Properties of High Density Diffusion Bonded Alloys
Ian W. Donaldson, (GKN Sinter Metals, Worcester, MA, USA)
Michael L. Marucci, (Hoeganaes Corporation, Cinnaminson, NJ, USA)
Abstract:
For P/M components, overall mechanical properties can be improved by increasing the
density coupled with alloy additions. This can be seen by the excellent properties
achieved for high performance applications with material compositions based on 1.75%
Ni and 4% Ni diffusion bonded steel powders processed to high densities. Through the
use of an advanced binder system, higher densities with subsequent increases in
mechanical properties can be achieved in a single compaction step. Further densification
can be achieved through the use of the double press, double sinter process coupled with
the warm compaction process. The static and dynamic mechanical properties of warm
compacted and double pressed, double sintered FD-0205 and FD-0405 with densities up
to 7.5 g/cm3 are presented.
Introduction:
Faced with global competitive pressures, a key for continued growth for the P/M industry
lies in the ability to improve on the performance characteristics and cost effectiveness of
materials and processing. One of the areas that was focused on with these key
objectives in mind was to achieve higher densities in a single compaction step through
warm compaction via heating the tools and powder which allowed for increased densities
of about 0.15 g/cm3 over conventional compaction1,2,3.
This technology, the ANCORDENSETM lubricant/binder system, required heating of the
powder and the tooling to about 150°C. The heating of the powder produced processing
complexity, so the objective was to eliminate this aspect in the compaction process. This
led to a recent advancement in warm compaction, which was the development of a new
binder/lubricant system, AncorMax DTM. This binder/lubricant system does not require
powder heating so it eliminates complexity from the compaction process. The
effectiveness of this system with processing FLN2-4405 and a Ni-Mo-Mn-Cu sinterhardening compositions were evaluated and revealed that it provided a means of
achieving a density increase between 0.05 g/cm3 at 415 MPa to greater than 0.1 g/cm3 at
825 MPa without the need to heat powder, with no effect on tensile strength as a function
of compaction method4,5.
The advantages of processing diffusion bonded materials to high densities have been
shown2,7. This paper provides the results of material compositions based on 1.75% Ni
and 4% Ni diffusion bonded steel powders processed to high densities via the new
binder/lubricant system.
Experimental Procedures:
Material, Specimen Preparation and Processing
The scope of the testing was carried out on premixes made using Distaloy 4600A and
Distaloy 4800A diffusion bonded base materials (1.75% Ni and 4% Ni, respectively). The
premixes were made utilizing pilot size equipment to produce 225 kg premixes. The
admixed composition was 0.55% graphite and 0.55% AncorMax D lubricant.
1
Standard test samples for transverse rupture, Charpy impact, and tensile testing were
compacted on tools modified to heat and maintain temperature at +/- 3°C. All samples
were compacted with a die temperature between 57°C to 63°C.
Compaction of the standard test specimens was performed between 415 and 825 MPa.
For the specimens compacted for the double press, double sinter (DPDS) processing,
compaction pressures of 550, 690 and 825 MPa were used. After compaction, the single
press, single sinter (SPSS) test specimens were sintered in a belt furnace at 1120°C or at
1260°C for 20 minutes in a N2 based atmosphere with 10 v/o H2.
For the DPDS samples, the first sinter was performed at 790°C for 20 minutes in a batch
furnace using a 25 v/o N2 - 75 v/o H2 atmosphere. Repressing was performed at
pressures of 550, 690 and 825 MPa for each of the first compaction pressures. After
repressing, the samples were either sintered in a laboratory belt furnace at 1120°C or
1260°C for 20 minutes in a N2 based atmosphere with 10 v/o H2.
Testing
Test specimens were processed and evaluated according to industry standard test
procedures for green density, and sintered TRS and tensile properties8. Tensile
properties were developed from flat, un-machined “dogbone” tensile bars according to
ASTM E8 and MPIF Standard 10. TRS and tensile testing was performed at a crosshead
speed of 0.1 in./min. (2.5 mm/min.). Impact testing was performed on un-notched Charpy
bars per MPIF Standard 40. A Rockwell hardness tester was used for apparent hardness
measurements in the Rockwell A scale. Optical metallurgical analysis was performed on
TRS bars processed from each compaction pressure and sintering temperature.
