“Light Weighting of Structural Automotive Components Using High Strength

Powder Metallurgy Aluminum Alloys”

Chaman Lall, Ph.D.

Metal Powder Products

Westfield, IN 46074, U.S.A.

ABSTRACT

The intense effort by automotive companies to improve gas mileage and meet mandated CAFÉ Standards

is aided by enabling vehicular mass reduction technologies. One area of such research is the application

of appropriate powder metallurgy (PM) materials such as the alloys of aluminum, titanium, and

magnesium. Each material option presents certain challenges that need to be overcome when

manufacturing components for high volume and cost sensitive applications. Aluminum alloys produced

by the PM technology have been used for several decades, but primarily in relatively low-stress

components. The Al-4%Cu alloy has been the most widely-used PM material for over two decades in the

form of automotive camshaft caps. This material offers yield strength and ultimate tensile strength values

near 160 and 200 MPa, respectively. Heat treatment permits attainment of higher strength values, but

sometimes with the accompanying embrittlement. This paper discusses process and alloy development

work on PM aluminum alloys, which seeks to produce components with yield strength values of over 300

MPa, along with reasonable ductility. These components can replace many wrought, cast, or PM steel

components with a reduction in mass of at least 50%.

INTRODUCTION

Lightweight materials in automotive applications present the industry an opportunity to improve gas

mileage and thereby meet the government-mandated CAFÉ standards (Corporate Automotive Fuel

Efficiency)1. While these standards have the admirable intent of reducing greenhouse gases and, thereby,

improve the environment and reduce global warming, the automotive industry faces many challenges in

achieving the set targets (54.5 mpg by 2025). One aspect of the industry’s efforts has been the significant

progress in engine technology which has resulted in smaller engines with as much, or greater, horsepower

than their predecessors2 - a 2.0 L DOHC 4-cylinder Suburu engine is able to deliver a remarkable 256 HP

output. Compare this to a much larger Subaru engine from 2002, the 3.0L DOHC 6-cylinder, which was

rated at “only” 212 HP. The second aspect is vehicular mass reduction wherever possible1,3-5, which

includes deployment of low density materials. In their list of best 10 engines for 2016, Ward’s picked

nine in which both the blocks and heads were made from aluminum alloys2.

A 10% reduction in the mass of a vehicle is estimated to result in about a 6-8% improvement in fuel

efficiency1. From a pollution perspective, a reduction in vehicle mass of 100kg results in a lowering of

CO2 exhaust emissions by about 8 g/km, or11.3 g/mile. The primary materials for enabling weight

reduction are: 1. Carbon fiber composites (30 -70% reduction), 2. Magnesium (50- -70% reduction), 3.

Aluminum (30 - 60% reduction), 4. Titanium (40 - 55% reduction), and 5. Advanced high strength steels

(15 - 25% reduction). Each of these material systems present their unique set of cost and manufacturing

challenges in the quest to offer cost-effective solutions to the automotive industry.

Aluminum alloys have been very effective in reducing vehicle mass - the best example being the Ford

F150 aluminum body introduced in 2014, although the 2005 Ford GT had an all-aluminum body, almost a

decade earlier4. The inherently low density (2.7 g/cc) of aluminum compared to iron (7.8 g/cc) gives the

material a significant advantage in offering lower mass parts. Ford has made prototypes of forged

aluminum con rods, with the potential to replace the current steel versions5. A 30% saving in

reciprocating and rotating mass is possible with the switch from wrought steel to aluminum. Of course,

the lower strength of aluminum alloys compared to steels must be carefully reviewed for such highly

stressed structural components.

A recent article6 on conventional powder metallurgy (PM) aluminum alloys lists typical mechanical

properties and densities for the most common alloys produced by this technology. By far, the PM

aluminum alloy in commercial use the most at this time is the AC-2014 material (nominal composition,

Al-4%Cu-1%Si-0.5%Mg). The primary use is the camshaft bearing cap application (“cam cap”, Figure

1a) which began in 19927, 8, and is still going strong, more than 2 decades later9 – more than 300 million

such components have been shipped to date by MPP alone. The alloy is fairly soft and ductile in the

sintered condition, but hardens, or “naturally ages”, at room temperature. This is why it is advisable to

repress, or “size”, the PM components within a day of performing the sintering operation. Some fairly

complex and precise details can be produced by the PM technology, as demonstrated by the larger

component (about 17 mm long) shown in Figure 1b.

