
(11 R. M. German, Sintering
/ I
(6)
(7)
‘Component Shape Retention in Supersolidus Liquid Phase Sintering of Prealloyed Powders’, Advances in Powder Metallurgy and Particulate Materials - 1998, Vol. 3, Metal Powder Industries Federation, (1998), pp. 12.33-12.43. A. Lal, G A. Shoales, and R.M. German, ‘Thermal Characterization to Study the Sintering of Bronze Powders,’ Proceedings of Thermal Conductivity 24, Technomic Publ., (1998), pp. 319-330. G.A. Shoales and R.M. German, ‘In Situ Strength Evolution during the Sintering of Bronze Powders,’ Metallurgical and Materials Truns, (1998), Vol. 29A, pp. 1257-1263. G.A. Shoales and R.M. German, ‘Combined Effects of Time and Temperature on Strength Evolution Using Integral Work-of-Sintering Concepts,’ Metallurgical and Materials Trans, (19991, vol. 30A, pp. 465470. A. Upadhyaya, R.G. Iacocca, and R.M. German, ‘Gravitational Effects on Compact Shaping and Microstructure during Liquid-Phase Sintering,’ J. Metals (JOMI, (1999),Vol. 51, no. 4, pp. 37-40.
Contact: Profi Randall M. German Department of Engineering Science P/M Lab, 118 Research West The Pennsylvania State University University Park, PA 16802-6809, USA Tel: +l-814-863-8025. Fax: +l-814-863-8211.
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S
intering involves heating a compressed powder to a temperature where atomic motion is stimulated. In the standard theory of sintering, interparticle bonds grow through atomic motion promoted by high temperatures. Early atomic diffusion models for sintering were formulated by Kuczynski starting in 1949. A teaching from these models is that very long times are required to induce densification of large particles. For example, 127 pm copper spheres change from 60% to 67% density after 300 hours at 1020°C. Practical full density sintering processes must be thousands of times faster. Based on new results, we now realize one reason for slow sintering is the anomalous character of the compact strength during heating. Common bulk engineering materials weaken when heated. However, sintering powders are different, since the powder compact strengthens while the parent material is weakening. During 250 m
200
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150
E 'j
100
s F
heating, inter-particle bonds grow between contacting particles that generate strength, even without densification. Thus, powders undergo strengthening for a good portion of the sintering cycle. It is this increasing strength that hinders sintering densiflcation. Consequently, if faster sintering is required, then softening is necessary to allow inherent sintering stresses to densify the compact. Large powders usually undergo little sintering densification. On the other hand, small powders, such as encountered in injection moulding, can be sintered to full density. The difference in behaviour depends on the sintering stress, atomic motion, and component strength - each of these will be taken up in sequence to establish the principles for full density sintering.
Sintering
Sintering is sensitive to the particle size, with small powders easily sintered to full density. The rate of sintering depends in part on the microstructure scale, as measured by particle size. Smaller powders have more surface energy that provides a larger driving force for sintering. A measure is the sintering stress, since the surface energy times the surface curvature (inverse of the particle size) gives units of stress. All powders have an inherent sintering stress that decreases as the particle size becomes larger. Most metals have surface energies between 1 and 2 J.me2. Since the microstructure scale depends on the particle size, estimates of the sintering stress are possible. For example, 10 pm stainless steel powder used in injection moulding reaches 6 MPa sintering stress during densification. For a larger powder, the sintering stress can be as small as 0.02 MPa. Thus, for metals the sintering stress probably ranges from 0.01 to 10 MPa. Note these are not high stresses, so most metals sinter bond (strengthen) but resist densification during sintering. The higher sintering stress is one reason small powders are easier to sinter dens&
Atomic 50 0 200
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Temperature, "C
_ FIGURE 1: The yield strength for bulk, annealed 316L versus test temperature. As with most engineering materials, thermal softening occurs with higher temperatures to a point where strength is lost at the melting range.
