High-temperature dry sliding wear behaviour of pre-aged 3-step T6-treated Al7075 hybrid matrix composite

Abstract

The focus of current work is to narrow the understanding of high-temperature dry sliding wear behaviour of Al7075 and its composite, which contains Si₃N₄ and 2 wt.% granite powder and is pre-aged and T6-treated in 3-steps. The composite is prepared by conventional stir casting. The time to attain peak hardness at 100°C, 120°C and 150°C is taken as the basis to fix the duration of each step in 3-step aging process. The alloy and composite were initially pre-aged and are followed by 3-step T6 treatment. The alloy and composite hardness due to this heat treatment were 185.6 and 197.6 VHN respectively. Further the treated alloy and composite were subjected to temperature dependent isothermal dry sliding wear by choosing the operating temperature as 35°C, 70°C, 140°C and 210°C. The results showed that, average wear rate of alloy was more as compared to composite for each of the temperature and normal load. An inverse relation is noticed among hardness and wear rate. But wear rate was found to vary in direct relation to the temperature. Also the co-efficient of friction showed an increasing trend with increase in temperature. It is found to be noticeably high for alloy than the composite for all temperature and normal load.

Keywords:

• composite
• step aging
• hardness
• high temperature wear
• SEM

1. Introduction

Al and Al alloys are commonly adapted materials in the aerospace and automotive sectors today. In conjunction with these alloys, aluminium MMCs have also become adaptable materials for meeting the demands of applications in current industries and also the market’s increasing demands. One distinguishing feature of these composites is the ability to alter their properties through heat treatment. Aluminium-based composites are favoured for application in aerospace, automotive, sporting goods, defence, electrical, thermal management, and general engineering industries due to their flexibility in changing current features. Al7075-based composites are the most suitable for applications as they have high strength to weight ratio, adaptability, and performability (Sowrabh et al., 2021). Because of their superior UTS to weight ratio, strong resistance to corrosion, and exceptional workability, such composites are the highly preferred materials for aerospace, marine, and automobile parts (Deaquino-Lara et al., 2011, 2015; Lal et al., 2014; Shen et al., 2014; Wu et al., 2014).

Because of their weak tribological characteristics, Al7075 and its composites have limited applicability in wear-resistant applications (Kumar et al., 2012; S. Liu et al., 2019; Loganathan et al., 2020; Manoj & Gadpale, 2019; Mistry & Gohil, 2017; Peng et al., 2018; Ramadoss et al., 2020; Rana & Badheka, 2019; Suganeswaran et al., 2020). Heat treatment is a method used for materials to increase grain development and strength (Alphonse et al., 2021). Dendritic structure, impurities, and intermetallic particles in wrought and cast alloys have a noticeable negative effect on mechanical behaviour and limit use in several applications. As a result, heat treatment as a critical stage in numerous processing sectors is being established in today’s industry to evade the aforementioned faults to the greatest possible extent.

Among all of these heat treatment methods, aging treatment is the most extensively preferred and effective method because, in addition to reducing or removing micro stress in alloy matrix after quenching, it also increases alloy strength and hardness by changing the fraction number, size, and distribution of intermetallic particles (Zhang et al., 2020). Heat treatment has a direct impact on the topographical response of the alloy to air conditions (Fageehi et al., 2021). The multi-step aging approach is a thermal treatment method that evolved from solution aging (Z. Liu et al., 0000). Longer durations in isothermal heat treatment (single step aging) of alloys for industrial use can be minimized by using multiple step aging to achieve identical mechanical properties (Lee et al., 2018; Omer et al., 2018; Osterreicher et al., 2018). In general, first step aging is a low-temperature process that results in fine dispersion of GP zones. Complete precipitation will be attained in the subsequent higher temperature phases (Lee et al., 2018; Stemper et al., 2021). A good processing procedure combined with a correct aging condition can greatly improve the microstructure and performance of the alloys (Jiang et al., 2021; S. Liu et al., 2020; Zhou et al., 2021). Metals and alloys cannot withstand high temperatures due to their limited properties and melting points (Akash et al., 2018). As a result, the working temperature of any material exposed to elevated temperature testing must be significantly lower than its melting point. If the material must withstand high temperatures, it must have special features such as higher refractoriness, lower thermal conductivity, reduced thermal expansion, and chemical stability. In both bulk and coating formats, high-temperature materials are of scientific and technological interest (B. Liu et al., 2010; Rama Rao & Padmanabhan, 2012; Toptan et al., 2010). It may provide significant benefits in aircraft and elevated temperature exposure parts. In high-temperature tribological qualities, aluminium alloy has high adhesion when sliding against steel, poor hardness, and inferior wear resistance. There has been much interest research recently in ultra-high temperature ceramics, which can tolerate high temperatures and improve both mechanical and tribological properties of the alloy without degrading the base properties, to enhance high-temperature wear behavior of aluminium alloy with the dispersion of discontinuous hard ceramic particles with alloy matrix (P. Li et al., 2012; Panda & Chandran, 2006). In this regard, the current study focuses on combining granite powder and Si₃N₄ particles as reinforcements with an Al7075 matrix to better understand the elevated temperature wear behavior of heat-treated composites.

