By Michael
J. Walter, Medical Alloy Specialist
Carpenter Technology Corp., Wyomissing, PA, USA
November 2006
For several
decades, orthopaedic medical implants have been manufactured mainly from
austenitic stainless steels, titanium and titanium alloys and cobalt-based
alloys. Selection of which alloy system to use for a specific application has
depended upon a variety of design criteria. These have included
biocompatibility, corrosion resistance, tensile strength, fatigue strength,
modulus, wear resistance, processing and cost.
The vast
majority of cobalt-based orthopaedic implants worldwide have been manufactured
using castings of ASTM F75 alloy. Castings in many instances have provided
desirable processing flexibility and lower initial costs. However, distinct
limitations have been associated with castings, such as coarse grain size,
non-uniform microstructural segregation and lower tensile and fatigue strength.
These drawbacks can be overcome by manufacturing cobalt-based implants from
“cobalt-chromium-molybdenum” wrought barstock.
Of the
three wrought Co-28Cr-6Mo alloys covered under ASTM F1537 and used for
orthopaedic medical implants, the lowest-carbon (0.14% max) Alloy 1 (UNS
R31537), has been utilized most frequently. This alloy has been traditionally
manufactured by conventional cast/wrought processing, but can also be
manufactured using powder metallurgy (P/M) processing.
Studies to
characterize the differences in barstock made by each of the two manufacturing
methods have shown distinct advantages for the P/M process. These benefits
include higher strength, improved fatigue resistance and enhanced
microstructural characteristics at both room and elevated temperatures.
Data
collected has confirmed that both methods of manufacturing wrought feedstock
are superior to casting.
Carpenter
conducted the study by manufacturing its version of Alloy 1, known as BioDur®
CCM® Alloy, by conventional cast/wrought processing. The mill also made the
same alloy by Carpenter’s Micro-Melt® powder metallurgy (P/M) process for
comparison purposes.
P/M
Benefits
When
compared with the conventionally produced cast/wrought alloy, bar stock made by
the P/M process was found capable of higher tensile and fatigue strength,
increased hardness, finer grain size and more uniform structure that is less
prone to segregation. These attributes were all found in bars in the typical
as-supplied warm worked, unannealed conditions.
The powder
processed alloy also provided these same relative benefits after exposure to
the elevated temperatures typically associated with annealing or forging of
orthopaedic implants.
The unique
advantages imparted by the P/M process, when carried over to machined and forged
components, may be expected to improve the performance and life of joint
replacement implants and fracture fixation devices such as total shoulder, hip,
knee and shoulder replacements.
Because of
the characteristics typically produced by the P/M process, Micro-Melt®
processed F1537 Alloy 1 can be produced in the warm worked or hot worked
unannealed conditions in smaller diameter bar and wire products than the
conventional cast/wrought alloy. Also, powder processed stock can be made
without the need for cold drawing and annealing, which can be detrimental to
fatigue strength.
With
fatigue strength superior to that of the cast/wrought alloy bar stock, P/M bar
stock could be considered for smaller diameter applications requiring higher
fatigue capability such as pins, rods and wire that are typical to some spinal
applications.
The powder
process should also allow for the production of fully wrought near-net shapes
for applications where higher tensile and fatigue strength are required than
that possible with castings.
Processes
Compared
The
conventional cast/wrought alloy is manufactured typically by vacuum induction
melting (VIM), electro-slag remelting (ESR) ingots, hot forging to billets, hot
rolling into wrought bar stock, then turning and grinding to finish condition.
Carpenter’s
Micro-Melt P/M process is as follows: vacuum induction melt a heat of high
purity gas atomized powder, screen the powder to a predetermined mesh size,
blend several heats to make one master blend, fill stainless canisters and hot
isostatically press (HIP) to full denseness, hot roll into fully wrought bar
stock, turn and grind to finish (Figure 1).