RESULTS AND DISCUSSION
AncorMax D is an organic binder/lubricant system that does not contain zinc developed to
provide increased densification over conventional lubricants when compacted in heated
tools (57°C to 63°C). The bonding process is utilized to disperse the lubricant uniformly
throughout the powder mix while bonding it to the powder, ensuring that the lubricant
stays properly dispersed and improving the flow rate. This combination allowed for a
reduction in the amount of lubricant while maintaining good lubrication during compaction.
The SPSS and DPDS mechanical properties for both materials developed for the warm
compacted specimens at both sintering temperatures along with the densities achieved
are shown below in Tables I and II. The green and sintered densities reported were
measured on the TRS samples.
One aspect of single compaction of P/M powder materials that must be realized is the
pore free density. This is the density of the green compact if all the porosity could be
removed from the compact, which is dependent on the density and percentage of each of
the premix additions. The practical density limit has been determined to be 98% of the
pore free density. For the FD-0205 material, the calculated pore free density is 7.478
g/cm3, based on 98.9% base iron at 7.86 g/cm3, 0.55% graphite at 2.3 g/cm3 and 0.55%
lubricant/binder at 1.0 g/cm3. For the FD-0405 material, the pore free density is 7.514
g/cm3 (based on a base iron at 7.9 g/cm3). Therefore, the practical density limit of 98% of
the pore free density is 7.329 g/cm3 for FD-0205 and 7.364 g/cm3 for FD-0405. A review
of the data reveals that the 98% pore free density was achieved between 690 and 825
MPa. The densification of the two materials via both SPSS and DPDS at 1120°C is
2
shown in Figure 1. As expected, the higher Ni content material had a slightly higher
density.
Compaction
Pressure
MPa
Green
Density
3
g/cm
Sintered
Density
3
g/cm
TRS
Tensile
Strength
MPa
0.2% Yield
Strength
MPa
Elong.
MPa
Apparent
Hardness
HRA
%
Impact
Energy
J
FD-0205: 1120°C Sinter
415
6.97
6.92
1170
50
599
368
2.0
14
550
7.23
7.17
1365
53
701
419
2.7
20
690
7.34
7.28
1488
55
690
409
2.9
23
825
7.36
7.34
1493
57
720
418
3.0
26
FD-0205: 1260°C Sinter
415
6.97
6.95
1184
50
652
370
1.6
13
550
7.24
7.19
1443
53
727
433
3.2
21
690
7.35
7.32
1551
56
751
455
3.8
30
825
7.38
7.38
1578
57
721
453
3.4
34
FD-0405: 1120°C Sinter
415
6.95
6.94
1278
52
658
394
2.0
14
550
7.19
7.20
1556
57
781
438
2.5
22
690
7.35
7.33
1673
59
832
434
2.9
32
825
7.40
7.38
1700
60
841
430
2.9
31
FD-0405: 1260°C Sinter
415
6.95
6.97
1501
53
757
450
2.0
17
550
7.22
7.23
1683
58
868
471
2.5
26
690
7.36
7.36
1884
59
991
505
2.9
32
825
7.36
7.42
1944
61
1008
527
3.0
42
TABLE I: As-Sintered Mechanical Properties for SPSS Warm Compacted FD-0205 and
FD-0405 Sintered at 1120°C and 1260°C in a 90-10 N2-H2 Atmosphere
Comp/Repress
Pressure
MPa
Green
Density
3
g/cm
Sintered
Density
3
g/cm
TRS
Tensile
Strength
MPa
0.2% Yield
Strength
MPa
Elong.
