Al-Ni in situ surface composites were fabricated by friction stir processing method. Friction stir processing produced a composite with nickel and NiAl3 as reinforcement particles in aluminium matrix. The particles were fine and were in the submicrometer size range. The separation distance between the particles was very small. Impression creep experiments were conducted on the samples both at friction stir zone and base material zone at various temperatures. Steady state creep rates were estimated, and activation energy for creep was calculated. It is observed that the friction stir zone offered a higher creep resistance compared to the base metal zone. Higher creep resistance is attributed to the dissolution of nickel atoms into aluminium matrix and the presence of fine nickel particles and NiAl3 precipitates. The measured activation energy indicated that the associated creep mechanism is the dislocation creep in the temperature range of 30–150°C, both in friction stir zone and base metal zone. At higher temperatures (150–180°C) the diffusion creep mechanism is suggested.
Aluminum and its alloys have versatile properties which make them suitable for use in a variety of applications [
Arora et al. [
To investigate the creep behaviour of these composites, impression creep testing is a suitable method [
Commercial pure Al sheet 5 mm thickness was used as the substrate material for friction stir processing. Base metal had coarse grains (average grain size ~160
Initial condition of the substrate and powder materials. (a) Microstructure of the Al substrate showing coarse grains; (b) initial morphology of the nickel powder used.
Using a precision sample cutting machine, the sample was cut perpendicular to the processing zone, and the cut surface was polished using standard metallographic techniques and etched to reveal the macrostructure. Figure
Macroview of the cross section of the friction stir processed sample (A), base metal zone (B).
The macrostructural study reveals two distinct zones, namely, friction stirred zone and base metal zone. They are identified as FSZ (marked by A in Figure
Impression creep experiments were carried out both at FSZ and BMZ using a tungsten carbide indentor. A sketch of the tungsten carbide indentor used is shown in Figure
Dimensions of the indentor used for impression creep experiments.
Figure
Micrograph showing distribution of the particles.
A magnified micrograph from Figure
Particle size of a large number of particles as observed at a magnification of 2000x was measured using SigmaScan software (Jandel Scientific), and it is plotted in Figure
Particle size distribution of the reinforcing particles observed at 2000x magnification.
XRD analysis of friction stir processed region.
Microstructural study along with EDX (Figure
Yadav and Bauri [
Arora et al. [
In the impression creep experiments, a calculated load was applied on the indentor positioned at the desired location in the sample (i.e., friction stir zone or base metal zone). The penetration depth was measured continuously as a function of time, and a creep curve of depth versus time is drawn. A curve for the conditions of room temperature and FSZ for the load of 5 kg is shown in Figure
Penetration depth of the indenter as a function of time. FSZ, room temperature, and 5 kg load.
Using the values of penetration depth and indenter diameter, indentation creep strain was estimated for each curve, following the approach presented by Sastry [
Steady state creep rates in friction stirred zone and base metal (load: 5 kg).
Temperature (°C) | Steady state creep rate (min−1) | |
---|---|---|
Base metal region | Friction stirred region | |
30 | 3 × 10−5 | 8.33 × 10−6 |
100 | 30.4 × 10−5 | 8.7 × 10−5 |
150 | 200 × 10−5 | 90 × 10−5 |
180 | 235.7 × 10−5 | 178.9 × 10−5 |
Variations of steady state creep rate (×10−5) as a function of temperature.
The following observations could be made from the data presented in Figure Creep rates are lower in the friction stirred region than the base metal region at almost all temperatures. Creep rates are low at lower temperatures but they increased exponentially with temperature both for base metal and friction stirred regions.
Activation energy for creep (
where
The activation energy determined is in the range of 140 kJ/mole to 170 kJ/mole in the temperature range of 150–210°C. It is very close to the activation energy for self-diffusion (143.4 kJ/mole) in aluminium [
Higher creep resistance offered by the friction stirred region compared to base metal region is ascribed to the following reasons: presence of nickel atoms into Al matrix by mechanical alloying, precipitation of Al3Ni precipitates, dislocations created by friction stir processing.
Higher level of creep resistance is exhibited by friction stir processed zone compared to base metal zone even at an elevated temperature. It suggests that second phase particles play an important role in the creep mechanism over the temperature range investigated. They act as barriers for dislocation movement. The hindrance to the dislocation movement is related to the distribution of particles and interface between the particles and the matrix [
Hybrid surface composite was made on a commercial pure aluminium by incorporating nickel particles using the friction stir processing route. The composite region consisted of fine nickel and nickel aluminide particles produced in situ during friction stir processing. The dispersed particles were fine and uniformly distributed in the processed region. Impression creep experiments were done using a tungsten carbide indenter of a 2 mm diameter at temperatures in the range of 30–180°C. Plots of indentation creep depth as a function of time were obtained, and these plots were used for estimating the steady state creep rate. It is observed that the steady state creep rate is lower in friction stirred zone compared to base metal zone at all temperatures. Activation energy for creep was estimated and it was observed that at the temperature range of 30–150°C, the dislocation creep is predominant, and at higher temperatures the diffusion creep is playing a major role.
The authors thank Dr. G. Phanikumar and Mr. H. K. Raffi, IIT Madras, for helping in carrying out friction stir processing.