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stress
motion
Atomic mobility is the other determinant of the sintering rate. It reflects the rate atoms vibrate and move to new positions. Atomic motion is very sensitive to temperature, increasing rapidly during heating, and is most rapid in the presence of a liquid phase. Improved sintering rates come with liquid phases because atomic motion in liquids is at least loo-times faster than in solids. Faster
see front matter 0 1999, Elsevier Science Ltd
All rights reserved
atomic motion is not enough to cause densification. For example, copper is used in many ferrous steels where it enhances sinter bonding, but has little benefit on densification. So, atomic motion by itself is not sufficient to induce full density. In alloys such as Fe-Cu-C, interparticle sinter bonds grow, the sintered compact is much stronger, but the porosity is unchanged by sintering. Thus, rapid atomic motion in a liquid phase is not sufficient to induce densification with a large particle size and low sintering stress.
Component strength The missing link in this problem of large particle sintering densification traces to the material strength at the sintering temperature. Wrought materials weaken at high temperatures, a feature known as thermal softening. Figure 1 plots the strength of annealed bulk austenitic stainless steel versus test temperature. Based on the extrapolation, at 1250°C it has a strength of 25 MPa. This strength is above the sintering stress even for PIM grades of powders according to the above estimations. Note that above 1300°C the strength falls below 10 MPa, and approaches stress. the range of the sintering Consequently, densification would be expected for a 10 pm stainless steel powder when sintered at temperatures above 13OO”C, in agreement with practice. Hence, the difficulty in sintering large particles to full density corn& from a low sintering stress and high in situ strength. A pressed compact starts with a typical green strength between 2 and 20 MPa. Unlike a bulk material that thermally softens, powders increase strength during heating due to sinter bonding. Unfortunately, until recently the in. situ strength evolution in sintering was unknown and only estimated. Accordingly, a special test apparatus was constructed for performing strength tests on compacts during sintering. Figure 2 shows a picture of the device, known as the Flaming Tensile Tester. The flaming aspect is because the furnace burns the reducing atmosphere around the upper ram. Early experiments determined that powder compacts were too weak for tensile testing so this version relies on a threepoint transverse rupture test. Recent studies have focused on polymer burnout events, which significantly weaken the compact. With respect to sintering, Figure 3 gives results from a study by Shoales and German for the in situ strength of die pressed bronze powder during heating at 10”C.min-l. Note the green strength is 10 MPa. Shown on this plot are the in situ and sintered strengths for compacts heated to various temperatures up to 850°C. The in situ strength was measured by fracturing compacts as they reached various test temperatures, while the sintered strengths were determined on parallel samples cooled from that point to room temperature. This latter curve is widely reported as part of sintering studies, showing more sintered strength with a higher temperature.
FlGURE 2: The Flaming Tensile Tester used to measure in situ strength during sintering for assessment of densification and thermal cycle effects on distortion. Gregory Shoales and Kimberly Comstock are shown operating the device. (Photo courtesy of Julian Thomas.) 750
m
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$ r F g u-l
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Temperature, ‘C
FIGURE 3: Strength versus sintering temperature for die compacted 26 pm bronze powder with a green density of 7.66 g.cmm3 heated at lO”C.min-‘. The green strength was 10 MPa, and by 400°C the in situ strength fell to 4 MPa, before sinter bonding caused the in situ strength to rise to a peak near 100 MPa at 600°C. At higher temperatures the strength declined, especially when a liquid phase formed. Also shown on this graph is the room temperature strength after heating to each temperature, giving evidence for the large thermal softening associated with the higher temperatures. For bronze powder, initial annealing occurs up to approximately 4OO”C, lowering the in. situ strength to 4 MPa. Beyond 400°C the strength climbs rapidly due to sinter bonding, and peaks at 600°C. As heating progressed, the compacts thermally soften but
MPR July/August
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MPa. However, significant shrinkage is delayed to temperatures above 800°C where the in situ strength falls to ranges approaching the sintering stress.