Figure
1. An example of the Micro-Melt® P/M powder metallurgy process.
Properties
Compared
The
conventional cast/wrought alloy is offered in the annealed, or more typically
in the hot worked or warm worked conditions. The powder processed alloy is
typically offered in either the annealed or, more commonly, the warm worked
condition. When manufactured to the same metallurgical condition (such as warm
worked), the P/M alloy typically exhibits higher yield and ultimate tensile
strength. Typical mechanical properties for the alloy manufactured in each
manner are shown in Figure 2.
|
Properties
|
Conventional
Alloy 1:
Hot Worked
|
Conventional
Alloy 1:
Warm Worked
|
Powder-Metallurgy
Alloy:
Warm Worked
|
|
0.2% Yield
(ksi)
Ultimate tensile strength (ksi)
Elongation
(%)
Reduction
in area (%)
Rockwell C Hardness (HRC)
|
135
187
28
23
42
|
150
199
25
21
44
|
162
206
28
24
46
|
Figure
2. Typical properties of the conventional cast/wrought Alloy 1 and P/M Alloy
1.
R. R. Moore
rotating beam fatigue testing was conducted at a test frequency of 6,000 rpm on
bar stock samples of both the P/M alloy and the conventional alloy in the warm
worked condition. The P/M alloy, with its higher tensile strength capability,
showed elevated estimated endurance limits (Figure 3).
|
Stress (ksi)
|
Conventional
Alloy 1 (number of
cycles in millions)
|
Powder-Metallurgy
Alloy 1 (number of
cycles in millions)
|
|
120
125
130
135
140
|
14.6
7.1; 12.1
8.4*; 8.8*
0.01
—
|
10.3
10.0
10.2
4.3; 11.7
4.7; 15.7
|
|
* = Point
of fracture
|
Figure
3. Typical fatigue properties of conventional Alloy 1 and P/M Alloy 1.
The fatigue
results from this study are significantly higher than those found in previous
tests to evaluate fatigue properties of annealed plus cold drawn Alloy 1 bar
stock. The unannealed bar stock (both cast/wrought and P/M) had significantly
higher fatigue properties when compared with annealed and cold drawn Alloy 1
bar stock.
Both the
higher tensile and fatigue strength capability of the P/M alloy are attributed
to the finer grain size and more uniform microstructure produced by the
Micro-Melt powder metallurgy process.
In the warm
worked condition, the P/M alloy has a slightly finer grain size than the
conventionally produced alloy. The standard cast/wrought alloy has an ASTM
grain size of 12.5 with an average grain dimension of 7µ and an average grain
area of 50µ2, as shown in Figure 4. The P/M alloy has
an ASTM grain size of 13.6 with an average grain dimension of 4.6µ and an
average grain area of 22µ2, as indicated in Figure 5.
Figure
4. Typical microstructure of conventional
Alloy 1; warm worked bar stock;
longitudinal section;
1000x; etchant HCL + H2O2 (3%).
Figure
5. Typical microstructure of P/M Alloy 1;
warm worked bar stock; longitudinal
section; 1000x;
etchant HCL + H2O2 (3%).
In further
comparison, the conventionally produced hot worked alloy has an ASTM grain size
of 11.5 with an average grain dimension of 8.7µ.
Effect
of Thermal Treatment
A study was
conducted to evaluate the effects of various thermal treatments on the
microstructure and hardness of the conventional alloy and the P/M alloy.
Samples of the conventional alloy were tested in the unannealed hot worked and
unannealed warm worked condition. Samples of the unannealed warm worked P/M
alloy also were tested. The samples received 30 minute air-cool heat treatments
using a temperature range from 1500ºF (815ºC) to 2100ºF (1149ºC).
Grain
structure and hardness were evaluated on the as-received samples and after each
heat treat cycle. Microstructure showed that the P/M alloy exhibited a finer
ASTM grain size in the as-received unannealed condition, and also maintained
that finer grain structure after each heat treatment evaluated (Figure 6).