2. Materials

Al 7075 has a high strength to weight ratio and toughness among the family, the weight percentages of Zn 5.4%, Mg 2.6%, and Cu 1.5% were major in this material. The density of Al7075 is 2.83 g/cc. Granite powder (GP) and Si₃N₄ have been chosen as reinforcements. The granite powder utilized has a density of 2.6 g/cc and a compressive strength of roughly 210 MPa. The average particle size of the granite particles is 40 m. Figure 1 depicts the SEM of GP particles as well as their EDAX spectrum.

Figure 1. SEM micrograph and EDAX spectrum of granite powder particles.

Si₃N₄ used has a density of 3.44 g/cc and particle size was in the range 20–30 µm with irregular shaped particulates. Figure 2 depicts the SEM- EDAX spectrum of Si₃N₄ particles.

Figure 2. SEM micrograph and EDAX spectrum of Si₃N₄ particles.

3. Stir casting

To take out dust, oil, and other foreign particles, the mould surfaces are polished by emery paper and washed with acetone. The surfaces are coated with a paste made of graphite, acetone, and water to facilitate removal of solidified castings and prevent molten material from adhering to die surfaces. The dies are then preheated in a muffle furnace for 2 hours at 550°C to aid in the consistent cooling of the melt as it is poured into them. The graphite crucible having Al7075 alloy slabs is placed in the electric furnace, and the furnace temperature is elevated to 800°C.

At this temperature, about 10 g of alkaline powder is added to the melt to remove the slag, followed by hexa chloro ethane (C₂Cl₆, 10 g) for melt degasification and breakdown the pores in it.

Granite dust and Si₃N₄ particles are poured into the melt gradually as stirring (150 rpm) continues at 800°C for 10–15 minutes. Further melt temperature is lowered to 600°C, maintained isothermally for another 10 minutes followed by reheating to 800°C and kept isothermal stirring for additional 10–15 minutes. After the degassing stage, the melt is put into the mould and allowed to harden in ambient air conditions. To cast the rectangular specimens, the melt of composite is poured at 800°C into preheated moulds. Figure 3 depicts the stir casting approach that was used.

Figure 3. Stir casting approach.

As can be observed in Figure 4 the reinforcements are uniformly spread in the matrix. Figure 5 illustrates the composites SEM-EDAX confirming the presence of major chemical components.

Figure 4. Morphology of distribution of reinforcements in the Al7075 alloy.

Figure 5. SEM and SEM-EDAX of the composite.

4. Heat treatment

The different heat treatment conditions employed in the current investigation have been shown in Table 1. The abbreviations used for each of these treatments are also shown in Table 1. Table 2 lists the designation used for alloy and composite samples subjected to age hardening under different aging parameters.