Mechanical view
-1 1 0
200
400
600
800
25
From such measures, a new view of sintering densification is possible based on the in situ mechanical response of the compact. It involves the simultaneous consideration of the sintering stress and compact strength evolution. A solid deforms when the stress exceeds the strength. At high temperatures, slow densification occurs by creep. Ignoring creep gives a simple notion for full density sintering in short times based on the in situ stress state. The sintering stress arises from the curved surfaces. Accordingly, sintering densification is determined by the component strength. If the compact is weak (thermally softened) compared with the sintering stress, then it densifies. Unfortunately, large particles have a low sintering stress and resist densification. Consider the sintering of stainless steel. Figure 1 showed the bulk strength of 316 stainless steel extrapolated to the melting temperature. In a powder compact, the green strength is low. As sinter bonds grow during heating, the compact strengthens. Densification requires compact softening but, as evident in Figure 3, much of the sintering cycle acts to promote strengthening through the growth of sinter bonds. This is an inherent problem in the sintering densification of large powders - they strengthen at low temperatures and remain too strong to densify. Only near the melting temperature will the compact sufficiently weaken to densify in response to the low sintering stress. Even for an initially loose powder, such as those formed by injection moulding, considerable strengthening occurs during heating, but the sintering stress is high so high sintering temperatures allow sufficient compact softening to induce densification.
ID
Densification of large powders
1000
,
Temperature, ‘C SGURE 4: Dilatometer measured sintering shrinkage versus sintering temperature for die compacted 26 pm bronze powder with a green density of 7.66 g.cmm3, heated at lO”C.min-‘. Note that most of the sintering process occurs with minor shrinkages, yet according to Figure 3, considerable strengthening occurs. Large shrinkage is delayed to a high temperature where the compact has undergone considerable thermal softening. (a)
100
20
1
50
1350
b)
1450 1400 Test temperature, “C
1
20
400
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450
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Test temperature, ‘C FIGURE 5: Plots showing the rapid loss of tensile strength and ductility for full density steel and aluminium alloys near the solidus temperature: (a) 0.2% C steel as reported by C.H. Yu et al., Mates Trans. Japan Inst. Metals, 1996, vol. 37, pp. 125 1- 1257, and (b) Al- 1% Si as reported by A. R. E. Singer and S.A. Cot&e/l, J. Inst. Metals, 1947, vol. 73, pp. 33-54. continue to have higher sintered strengths up to 800°C. For these same compacts, dimensional change shows a different view of sintering. Parallel dilatometer and differential thermal analysis tests show first melt formation at 861°C. Dilatometer measured shrinkage (in situ dimensional change) is plotted versus temperature in Figure 4 for a lO”C.min-’ heating rate. Shrinkage initiates near 6OO”C, the point where the first in situ softening occurs. At this point the sintered strength at room temperature is already more than 400
20 MPR July/August 1999
The in situ strength evolution findings show the importance of temperature in softening the sintering compact. The pathway shown here for bronze, where strength increases before densification, has been verified for several other materials. For example, conventional mixed water atomized iron and copper powders with graphite (2 wt% Cu and 0.8% C), compacted to 7.37 g.cmm3gives an in situ strength at 1100°C (after heating at 10”C.minl) near 56 MPa. Data on the high temperature strength of steel predicts 63 MPa at 1120°C for bulk steel, so the 56 MPa measured strength is reasonable. This strength is probably lOO-fold larger than the sintering stress for the typical large water atomized iron powder. Accordingly, only slow creep densification will occur in sintering. Thus, the fundamental problem in sintering large powders to full density is the in situ strength. As particle size increases, the sintering stress decreases, yet sinter bonding gives a
been applied to several systems. Unfortunately, details on these results are proprietary to the research sponsors, but the concepts are well developed using both mixed elemental and prealloyed large powders.