MPa
Apparent
Hardness
HRA
%
Impact
Energy
J
FD-0205: 1120°C Sinter
550 / 550
7.21
7.29
1553
57
781
439
3.4
35
690 / 690
7.31
7.43
1756
58
772
500
3.0
45
830 / 830
7.39
7.49
1836
58
824
460
3.8
59
FD-0205: 1260°C Sinter
550 / 550
7.23
7.29
1272
58
813
485
3.6
32
690 / 690
7.31
7.44
1295
60
804
485
3.1
48
830 / 830
7.37
7.51
1353
58
826
505
3.8
42
FD-0405: 1120°C Sinter
550 / 550
7.21
7.3
1637
60
867
453
3.0
33
690 / 690
7.27
7.45
1731
62
897
443
3.3
56
830 / 830
7.39
7.52
2002
62
908
455
3.7
68
FD-0405: 1260°C Sinter
550 / 550
7.21
7.30
1680
60
1003
535
2.7
31
690 / 690
7.31
7.46
1663
62
995
516
2.6
37
830 / 830
7.38
7.52
1622
62
1040
555
3.3
41
TABLE III: As-Sintered Mechanical Properties for DPDS Warm Compacted FD-0205 and
FD-0405 Sintered at 1120°C and 1260°C in a 90-10 N2-H2 Atmosphere
3
7.55
Sintered Density (g/cm3)
7.45
7.35
7.25
7.15
7.05
6.95
6.85
400
450
500
550
600
650
700
750
800
850
900
Compaction Pressure (MPa)
FD-0205 SPSS
FD-0405 SPSS
FD-0205 DPDS
FD-0405 DPDS
Figure 1: As-Sintered Density of SPSS Warm Compacted and Sintered FD-0205 and FD0405 at 1120°C in a 90-10 N2-H2 Atmosphere
The results revealed that mechanical properties generally increased with increasing
density, regardless of whether it was SPSS or DPDS. Figure 2 shows TRS and UTS as a
function of density, with a linear increase with increasing density at 1120°C. Figure 3
shows Charpy Impact energy as a function of density. An exponential relationship with
density was shown. There was a decrease in SPSS strength shown at the highest
compaction pressures, which is attributed to the higher elastic recovery under removal of
the compaction load and ejection from the die, leading to some separation between the
particles. It was noted that there was more variation in the high temperature sintering
results. Further investigation is required to determine the reason.
2000
R2 = 0.9222
1800
1600
R2 = 0.9197
1400
1200
TRS
1000
R2 = 0.943
800
600
400
6.80
R2 = 0.8325
UTS
6.90
7.00
7.10
7.20
7.30
7.40
7.50
7.60
Densit y (g/cm3 )
FD-0205 TRS
FD-0405 TRS
Figure 2: As-Sintered TRS and UTS as a Function of Density for Warm Compacted and
Sintered FD-0205 and FD-0405 at 1120°C in a 90-10 N2-H2 Atmosphere
4
80
70
R2 = 0.9299
Charpy Impact (J)
60
50
R2 = 0.8808
40
30
20
10
0
6.70
6.80
6.90
7.00
7.10
7.20
7.30
7.40
7.50
7.60
Sinter Density (g/cm3)
FD-0205
FD-0405
Figure 3: As-Sintered Charpy Impact Energy as a Function of Density for Warm
Compacted and Sintered FD-0205 and FD-0405 at 1120°C in a 90-10 N2-H2 Atmosphere
For the samples sintered at 1120°C, the mechanical properties increase with increasing
density. As shown in figures above, a relatively strong linear relationship for TRS, UTS
and Charpy Impact as a function of density was determined with R2 values generally
greater than 0.88, regardless of compaction or repress compaction pressure. These
results indicate that the densification method did not negatively affect the linearity of
density to tensile properties and exponential relationship with impact typically found with
PM steels.
The microstructural phases present in sintered materials, along with interparticle bonds,
porosity morphology and impurity levels affect the mechanical properties. At low density,
the main determinant is the interparticle strength. At the high densities that were present
in the samples, there is a complex interdependence of the mechanical properties on the
pore morphology and interparticle bonds. In the as-polished microstructure, the pore
morphology and distribution were examined. More pore rounding was evidenced at the
higher sintering temperature. A decrease in pore size with compaction and repress
pressures was found as represented in Figure 4.
Figure 4: As-Polished FD-0205 Sintered at 1120°C. Left – 7.28 g/cm3, right – 7.43 g/cm3.
5
The etched microstructures from Figure 4 at 1120°C for the two densities for FD-0205 are
shown in Figure 5. The microstructure consists of divorced pearlite, fine pearlite, nickelrich regions, bainite and martensite around the nickel-rich regions. The presence of
martensite around the pores would lead to some loss in toughness. Pore rounding and
more pronounced nickel diffusion was evident in the high temperature samples.
Figure 5: FD-0205 Sintered at 1120°C – from Figure 4.
2% Nital / 4% Picral
CONCLUSIONS
1. The new lubricant/binder system is viable for warm compaction of FD-0205 and
FD-0405 to achieve high density in a single compaction step.
2. A density greater than 96% of theoretical density can be achieved through the
combination of warm compaction and DPDS.
3. Regression analysis revealed that for parts sintered at 1120°C, the mechanical
properties increased as a function of density, regardless of densification method.
ACKNOWLEDGEMENTS
The authors wish to thank Mr. T. Murphy from Hoeganaes Corporation for his support.
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