Table 1. Heat treatment conditions and their abbreviations

Heat treatment condition	Abbreviation
Solution treatment	ST
Aging treatment	AT
Pre-aging treatment	PAT
First-step aging treatment	FSAT
Second-step aging treatment	SSAT
Third-step aging treatment	TSAT

Table 2. Alloy/Composite sample designation for different age hardening parameters

Sample ID		Heat treatment parameter
Alloy	Composite
A1	C1	ST-470°C, 2h; AT-100°C
A2	C2	ST-470°C, 2h; AT-120°C
A3	C3	ST-470°C, 2h; AT-150°C

4.1. Peak aging treatment of as cast alloy and composite

The casted alloy and composite samples are subjected to solution treatment and aging treatment in the order illustrated in Figure 6, to determine the time necessary to achieve peak hardness and matching peak hardness (VHN) (Gowrishankar et al., 2019; Shivaprakash et al., 2019, 2022). Casted alloy and composite samples are aged at 100°C, 120°C, and 150°C, respectively. Table 3 shows the attained peak hardness for individual aging temperature at varied time durations for as cast alloy and composite throughout aging.

Figure 6. Heat treatment process for finding peak hardness of alloy and composite.

Table 3. Peak hardness number and time to reach peak hardness of cast composite samples

Sample	Time for peak hardness t (h)	Peak hardness (VHN)
A1	tA1 = 6.5	142.9
A2	tA2 =4.5	139.4
A3	tA2 =3.0	137.9
C1	tC1 = 5.5	153.5
C2	tC2 =3.5	149.6
C3	tC3 =2.5	145.8

4.2. Step aging

The period for aging in step aging is computed based on the number of steps. In 3-step aging, the time of aging at each step is 1/3^rd of single step aging at that temperature, so the entire duration remains unchanged. The time for each step is kept constant so that at lower temperatures, the peak aging duration is taken, and vice versa. Table 4 shows the designations of the alloy and composite samples subjected to different step aging conditions.

Table 4. Alloy/Composite sample designation for different step aging parameters

Sample ID	Step aging parameter
AT	PAT-500°C, 8 h; ST-470°C, 2 h; FSAT-100°C,1 h; SSAT-120°C, 1.5 h; TSAT-150°C, 2.17 h
CT	PAT-500°C, 8 h; ST-470°C, 2 h; FSAT-100°C,84 h; SSAT-120°C, 1.17 h; TSAT-150°C, 1.84 h

4.3. Combined pre age hardening and 3-step aging treatment

Alloy and composite samples are pre-aged, then aged in 3-steps as per heat treatment pattern shown in Figures 7 and 8 (Sowrabh et al., 2023). The pre-aging is done at 500°C for 8 hours and then the furnace is cooled. The lengths of each phase are determined from outcomes of the peak aging treatment. Table 5 summarises split up and overall length for three-phase aging.

Figure 7. Heat treatment pattern for determining VHN of A_T.

Figure 8. Heat treatment pattern for determining VHN of C_T.

Table 5. Steps and total duration of A_T and C_T subjected to pre aging and 3-step aging treatment

Sample	First step duration (h)	Second step duration (h)	Third step duration (h)	Total aging duration (h)
AT	1	1.5	2.17	4.67
CT	.84	1.17	1.84	3.85

5. Mechanical testing

5.1. Hardness test

The Vickers hardness test is performed on cast metal and composite according to ASTM E92–17. The hardness test results are reported in Table 6. A diamond indenter is employed in this test, and a 100 gm(f) load is applied for 15 seconds. The 320, 400, 600, 1000, 1500 and 2000 grit emery was used to polish the test specimen necessarily in the same order to reduce the machining scratches and the effects of surface defects if any on the sample, followed by fine polishing using a velvet cloth on surface with disc speed of 545 rpm.

Table 6. Hardness of 3-step aged specimens

Sample	Hardness (VHN)
AT	185.6
CT	197.6

While fine polishing, a diamond suspension of 3 and 1 µm size is employed in series on fabric. The test was performed in ambient weather, and the hardness of each sample was assessed at five different locations on its surface to calculate the average VHN. The Spider web approach established by authors (Shivaprakash et al., 2022) was adopted in the current investigation to precisely estimate the hardness of the surface.