(a) (b)

Figure 1 - Powder metal aluminum dual overhead camshaft bearing caps: (a) “Cam caps” covering

single shaft, and (b) “Double-hump cam cap” bridging both camshafts. (Courtesy, Metal Powder

Products Company, GM and MPIF10).

This highly popular PM alloy has been the subject of much research work 11-18 in order to understand the

sintering and strengthening mechanisms that provide the material its set of properties. During sintering,

Mg is considered to aid the reduction of aluminum oxide covering the metal powders11-14 while nitrogen

provides additional assistance by forming AlN in preference to the metal oxide15-18. At high green

densities, the presence of Mg is necessary to sinter aluminum in an inert atmosphere such as Ar. In

contrast, at low green densities and sintering in a nitrogen atmosphere, the latter forms AlN, effectively

“gettering” the aluminum compact. The level of oxygen in the sintering atmosphere is also an important

factor; in an elegant experiment a low density compact, not containing Mg, was sintered in nitrogen and

found to be well sintered internally, but not so near the outer edges16. X-ray diffraction patterns verified

that the lighter phase seen in the inner sections was indeed AlN, while this phase was not observed in the

outside “picture frame” area, nor in any section of test samples sintered separately in pure argon. The

inner portions of the test sample combined with nitrogen to form the nitride, while the restrictive inner

passages kept the partial pressure of oxygen low by not permitting supply of fresh oxygen to promote

oxide formation.

The role of magnesium and nitrogen in the sintering of aluminum alloys is very important, because in

their absence the task of reducing aluminum oxide from the surface of powder particles is problematic.

From a thermodynamics perspective, Al2O3 has a very high standard free energy of formation and under

conventional sintering atmospheres which use hydrogen to drive oxide reduction, the dew points and

partial pressures of oxygen would need to be impractically low19-21 (at 600OC, about -140OC and less than

10-50, respectively).

Figure 2. The aging response of wrought Al-Cu alloys22: (a) at 130 OC, and (b) at 190 OC

As in the corresponding wrought alloys of Al-Cu, the primary strengthening mechanism is the formation

of Guinier-Preston (GPI and GP-II) zones, which are concentrations of Cu-rich plates along {100} type

lattice planes of the f.c.c. crystal structure of the α-aluminum phase 22- 24. GP zones form even at room

temperature right after sintering, causing matrix hardening (“natural aging”), although heating slightly

will speed up this process (“artificial aging”). Application of too much thermal energy and or long times

can cause “over-aging, a condition in which the GP zones are replaced by the ordered structure of CuAl2,

θ-phase 22. As shown in Fig 2 for a wrought Al-Cu alloy, hardness increases initially as GP zones are

formed and then starts to decrease as the ordered phase precipitates out. A more recent study24 resulted in

similar behavior for the PM Al-4%Cu alloy, as well as the alloys of Al-Si and Al-Zn (Fig. 3).

Published industry standards25 indicate modest mechanical strength levels for the Al-4%Cu alloy; UTS of

about 200 MPa, yield of about 160 MPa, along with an elongation of 1%, and an apparent hardness of

HRE 60. This data is for test samples that have been compacted, sintered, sized, and naturally aged

(designated T2 condition) for at least seven days. This material is the control group for this current work

and to which properties will be compared.

Figure 3. Aging response24 for conventional PM aluminum alloys: (a) A-2014, (b) A-4032,

and (c) A-7075

EXPERIMENTAL

Base materials used in these studies were primarily supplied by AMPAL, a division of U.S. Metal

Powders. All powder blends were processed in a similar manner involving compaction, sintering, and

sizing. Some were selected for additional proprietary post-sinter treatments, such as thermal and surface

modifications. Nominal compositions of the material blends are shown in Table I. Acrawax C (a form of

ethylene bi-stearamide wax) was used in all blends at a level of about 1.5% by weight. MPP designations

are assigned to the closest major additive, per The Aluminum Association teal book system. Where there

is no exact chemical match, an arbitrary value is assigned to the second digit, again, in line with the

association guidelines (at this time, these are not registered with The Aluminum Association). All

components and test bars were processed in an industrial manufacturing environment using

installed production equipment.

