Due to their light weight, high corrosion resistance and good heat conductivity, aluminium alloys are used in many industries today. They are suitable for manufacturing many automotive components such as clutch housings. These alloys can be fabricated by powder metallurgy and casting methods, in which porosity is a common feature. The presence of pores is responsible for reducing their strength, ductility and wear resistance. The present study aims to establish an understanding of the tribological behavior of high pressure die cast Al A380M and powder metallurgy synthesized Al 6061. In this study, dry sliding wear behavior of Al A380M and Al 6061 alloys was investigated under low loads (1.5 N – 5 N) against AISI 52100 bearing steel ball using a reciprocating ball-on-flat configuration and frequency of 10 Hz. Wear mechanisms were studied through microscopic examination of the wear tracks. This study revealed that due to combined effect of real area of contact and subsurface cracking, wear rate increased with increasing porosity content. The difference in friction and wear behavior between received Al A380M and Al 6061 is attributed to their hardness differences.
In automotive applications, aluminium is an attractive alternative to ferrous alloys due to its high strength to weight ratio and high thermal conductivity. The combination of light weight, high strength, good corrosion and impact resistance made aluminium alloys suitable candidates [
High pressure die casting (HPDC) is a common method for producing aluminium parts. It is fully automatic, large volume, high productivity process for the production of complex, thin walled near net shape castings. Due to smooth surface finish and excellent dimensional tolerance, most high pressure die castings require no machining except the removal of flash around edges and possible drilling and tapping holes. More recently, near net shape processing of aluminium by classical press and sinter powder metallurgy has emerged as unique and important metal forming method [
However, a certain amount of porosity is common in both of these manufacturing techniques. The influence of porosity on the wear behavior is not clearly understood [
Commonly, the presence of pores has a detrimental effect on the wear performance of materials [
Hence, a detailed understanding of the effect of porosity is necessary to assess wear properties of cast and P/M Al alloys. In this study, an attempt has been made to identify the relationship between surface porosity and wear behavior of aluminium alloys. In addition, wear behavior of different microstructures (due to different production methods) was investigated. A series of reciprocating wear tests were conducted under different loads and wear mechanisms were identified.
High pressure die cast A380M aluminium clutch housing was obtained from Magna Powertrain and 6061 Al powder was obtained from Ecka granules. Particle size analysis on the raw 6061 Al powder was performed using Malvern particle size analyzer (model 2600c) equipped with MASTER particle sizer 3.1 analytical software. Here tests were conducted using a focal length of 100 mm and a beam length of 300 mm. The average particle size of the 6061 Al powder is 70.9 µm. The morphology of the powder is shown in
Elements (%) | Al | Mg | Si | Fe | Cu | Zn | Mn |
---|---|---|---|---|---|---|---|
Al 6061 | 97.5 | 1 | 0.6 | 0.5 | 0.1 | 0.2 | 0.1 |
Al A380M | 85.2 | 0.07 | 8.35 | 0.06 | 0.05 | 0.75 | 2.15 |
The 6061 Al powder was weighed using a Denver Instruments APX-1502 scale and placed in Nalgene bottles. A total of 0% and 1.5% lubricant (Lico wax C) was added to the powder and blended in a Turbula Model T2M mixer for 40 minutes to ensure homogeneity. Rubber molds were filled with the blended powders and sealed with electrical tape. The sealed molds were then transferred to a cold isostatic press (CIP) chamber for wet bag pressing. The chamber was filled with a mixture of water and water soluble oil (20:1). Using a high pressure air- operated piston type pump, the pressure within the pressure chamber was increased to 200 MPa and maintained for a dwell time of 5 minutes. The pressure was then released and the compacts were removed. To increase density, green compacts were subsequently sintered in a Linderburg Blue 3-Zone Tube Furnace Model STF 55666C-1. Once the samples were placed in the furnace the pressure in the tube was reduced to a value less than 9.9 × 10−2 Torr and then backfilled with nitrogen. This was repeated to minimize oxygen contamination in the atmosphere. Nitrogen was then allowed to flow continuously at a rate of 9.4 L/min. Heating of samples progressed in three stages: de-lubrication, sintering and post-sintering cooling. After ensuring that all the oxygen had been evacuated and that the nitrogen was flowing properly, the temperature was quickly ramped to 400˚C and held for 20 minutes. After the 20 minutes dewax stage, the furnace was then ramped to the final sintering temperature of 620˚C and held for a time period of 30 minutes. The power to the furnace was then turned off and the samples allowed to cool to 580˚C.