5.2. Wear test

The wear tests were conducted on A_T and C_T samples as per ASTM G-99. Figure 9 depicts the tribometer details. The single test lasted 30 minutes. Prior to each test, the test specimen contact surface and disc surface were polished with 600 grit silicon carbide emery paper to ensure smooth contact.

Figure 9. TR 20 series neo pin-on-disc tribometer showing (a) Assembly (b) Environmental chamber (c) Wear disc.

Prior to and after test trial, the specimens were washed using an ethanol solution. The specimen mass was measured every 5 minutes during the test to know the loss in mass using a high-precision electronic weighing system with a resolution of 0.001 mg. In addition, the disc track and specimen surface were cleaned on a regular basis with a soft cotton cloth to avoid the entrapment of wear debris. A wear specimen of d = 8 mm and L = 27 mm was maintained with its axis normal to the disc surface, and one end of the pin moved against the disc in a dry friction state under a constant axial load applied with a dead weight. A sliding velocity of 0.4188 m/sec (200 rpm) is selected for testing specimens. Normal loads of 20, 30, 40, and 50 N were used. The test was repeated isothermally at all temperatures shown in Table 7.

Table 7. Wear testing parameters

Test duration (min)	Load (N)	Temperature (^0C)	Track diameter (mm)	Disc speed (rpm)
30	20; 30; 40; 50	35	40	200
		70
		140
		210

The wear rate (Wr) was computed on loss mass basis of specimen, which was noted at the end of each 5 minutes of the test under one perticular load, temperature, speed, and track diameter condition. Tables 8, 9, 10, 11, 12, 13, 14, 15 reflect the findings of the wear test.

Table 8. Outcomes of wear testing on pre aged and 3-step aged alloy (A_T) under the normal load of 20 N, 200 rpm disc speed and at 35, 70, 140 and 210°C

20 N, 200 rpm, 35^0 C					20 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
1	300	0.472	0.126	0.796	2	300	0.474	0.126	1.592
2	600		0.251	0.796	4	600		0.251	1.592
4	900		0.377	1.061	6	900		0.377	1.592
5	1200		0.503	0.995	7	1200		0.503	1.393
7	1500		0.628	1.114	9	1500		0.628	1.433
8	1800		0.754	1.061	11	1800		0.754	1.459
20 N, 200 rpm, 1400 C					20 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
4	300	0.477	0.126	3.184	5	300	0.479	0.126	3.980
7	600		0.251	2.786	8	600		0.251	3.184
8	900		0.377	2.122	11	900		0.377	2.918
10	1200		0.503	1.990	14	1200		0.503	2.786
12	1500		0.628	1.910	16	1500		0.628	2.547
14	1800		0.754	1.857	17	1800		0.754	2.255

Table 9. Outcomes of wear testing on pre-aged and 3-step aged alloy (A_T) under the normal load of 30 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

30 N, 200 rpm, 35^0 C					30 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
2	300	0.554	0.126	1.592	4	300	0.556	0.126	3.184
3	600		0.251	1.194	5	600		0.251	1.990
5	900		0.377	1.327	7	900		0.377	1.857
7	1200		0.503	1.393	8	1200		0.503	1.592
8	1500		0.628	1.273	10	1500		0.628	1.592
10	1800		0.754	1.327	12	1800		0.754	1.592
30 N, 200 rpm, 1400 C					30 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
6	300	0.559	0.126	4.776	7	300	0.562	0.126	5.571
7	600		0.251	2.786	9	600		0.251	3.582
8	900		0.377	2.122	10	900		0.377	2.653
11	1200		0.503	2.189	12	1200		0.503	2.388
14	1500		0.628	2.229	14	1500		0.628	2.229
16	1800		0.754	2.122	17	1800		0.754	2.255

Table 10. Outcomes of wear testing on pre aged and 3-step aged alloy (A_T) under the normal load of 40 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