Normal industry test method protocols were used to evaluate mechanical properties, such as ultimate

tensile stress (UTS), yield stress, elongation, hardness, etc. Axial fatigue stress values were determined

from round test bars machined from Izod impact bars. Some selected parameters required for finite

element analysis (FEA) and computer simulations were also measured.

Table I Nominal Alloy Compositions of Selected PM Aluminum Alloys.

RESULTS AND DISCUSSION

Table II shows the mechanical properties of the PM materials prepared for this study. No TRS value is

shown for pure aluminum as the ductility is too high for this test method. These properties are in general

agreement with published literature for materials sintered in a nitrogen atmosphere9,14, 23, 26. As one might

expect, the specific properties are a function of chemistry, compact pressure, sintering protocol, and post-

sinter processing. Sintering is a complex process with various phenomena in play, e.g. additive

dissolution, oxide reduction, nitriding, transient liquid formation, particle re-arrangement, densification,

etc., as discussed by various researchers11-18. As discussed above, the high stability of aluminum oxide,

Al2O3, necessitates the need for Mg as an additive and/or nitrogen in the sintering atmosphere. The former

aids oxide reduction by forming the spinel MgAl2O4., while the latter adds support to the same goal of

oxide reduction by the preferential formation of AlN.

Table II Measured Mechanical Properties of Selected PM Aluminum alloys.

Material Alloy Series %Cu %Si %Mg %Zn % Others

Pure Al 1000 - - - - < 0.5

Al-4%Cu MPP AC-2014 4 0.7 0.5 - < 1.5

Al-2.5%Cu MPP AC-2236 2.5 - 1.5 - < 1.5

Al- 2%Mg MPP AM-6225 4 0.5 2 - < 1.5

Al-14%Si MPP AS-4132 2.5 14 0.7 - < 1.5

Al-8%Si MPP AS-4144 2 8 1 - < 1.5

Material Alloy Series

UTS

(MPa)

Yield

(MPa)

Elong.

(%)

App.

Hardn.

(HRB) TRS (MPa)

Density

(g/cm^3)

Pure Al 1000 170 85 20 HRH 32 - 2.55

Al-4%Cu MPP AC-2014 200 165 2 20 325 2.50

Al-2.5%Cu MPP AC-2236 325 300 3 50 585 2.60

Al- 2%Mg MPP AM-6225 275 195 3 40 450 2.55

Al-14%Si MPP AS-4132 210 150 1 32 290 2.60

Al-8%Si MPP AS-4144 165 145 1 30 275 2.60

As expected, the pure aluminum powder processed by the PM technology results in a low strength, but

highly ductile product. Add to this the very low apparent hardness, and it is clear that the material is

highly formable and malleable. Repressing after sintering causes plastic deformation and strain hardening.

Combining aluminum with Cu, Si, and Mg as primary additives, doubles the yield strength of the pure

metal, along with increases in tensile strength and apparent hardness. In comparison to PM steels, the

apparent hardness values are relatively low, even with these additives, so that sizing is comparatively

straightforward. Note that most commercial PM alloys are sized because of the high shrinkage and

potential component distortion during the sintering process. The plastic deformation during the sizing

operation creates work hardening by the initiation and movement of dislocations on the slip planes of the

f.c.c. lattice structure. Combining the close-packed planes along with 3 slip directions generates 12 slip

systems for the aluminum alpha phase. As these interact and intersect with each other, subsequent

dislocation motion is impeded, resulting in the observed increases in hardness.

As a further comparison to PM steels, it is instructive to consider Figure 4, which shows the significant

variation in shrinkage that occurs as a function of starting density (which in turn is a reflection of the

compaction pressure). In contrast, in most PM ferrous alloys, the effect of compaction pressure on the

final sintered density is minimal, because the green density curve simply moves up a slight amount. In

agreement with prior published work, the PM aluminum alloy shrinkage can be quite high at low green

densities.

Figure 4. Green and sintered density variations as a function of compaction pressure.

Dilatometry curves for high purity aluminum sintered in a pure nitrogen atmosphere18 show a rapid

shrinkage as soon as sintering temperature is reached, followed by a continuous shrinkage with time, even

after 4 hours at temperature. This study re-affirmed the significant role of nitrogen in the sintering of

aluminum and its alloys. It was found that the higher the purity level of nitrogen, the more extensive the

degree of sintering, and the greater the amount of transient liquid phase formed during the sintering stage.