Dimensional control
Sintering shrinkage, % +
L
Distortion
- - Deflection
-v- Rate
FIGURE 6: Distortion as measured by dimensional change after sintering ‘distortion’, in situ video imaging of a cantilever section ‘deflection’ and time dependent creep ‘rate’ versus the sintering shrinkage for initially loose (5.2 g.cmm3)22 pm bronze powder (heating rate = lO”C.min-‘). The results illustrate densification occurs prior to distortion. 20 ,
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0
0
10
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30
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In situ strength, MPa FIGURE 7: A plot of the sintering shrinkage versus in situ strength for 22 pm bronze powder heated at 5”Cmin-‘. Here rapid densification corresponds to the strength falling below 10 MPa. high strength at the sintering temperature. In short, the sintered material is too strong at the sintering temperature to respond to the sintering stress and fails to densify. So, how can we sinter large powders to full density? The solution is to weaken (at the peak sintering temperature) the sinter bonds that form during heating. One route is via liquid phases that penetrate the grain boundaries. Bulk materials undergo significant weakening when a liquid penetrates the grain boundaries. Tensile tests on iron and aluminiurn alloys near the solidus temperature give strengths well below 1 MPa when a grain boundary liquid forms. Figure 5 shows two examples of such results taken from Yu et aZ., and Singer and Cottrell. Both alloys exhibit a sudden strength loss and ductility loss near the solidus. Sometimes this phenomenon is termed liquid metal embrittlement. In this manner we can rationalize the beneficial effects of boron on stainless steel sintering, where rapid densiflcation occurs with the formation of a grain boundary liquid. Likewise, supersolidus liquid phase sintering depends on the formation of grain boundary liquid films to promote rapid densification. Alloying additions that form liquid phases on the grain boundaries weaken the structure during sintering and promote densification, independent of the particle size. This principle has
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One major difficulty with attaining a low in situ strength for sintering densiflcation is concomitant distortion. To investigate this problem, experiments were conducted with loose 22 pm bronze powder where the initial strength was as low as possible. Above 400°C strength evolution followed the same pattern as observed with the die compacted bronze, but the strength was not measurable up to 400°C. Beyond that temperature, the lower density loose powder exhibited a lower strength - for example, it was 51 MPa at 800°C (versus 100 MPa for the die compacted bronze). Curiously, this powder did not distort once sinter bonding created in situ strength. Most important, even with a lower strength it did not distort until temperatures above 900°C. This is most evident in Figure 6, which shows densification occurs before significant distortion. These data are from the thesis of La1 using loose 22 pm bronze powder heated at 10”C.min-l. Shrinkage was measured using dilatometry. Distortion after sintering was quantified using the difference in point-to-point (anisotropic) final dimensions. Deflection and its rate were measured in situ via video imaging. All three measures show resistance to distortion up to near full density, in spite of starting with loose powder. The in situ strength evolution followed a temperature dependence similar to that shown in Figure 3. Figure 7 plots sintering shrinkage versus in situ strength for this 22 ,um bronze powder heated at 5”C.min-l, with notations of the fractional density (starting from 58% loose powder density). Significant densification occurs as the compact thermally softens. It is characteristic of most systems that densification occurs before distortion (beyond distortion from non-uniform heating or green density gradients). For the loose powder experiments, the sintering stress was calculated for each point where the in situ strength was measured, leading to the plot shown in Figure 8. Two critical regions are marked, one where densification occurred and the second where distortion became evident. Most of the sintering shrinkage occurred before the first detectable evidence of distortion. Distortion occurs when the compact strength falls below 1 MPa (below 150 psi). Note that strength measures were conducted up to 95O”C, where the compact distorted under its own weight with an in situ strength of 0.002 MPa. Other than green density and heat transfer problems, it is excessive compact weakening at high temperatures that causes a loss of dimensional uniformity. The sintering stress is hydrostatic and only tends to spheroidize a compact, but gravity is not isotropic. The larger the gravitational stress, the larger the expected distortion. Such a finding was verified using a variety of height and diameter compacts. The
distortion data rationalize when viewed on a height to diameter ratio basis, as shown in Figure 9 where the compacts were intentionally heated to induce distortion. A simple consequence is to keep the height small to minimize gravitational stresses. The key question is how to soften large powders with a grain boundary liquid while not lowering the strength to a point where gravity causes distortion? The response to this is to control temperature and compact strength or to keep the component squat to reduce the height. Consequently, new thermal cycles become evident for the optimization of dimensional control in full density, large powder structures. These findings point to some exciting new opportunities for powder metallurgy. This fundamental research on strength evolution during sintering and gravity effects on distortion has isolated several new alloys and sintering cycles that allow full density sintering with large powders.