Figure
6. Effect of thermal treatment on ASTM grain size
for the P/M alloy (MMCCM)
and conventional Alloy 1.
Data
developed from the evaluation clearly indicates that the powder processed alloy
maintains a consistently finer grain size than the cast/wrought alloy
throughout the heat treatment range, especially after exposure to temperatures
above 1900ºF (1038ºC).
The
dramatic differences in grain size capability between the P/M alloy and the
cast/wrought alloy can be readily discerned in Figure 7, showing
structure for the conventional cast/wrought alloy and Figure 8, showing
structure for the P/M alloy.
Figure
7. Cast/wrought Alloy 1; warm worked + 2100ºF; longitudinal
section; 100x;
etchant HCL + H2O2 (3%); ASTM grain size 4.5;
avg. grain area 11,000 µ2.
Figure
8. P/M Alloy 1; warm worked + 2100ºF; longitudinal section; 100x;
etchant HCL
+ H2O2 (3%); ASTM grain size 7.0; avg. grain area 2,000 µ2.
Of
particular interest is the grain size difference noted in Figs. 7 and 8 after a
2100ºF/30 minute cycle. This is a relatively common forging temperature used
during the processing of orthopaedic implants. After exposure to a temperature
of 2100ºF (1149ºC), cast/wrought Alloy 1 developed a grain size of ASTM 4.5
with an average grain area of approximately 11,000µ2. In contrast,
the Micro-Melt P/M alloy developed an ASTM grain size of 7 with an average
grain area of approximately 2,000µ2.
More
Advantages
Additional
findings further emphasize the unique characteristics of the P/M alloy. At
1900ºF (1038ºC), which is within the carbide precipitate range for the F1537
Alloy 1 tested, a significant difference was observed in the nature of the
carbide precipitate between the cast/wrought alloy and the P/M alloy.
As can be
seen in Figures 9 and 10, the cast/wrought alloy developed a banded
carbide precipitate while the P/M alloy tended to have more uniformly dispersed
carbide precipitate. This shows that the P/M process greatly decreases the
likelihood for localized segregation and possible banding, which can occur in
the cast/wrought alloy at times.
Figure
9. Cast/wrought Alloy 1; warm worked + 1900ºF; longitudinal
section; etchant
HCL + H2O2 (3%); carbide precipitate banding.
Figure
10. P/M Alloy 1; warm worked + 1900ºF; longitudinal section;
400x; etchant HCL
+ H2O2 (3%); uniform carbide precipitate.
The
precipitate in both the cast/wrought and P/M material was completely solutioned
at 1950ºF (1066ºC). Once solutioned, the carbide does not tend to
re-precipitate if exposed again to temperatures in the 1600ºF (871ºC) to 1900ºF
(1038ºC) range.
In addition
to microstructure evaluations, surface-to-center hardness profiles were
also completed on each sample in the as-received unannealed condition, and also
after each heat treating cycle. As a result, the P/M alloy was found to have
consistently higher hardness on the surface, at mid radius (Figure 11)
and at the center in the as-received, unannealed condition as well as after
each heat treating cycle, when compared with both the hot worked and warm
worked cast/wrought alloy.
Figure
11. Midradius hardness profile of P/M Alloy 1 (HRC MMM) vs. hot
worked (HRC
MHW) and warm worked (HRC MWW) cast/wrought Alloy 1.
Conclusion
As
previously described, the unique attributes developed by the Micro-Melt® powder
metallurgy (P/M) process results in an Alloy 1 F1537 bar material that exhibits
higher strength, enhanced fatigue resistance, increased hardness, improved microstructural
uniformity and finer grain size in the unannealed condition as well as after
exposure to elevated temperatures. These benefits allow the material to be
manufactured to smaller diameters without the need for cold working and
annealing, which can be detrimental to grain size and, subsequently, fatigue
strength. In addition to smaller diameter, the alloy also lends itself to the
manufacturing of special shapes that could replace castings in certain
applications.