Thermo-gravimetric (TG) and differential scanning calorimetry (DSC) data indicated weight gains and

exothermic reactions that were related to the purity of the nitrogen sintering atmosphere. The point being

that during the sintering of the pure metal under nitrogen, liquid phases are formed even though there are

no other alloying elements present.

Just as in the case of PM steels, complex interactions and reactions are occurring during the PM sintering

operation while the material system is trying to attain an equilibrium state. Most are reversible reactions

so that Le Chatelier’s principle applies to influence the direction of those reactions and the state of

equilibrium. A simple example is the reversible reaction in which a metal (reactant) in the presence of

oxygen gas (second reactant) will form the metal oxide (reaction product). If the metal oxide is

continuously removed from the right hand side of the equation, the reaction will favor further formation

of the oxide product and tend to completion to the right hand side of the equation.

Beyond the normal oxidation-reduction and the additive dissolution phenomena, many other interactions

are abundant in aluminum. As reviewed above11-17, Mg plays a key role during the sintering of aluminum

alloys. The nitrogen atmosphere, which for the most part in PM steels can be considered inert, is certainly

not so for PM aluminum. So, while transient liquid phases can transcend to the solidified state after a

relatively short time, the formation of AlN can continue for prolonged periods, since the sintering

atmosphere is not inert to the aluminum alloy system. The constant supply of nitrogen in the sintering

atmosphere ensures that the direction of the reversible reaction is to the side that has the aluminum nitride

reaction product.

Beyond Mg and N, other elements can affect the properties of the PM product11,14,15,26-28. Even trace

amounts of tin (0.05%) can have a beneficial effect on the sintering behavior of Al-Cu alloys, tending to

offset the growth that occurs as a result of Cu additions. Greater amounts of Sn (0.2%) lead to significant

densification, almost 99 % of full density28. This excellent study by Boland et al also confirmed that Cu

formed a solid solution in the α-aluminum matrix, but that Mg and Si were only present at the grain

boundaries. The significance of this in the AC-2014 alloy is that while Si and Mg can affect intergranular

behavior (particle shape, densification, pore removal), they cannot affect the primary strengthening

mechanism in the Al-Cu system. The absence of Mg and Si within the grains means that the normal

sequence of GP zones followed by the precipitation of CuAl2 (θ-phase) will prevail as the dominant

method of strengthening this alloy.

It is worth noting that in this same study28 a new formulation (PM-2324, containing Al-4.5%Cu-

1.5%Mg), was examined by EDS (Energy Dispersive Spectroscopy) with interesting results. While high

levels of Mg (3.7-4.7%) were measured at the grain boundaries, almost 1% Mg was detected within the

grains, contrary to the AC-2014 alloy which contains about 0.8% Si. It is well known that Si has very

limited solubility in α-aluminum, so this it is not surprising that the element was not detected within the

grains but was segregated to the grain boundaries. It appears that in the AC-2014 alloy, the Si was

“binding” with the Mg to form M2Si (β-phase) and retaining the Mg at the grain boundaries. Indeed, the

researchers used this fact to good effect in that the ability to have Mg present within the grains permitted

modification of the normal CuAl2 precipitation mechanism. The additional Mg within the grains, along

with Sn additions promoted the S-phase (Al2CuMg) in the new alloy. This phase does not exist in the

conventional AC-2014 alloy. The sintered PM-2324 alloy resulted in enhanced mechanical properties;

UTS of 268 MPa, yield of 194 MPa, elongation of 2.6%, and an apparent hardness of 93HRE, at a density

of 2.72g/cm3. Sizing promoted strain hardening and additional strength improvements.

The AC-2014 alloy can be strengthened by heat treatment. One such protocol is T6, which consists of

solutionizing for at least one hour at about 500 OC, quenching into water, then artificially aging at 160 OC

or higher for about twenty hours14, 19. The T4 thermal treatment is similar, except that the material is aged

“naturally” at room temperature over a period of days or weeks.