The green and sintered densities of samples were determined in accordance with MPIF standard 42. In order to determine the unsintered (green) density, samples were weighed in air (Wair) and the water temperature was recorded. The green density was calculated using
in
HPDC A380M Al and sintered 6061 Al specimens were prepared for metallographic examination using 240, 320, 400 and 600 SiC abrasive papers and 0.1, 0.3 and 0.05 alumina suspensions for polishing. Olympus BX51 microscope, equipped with bright field objectives was used to analyze the specimens at high resolution. Surface porosities of samples were calculated using image analysis software. A series of images were taken to cover the whole surface area of the sample. Porosity was identified based on their gray-level intensity differences compared to the matrix. Gray-level threshold settings were selected to permit independent detection of porosity, using the “flicker method” of switching back and forth between porosity and the matrix. Second phase particles and dendrites may be counted as porosity because their gray-level range is similar to that of porosity. The gray-level thresholds as well as boundary conditions (i.e., aspect ratio, minimum radius and area) were set to avoid second phase particles and dendrites detection. A counting protocol was chosen to correct for edge effects so that a porosity lying across a field boundary is counted only once. For each field the area fraction of porosity was calculated by dividing the area of covered by porosity by the total field area.
Dry reciprocating wear tests were performed using a Universal Micro-Tribometer. This test method utilizes a ball upper specimen that slides against a flat lower specimen in a linear, back and forth sliding motion having a stroke length of 5.03 mm. All tests were conducted at room temperature and at a relative humidity of 40% - 55%. The load is applied downward through the ball counter-face against a flat specimen mounted on a reciprocating drive. The tester allows for monitoring the dynamic normal load and friction force during the test. A 6.3 mm diameter AISI 52100 bearing steel ball with a hardness of HRA 83 was used as a counter-face material. The ball was mounted inside a ball holder, which is attached directly to a suspension system. The suspension system is attached to a load sensor that controls and records forces during the test. The weight of the specimen was measured before and after each wear test to determine individual weight loss at selected time intervals. Six different loads (1.5, 2, 2.5, 3, 4, 5 N) were employed; each tested under 10 Hz frequency and for 120 minutes. After wear tests, worn surfaces of wear tracks were examined using optical profilometry to calculate the volume loss. Image analysis software calculates volume loss from the differences between the interpolated reference plane and the actual worn surface. Scanning electron microscope was used to determine possible wear mechanisms.
Rockwell hardness tests were carried out in all Al A380M and Al 6061 specimens. Tests were performed on a Leco R600 Rockwell hardness tester using the “H” scale under a load of 60 kg and a diamond indenter. Nanoin- dentation experiments were conducted using a nanoindentation system (developed by CETR, USA). The instrument uses a Berkovich diamond pyramid with an angle of 65.3˚ between the tip axis and the faces of the triangular pyramid. The total penetration depth consists of a plastic component and an elastic recovery component which occurs during the unloading. Maximum indentation depth (hmax) can be expressed as:
Al 6061 | Green density (g/cc) | Sintered density (g/cc) | Volume porosity (%) |
---|---|---|---|
No Wax | 2.49 | 2.52 | 6.5 |
1.5% Wax | 2.24 | 2.32 | 13.8 |
where p and h are load and indentation depth, respectively. hmax, Pmax and slope at maximum load dp/dh are determined from the load versus displacement profile. The relationship between hardness H and the maximum applied load (Pmax) is as follows:
where A is the area of contact for Berkovich indenter and is given by,
where hc is the contact depth. The elastic modulus can be expressed as
where
here, E2 and V2 are elastic modulus and Poisson’s ratio of the test material respectively, and E1 and V1 are the same parameters for Berkovich indenter.
The size, shape and amount of pores in compacts are largely dependent on processing parameters like the amount of lubricant used and compaction pressure. Different surface characteristics were observed with and without adding wax. For Al 6061 specimens, surface porosity ranges from 3.5% for no wax to 10.3% for 1.5% wax (
Porosity in high pressure die cast aluminium component is caused by the combined effects of solidification shrinkage and gas entrapment. Shrinkage takes place when the metal is solidifying inside the die. This shrinkage may form voids, known as shrinkage porosity. Gas porosity is caused by the entrapment of air during the casting process. Shrinkage porosity tend to be large and irregular in shape while gas porosity is small and spherical in shape. The as received high pressure die cast Al A380M alloy exhibits 0.6% gas porosity and 1.5% shrinkage porosity. These two types of porosity are combined to give the final porosity content of 2.1%.
It is noticeable from
Specimen | Al grain size (µm) | Pore size (µm) | Pore shape | Surface Porosity (%) | Pore distribution |
---|---|---|---|---|---|
Al 6061 (No wax) | 45 | 12 | Round | 3.5 | Uniform |
Al 6061 (1.5% wax) | 45 | 20 | Round | 10.3 | Uniform |
Al A380M | 25 | 31 | Irregular | 2.1 | Non-uniform |
the amount of porosity is lower and non-uniformly distributed.