40 N, 200 rpm, 35^0 C					40 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
3	300	0.590	0.126	2.388	5	300	0.593	0.126	3.980
4	600		0.251	1.592	6	600		0.251	2.388
6	900		0.377	1.592	8	900		0.377	2.122
8	1200		0.503	1.592	11	1200		0.503	2.189
10	1500		0.628	1.592	13	1500		0.628	2.069
12	1800		0.754	1.592	14	1800		0.754	1.857
40 N, 200 rpm, 1400 C					40 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
7	300	0.594	0.126	5.571	9	300	0.595	0.126	7.163
8	600		0.251	3.184	10	600		0.251	3.980
11	900		0.377	2.918	13	900		0.377	3.449
13	1200		0.503	2.587	15	1200		0.503	2.985
14	1500		0.628	2.229	16	1500		0.628	2.547
16	1800		0.754	2.122	19	1800		0.754	2.520

Table 11. Outcomes of wear testing on pre aged and 3-step aged alloy (A_T) under the normal load of 50 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

50 N, 200 rpm, 35^0 C					50 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
4	300	0.607	0.126	3.184	5	300	0.609	0.126	3.980
6	600		0.251	2.388	8	600		0.251	3.184
8	900		0.377	2.122	11	900		0.377	2.918
10	1200		0.503	1.990	13	1200		0.503	2.587
13	1500		0.628	2.069	14	1500		0.628	2.229
14	1800		0.754	1.857	16	1800		0.754	2.122
50 N, 200 rpm, 1400 C					50 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
7	300	0.611	0.126	5.571	9	300	0.613	0.126	7.163
9	600		0.251	3.582	11	600		0.251	4.378
13	900		0.377	3.449	14	900		0.377	3.714
15	1200		0.503	2.985	17	1200		0.503	3.383
16	1500		0.628	2.547	19	1500		0.628	3.025
19	1800		0.754	2.520	21	1800		0.754	2.786

Table 12. Outcomes of wear testing on pre aged and 3-step aged composite (C_T) under the normal load of 20 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

20 N, 200 rpm, 35^0 C					20 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
1	300	0.452	0.126	0.796	2	300	0.456	0.126	1.592
1	600		0.251	0.398	3	600		0.251	1.194
3	900		0.377	0.796	5	900		0.377	1.327
4	1200		0.503	0.796	6	1200		0.503	1.194
5	1500		0.628	0.796	6	1500		0.628	0.955
6	1800		0.754	0.796	8	1800		0.754	1.061
20 N, 200 rpm, 1400 C					20 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
4	300	0.459	0.126	3.184	6	300	0.462	0.126	4.776
5	600		0.251	1.990	7	600		0.251	2.786
7	900		0.377	1.857	9	900		0.377	2.388
8	1200		0.503	1.592	10	1200		0.503	1.990
9	1500		0.628	1.433	12	1500		0.628	1.910
10	1800		0.754	1.327	13	1800		0.754	1.725

Table 13. Outcomes of wear testing on pre aged and 3-step aged composite (C_T) under the normal load of 30 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

30 N, 200 rpm, 35^0 C					30 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
2	300	0.549	0.126	1.592	4	300	0.551	0.126	3.184
3	600		0.251	1.194	5	600		0.251	1.990
4	900		0.377	1.061	5	900		0.377	1.327
4	1200		0.503	0.796	6	1200		0.503	1.194
5	1500		0.628	0.796	7	1500		0.628	1.114
7	1800		0.754	0.929	9	1800		0.754	1.194
30 N, 200 rpm, 1400 C					30 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
6	300	0.553	0.126	4.776	7	300	0.555	0.126	5.571
5	600		0.251	1.990	5	600		0.251	1.990
6	900		0.377	1.592	6	900		0.377	1.592
5	1200		0.503	0.995	7	1200		0.503	1.393
7	1500		0.628	1.114	6	1500		0.628	0.955
9	1800		0.754	1.194	8	1800		0.754	1.061

Table 14. Outcomes of wear testing on pre aged and 3-step aged composite (C_T) under the normal load of 40 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