After quenching, the aluminum alloys are very soft and malleable. Precipitation hardening occurs during

the artificial aging stage, with the phenomena described earlier22-24. Again, this is in contrast to the

common PM ferrous alloys which are not precipitation hardened, but gain their strength from the

formation of metastable phases such as martensite and bainite, during the quench. A subsequent

tempering or “draw” treatment is used to soften the product to a level appropriate for the intended

application. In the absence of that temper treatment, the product can be very brittle.

The T6 thermal treatment strengthens the Al-4%Cu alloy, but decreases ductility. At a density of 2.55

g/cm3 the property values are: UTS about 300 MPa, elongation <1%, and apparent hardness about 30

HRB. The yield and UTS points are close to each other (within 10 MPa). These mechanical property

values are in reasonable agreement with prior work6, 14.

The property values for the Al-14%Si alloy shown in Table II can also be increased to similar levels by

use of the T6 thermal treatment, but at the expense of ductility. The Al-2%Mg alloy in Table II processed

by MPP seems to have UTS and yield values either matching or exceeding the heat treated (T6) 6000

series alloys6.

The Al-8%Si alloy in Table II was specially developed for an automotive application in which all the

prevalent materials were exhibiting “unacceptable” levels of wear. Several combinations of alloy systems

and composites were suggested and prototypes prepared for functional testing. Of all the test samples

prepared and tested, including cast and wrought materials, this alloy was far superior with regard to wear

performance in this application. The components were camshaft caps (similar in appearance to the caps

shown in Fig 1a) for start-stop engines where the engine is turned off if the vehicle is stationary for a

predetermined interval. The frequent drainage of oil and the engine re-starts were causing some limited

wear - leading to internal pressure drops. These pressure drops were not acceptable from an engine

performance perspective. The “islands’ of Si phase in a background of alpha aluminum, are

conceptualized9 to provide the structural support for the bearing function, while slight wear of the

aluminum creates “pockets” for oil retention. In PM parts, the pores also act as natural reservoirs for oil.

The pockets of worn aluminum are thought to retain oil, which can act as a cushion and cause a

“hydroplaning” action between the heat treated steel camshaft and the PM aluminum cam caps. Vehicles

incorporating these Fuel Management Systems (FMS) have been operating successfully for several years,

without any known cam cap issues.

Table III Mechanical and Thermal Properties of MPP Aluminum Alloy AC-2236 (Al-2.5%Cu), in

Comparison to Wrought A-2014 Alloy in the T6 Thermal Condition

Material Alloy Series

UTS

(MPa)

Yield

(MPa)

Elong.

(%)

App.

Hardn.

(HRB)

Young's

Modulus, E

(MPa)

Shear

Modulus, G

(MPa)

Poisson's

Ratio

Fatigue

Limit,

10M cycles

(MPa)

Thermal

Cond.

(mW/mm

deg.C)

Density

(g/cm^3)

Al-4%Cu A-2014 415 315 5 75 72 27 0.33 124 151 2.78

Wrought T6 Room Temp

Al-2.5%Cu MPP AC-2236 325 300 3 50 70 25 0.33 85 145 2.60

Fully processed Room Temp

The Al-2.5%Cu alloy (AC-2236) in Table II is also a specialty alloy developed by MPP. With proprietary

post-sinter processing, the material has the complete set of properties shown in Table III, which are

desirable for many structural applications. The values are comparable to the wrought Al-4%Cu alloy in

the T6 thermal condition, and slightly higher than the PM-2324 in the sized condition, referenced above28.

These properties are also comparable to the low C PM ferrous alloys in the sintered condition. The

combination of high strength with reasonable ductility in a PM aluminum alloy opens up the opportunity

to replace low C steels in some structural applications with this material, at one-third the density of steel.

Figure 5 shows the high cycle fatigue (HCF) data for this specialty alloy as well as the AC-2014 alloy, as

processed by MPP. The latter material exhibits an axial fatigue strength in these stress-controlled tests of

about 50 MPa (test suspended at 10M cycles). When fatigue tested to the same level, the new high

strength alloy displays a value of about 85 MP. This is of the order of a 70% increase over the AC-2014

alloy, when measured at room temperature. Furthermore, that fatigue strength level is apparently

maintained throughout the test temperature range of – 40 OC to + 150 OC. This temperature range was

selected in consultation with automotive design engineers who are seeking to apply these PM aluminum

materials for specific components in their vehicles. An earlier fatigue study29 on the AC-2014 alloy, as

well as alumina-reinforced composites, concluded that up to 10% alumina was beneficial. Surprisingly,

the sintered test samples performed better than the heat treated samples. Further verification test work is

continuing on the new AC-2236 alloy, as well as additional strain-controlled fatigue testing.