To investigate the possible factors effecting the hardness of A380M Al and 6061Al, a series of Rockwell hardness measurements were conducted and plotted in
Furthermore, nanoindentation tests were performed on all Al specimens and nanohardness and Young’s modulus were calculated using Oliver and Pherr method [
Specimen | Young’s modulus (GPa) | Hardness (GPa) |
---|---|---|
Al 6061 (3.5% porosity) | 57 | 0.67 |
Al 6061 (10.3% porosity) | 51 | 0.64 |
Al A380M (2.1% porosity) | 69 | 1.00 |
constant max load of 100 mN, A380M Al curve shows a max depth of 2.1 µm while 6061 Al having 3.5% and 10.3% porosity show max depths of 2.7 and 3.0 µm respectively. For the 6061 alloy, the increase in the max depth with porosity is consistent with the above argument. As outlined in the experimental section, there is an inverse relationship between contact depth (hc) and nanohardness and Young’s modulus. Furthermore, the relationship between maximum indentation depth (hmax) and the contact depth (hc) is given by the following equation:
where, he is the elastic depth upon unloading. The observed increase in hmax with increasing porosity in 6061 Al is accompanied by a corresponding increase in hc. Thus, it can be concluded from this argument that, as the contact depth increases, contact area
are plotted in
The stress intensity is particularly high near pores which act as sources for cracks during wear. The stress intensity increases with increasing normal load. At low load, the pores beneath the worn surface remain stable and can not propagate significantly. As a result, subsurface deformation and strain are relatively low. However, with increasing load, pores beneath the worn surface become unstable and cracks originated from these pores can propagate significantly. Consequently, areas surrounding pores become failure-prone.
The effect of porosity on wear resistance depends not only on total porosity content, but also on pore distribution and connectivity. When the pores are uniformly distributed, cracks can propagate at high rate as pores can easily link up with each other and forming a wide network of cracks. This effect ultimately contributes to the fracturing of material and increasing wear rate. Pores in 6061 Al are more uniformly distributed than A380M Al (see
The coefficient of friction is plotted as a function of load in
For Al 6061 alloy, at a given normal load of 3 N, the coefficient of friction increases 40% when surface porosity increases from 3.5% to 10.3%. In porous surface, the probability of generating wear debris increases as a consequence of increased asperity-asperity contact. Therefore, the rise in coefficient of friction with increasing porosity might be attributed to the formation of more asperity-asperity contact during sliding. Yalcin [
found similar results (increase in coefficient of friction with increasing porosity). He further suggested that, under dry friction conditions, an increase in the coefficient of friction results in an increase in the mass loss of the porous material.
Worn surfaces of Al A380M and Al 6061 specimens were analyzed using optical profilometer and scanning electron microscope.
The variation in volume loss with applied normal load (calculated from profilometry scans in
Scanning electron microscopy images of worn surfaces were examined to identify possible wear mechanisms.
A second mechanism contributing to the observed wear is delamination. Plastic deformation leads to changes in the microstructure of the subsurface, making the material unstable to local shearing causing delamination [
EDS analysis was conducted on the worn surface of Al A380M alloy and is shown in
worn surface is indicative of oxidative wear. As sliding takes place, the increase in temperature at the interface promotes the formation of oxides. Absence of Fe implies that, there is no material transfer from the counterface (AISI 52100 bearing steel ball). As a result, mechanical mixing of materials did not take place between the two sliding surface.
In case of Al 6061, wear tracks of the samples were characterized by surface deformation and heavy damage in the form of longitudinal grooves extending parallel to the sliding direction which are clearly observed in
In the present work, the effect of porosity on tribological properties of Al A380M and Al 6061 was studied. The following conclusions can be drawn:
a) Wear resistance is not only a function of pore size but significantly affected by pore distribution. For a given amount of porosity and pore size uniform pore distribution results in accelerated wear.
b) Indentation hardness of porous materials leads to high maximum penetration depth and high contact depth.
c) There is an inverse relationship between the hardness and porosity content of Al alloys. Hardness decreases with an increase in surface porosity. For Al 6061 specimens, 20% reduction in hardness was observed as the surface porosity increased from 3.5% to 10.3%.
d) Coefficient of friction decreases considerably with increasing load but increases with increasing porosity percentage. For Al A380M alloy, coefficient of friction decreases 36% when the applied normal load increases from 1.5 N to 5 N. In case of Al 6061 alloy, at a given normal load of 3 N, the coefficient of friction increases 40% as surface porosity increases from 3.5% to 10.3%.
e) A predominance of abrasive and oxidative wear was identified on both Al A380M and Al 6061 alloy. In addition, delamination wear was found in Al A380M alloy.
The authors acknowledge the financial support provided by Auto21 and Mr. Randy Cooke for his assistance in sample preparation.