100
a"
10
z r_ F E ;
1
0.1
x c 0.01
0.001 0.001
1
0.1
10
100
Sinteringstress,MPa
FIGURE 8: The sintering pathway shown in terms of the calculated sintering stress and in situ sintering strength for the same 22 pm bronze powder heated at 5”Cmin-‘. Rapid densification without distortion occurs over a narrow range of stress and strength conditions.
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Summary Basic stress-strength mechanical ideas are applied to the sintering of large powders. Unlike bulk materials that thermally soften with heating, the pathway for powder compacts is more complicated. A combination of annealing, sinter bonding, and thermal softening determines the resistance to densification for both large and small powders. Small powders have higher sintering stresses and prove easier to densify when the in situ strength falls at high temperatures. On the other hand, large powders have lower sintering stresses and the in situ strength is too large to allow much densification with the same sintering cycles. Liquid phases that penetrate grain boundaries provide a means to lower strength during sintering. With large powders the sintering stress is low, so only a low in situ strength will allow densification. From these principles, several alloys have been designed for full density processing. Further, thermal cycles emerge to manipulate strength for improved dimensional control. New commercial products are emerging from this new understanding of sintering densiflcation.
This paper resulted from collaborations with members of the Sintering Study Group at Penn State. Valuable contributions were made by Dr Anand La1 (now with Motorola), Dr Gregory Shoales (now with the US Air Force Academy), Dr Jason Liu and Dr Ronald Iacocca (both with the P/M Lab at Penn State), and Dr Anish Upadhyaya (now with Widia Carbide). Funding for application-specific research was from many sponsors, while fundamental research support was provided by NASA and the National Science Foundation.
Background references Theory and Practice,
John Wiley and Sons, (1996). (2) A. Lal, Mechanisms and Mechanics Loss
During
Supersolidus
J
0.5
Liquid
of Shape Phase
10
1.5
2.0
Aspect ratio, hwghtidiameter
FIGURE 9: Results from initially loose 22 pm bronze powder compacts with varying height to diameter ratios, sintered at 975°C where measurable distortion occurs (heating rate = S”C.min-’ sintering time = 30 min). A case is easily made for short, squat compacts to reduce distortion in sintering. Ph.D. Thesis, Engineering Science and Mechanics Department, The Pennsylvania State University, (1999). (3) A. Lal, J. Liu, G.A. Shoales, and R.M. German, Sintering,
(4)
(5)
Acknowledgements
(11 R. M. German, Sintering
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(6)
(7)
‘Component Shape Retention in Supersolidus Liquid Phase Sintering of Prealloyed Powders’, Advances in Powder Metallurgy and Particulate Materials - 1998, Vol. 3, Metal Powder Industries Federation, (1998), pp. 12.33-12.43. A. Lal, G A. Shoales, and R.M. German, ‘Thermal Characterization to Study the Sintering of Bronze Powders,’ Proceedings of Thermal Conductivity 24, Technomic Publ., (1998), pp. 319-330. G.A. Shoales and R.M. German, ‘In Situ Strength Evolution during the Sintering of Bronze Powders,’ Metallurgical and Materials Truns, (1998), Vol. 29A, pp. 1257-1263. G.A. Shoales and R.M. German, ‘Combined Effects of Time and Temperature on Strength Evolution Using Integral Work-of-Sintering Concepts,’ Metallurgical and Materials Trans, (19991, vol. 30A, pp. 465470. A. Upadhyaya, R.G. Iacocca, and R.M. German, ‘Gravitational Effects on Compact Shaping and Microstructure during Liquid-Phase Sintering,’ J. Metals (JOMI, (1999),Vol. 51, no. 4, pp. 37-40.
Contact: Profi Randall M. German Department of Engineering Science P/M Lab, 118 Research West The Pennsylvania State University University Park, PA 16802-6809, USA Tel: +l-814-863-8025. Fax: +l-814-863-8211.
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