Figure 5. High Cycle Fatigue (HCF) Data for Two Selected MPP PM Aluminum Alloys

At these levels of mechanical properties, there is significant impetus to consider PM aluminum alloys for

lighweighting various components in automobiles, such as, fittings, flanges, transmission carriers,

connecting rods 30, main bearing caps, variable valve timing (VVT)/variable cam timing (VCT) sprockets,

gerotors 31, as well as many others. The lightweighting opportunity is several kg per vehicle.

In one specific automotive application, currently under development, this new alloy is being tested and

thus far has passed all functional tests, with no signs of failure. The light-weighting opportunity is over

2kg per engine.

Kraus et al32, studied the powder forging of PM aluminum 7075 alloy (Al-7%Zn-2.5%Mg-1.7%Cu)

compacts that had been processed by CIP (Cold Isostatic Pressing). By combining a change in PSD

(Particle Size Distribution) with additions of Sn, reasonably high levels of mechanical properties were

obtained. These values (UTS of about 615 MPa, yield of about 585 MP, elongation of about 8%, Young’s

Modulus of about 75 GPa, and an apparent hardness of 90 HRB) are comparable to those of the wrought

version of the alloy. The parts were hot-forged at 470 OC, and subjected to a T76 thermal treatment. At

such high levels of mechanical performance, opportunities exist for conversion from steel to PM

aluminum for purposes of mass reduction.

The con rod application30 is intriguing but, however, quite a challenge for the PM aluminum industry,

because of the very high stress requirements of some of the high-end turbo-charged engines. As a case in

point, a recent study33 of the HS170M material (Fe-3.25% Cu-0.64% C-0.32% Mn), considered to be the

strongest powder forged ferrous material available in the industry, listed room temperature properties as

UTS of 1202 MPa, yield strength of 855 MPa, and elongation of 11.5%. When tested at 150 OC, this

powder-forged grade had even higher mechanical strength, while competing wrought grades exhibited

lower values than those at room temperature. This contrasting increase in strength of the powder-forged

material was shown to be a result of nano-scale precipitates optimally forming at 120 OC. The level of

mechanical properties attainable in PM forged steels is very high and presents an admirable, and

challenging, target for future PM aluminum alloy R&D work.

A European study34 on wrought aluminum con rods for a 1.8L four-cylinder engine indicated about a 50%

decrease in mass by converting from the steel versions. The ultra-high strength wrought aluminum alloys,

based on the EN AW-6082 alloy, exhibited yield strength values near 700 MPa. Furthermore, the study

also showed improvements in acoustic and vibration behavior by optimizing the geometry to re-position

the center of gravity of the con rods. The final configuration was also customized for the forging process

and the stress distributions that the components would need to endure in service.

CONCLUSIONS

The current work has reviewed the properties of PM aluminum materials in predominant commercial use

at this time. Using the Al-4%Cu alloy as the control group, significant property improvements have been

attained by changing compositions as well as processing protocols. The main conclusions of this program

are:

1. The PM Al-4%Cu alloy (AC-2014) provides a set of mechanical properties suitable for structural

components that are moderately stressed.

2. Post-sinter thermal processing to the T6 condition improves the strength of this alloy, but

ductility (as measured by tensile elongation) is reduced significantly.

3. The PM Al-2%Mg alloy (AM-6225), as processed in this study, exceeds the strength and ductility

of T6 condition 6000 series alloys.

4. The new high strength PM alloy Al-2.5%Cu (AC-2236) processed in this study offers an

excellent combination of strength and ductility for highly stressed components.

5. That combination matches the strength of many sintered PM steels but at a much lower density –

thereby presenting an opportunity to lightweight components.

6. An additional PM alloy (Al-8%Si, AS-4144) was developed for the automotive industry and has

demonstrated excellent wear resistance in the marketplace. After several years of service, no

functional issues have been reported, and no other material (either PM or wrought) has been able

to match the bearing performance of this new alloy.

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