40 N, 200 rpm, 35^0 C					40 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
1	300	0.585	0.126	0.796	3	300	0.587	0.126	2.388
3	600		0.251	1.194	6	600		0.251	2.388
4	900		0.377	1.061	6	900		0.377	1.592
5	1200		0.503	0.995	7	1200		0.503	1.393
7	1500		0.628	1.114	9	1500		0.628	1.433
9	1800		0.754	1.194	11	1800		0.754	1.459
40 N, 200 rpm, 1400 C					40 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
4	300	0.589	0.126	3.184	6	300	0.592	0.126	4.776
5	600		0.251	1.990	7	600		0.251	2.786
6	900		0.377	1.592	8	900		0.377	2.122
7	1200		0.503	1.393	9	1200		0.503	1.791
9	1500		0.628	1.433	11	1500		0.628	1.751
11	1800		0.754	1.459	14	1800		0.754	1.857

Table 15. Outcomes of wear testing on pre aged and 3-step aged composite (C_T) under the normal load of 50 N, 200 rpm disc speed and at 35°C, 70°C, 140°C and 210°C

50 N, 200 rpm, 35^0 C					50 N, 200 rpm, 70^0 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
3	300	0.601	0.126	2.388	5	300	0.603	0.126	3.980
5	600		0.251	1.990	7	600		0.251	2.786
6	900		0.377	1.592	9	900		0.377	2.388
8	1200		0.503	1.592	11	1200		0.503	2.189
10	1500		0.628	1.592	14	1500		0.628	2.229
11	1800		0.754	1.459	14	1800		0.754	1.857
50 N, 200 rpm, 1400 C					50 N, 200 rpm, 2100 C
Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)	Δm x 10−3 (gm)	t (sec)	Avg. COF	SD x 103 (m)	Wr x 10−7 (N/m)
7	300	0.604	0.126	5.571	9	300	0.606	0.126	7.163
9	600		0.251	3.582	11	600		0.251	4.378
11	900		0.377	2.918	13	900		0.377	3.449
14	1200		0.503	2.786	16	1200		0.503	3.184
16	1500		0.628	2.547	18	1500		0.628	2.865
17	1800		0.754	2.255	19	1800		0.754	2.520

6. Results and discussion

6.1. Implication of heat treatment on hardness of alloy/composite

In comparison to single-step aging, step aging is an alternative aging method used to artificially age nonferrous metals for increasing their strength and hardness. The aging procedure in step aging may consist of 2 to 3 steps, based on the aging dynamics of the alloy system under study. Whereas aging at a higher temperature provides modest property enhancement at a shorter aging duration, aging at a lower temperature needs a very lengthy aging duration with remarkable property improvement. For step aging, a range of temperatures is chosen rather than aging at one temperature. When using a single-step aging process, “t” refers to the number of hours needed to reach peak hardness at the specified aging temperature. Therefore, at lower aging temperatures, “t” is longer, whereas at higher aging temperatures, “t” is shorter. Step aging, which consists of three processes, is used to try and improve the property. Time for aging in three stages is regarded as t/3. In the beginning, aging temperature is provided with one step aging. After the initial pre-aging, 3-stage aging is provided in various combinations of time. Figure 10 compares the variance in hardness of specimens under peak and three-step aging with pre-aging. In contrast to greater aging temperatures, lesser aging temperatures have produced the highest hardness, as can be seen in Figure 10. This is due to rise in intermediate zones during precipitation, a rise in finer intermetallic, and a decline in interparticle distances (J. Li et al., 2008; Osterreicher et al., 2020; Wolverton, 2001). After peak aging circumstances, the hardness value declines as a result of excessive aging, which causes precipitates to coarsen. The alloy generally becomes softer as it ages. The increased rate of solid atom diffusion throughout the matrix results in an increase in aging rate as temperature rises.

Figure 10. Peak hardness for different samples.

For industrial reasons, a longer peak age treatment period is not usually preferred. As a result, multi-stage heat treatments have been advocated as a quicker way to achieve equivalent mechanical properties (Lee et al., 2018; Omer et al., 2018; Osterreicher et al., 2018). These treatments rely on two steps, the first at a low temperature to finely distribute GP zones and the second at a higher temperature to finish precipitation (Lee et al., 2018; Stemper et al., 2021). Due to combined pre aging and 3-step aging process, the alloy hardness has increased to 185.6 VHN and so as the composite hardness to 197.6 VHN. The enhanced hardness is always the result of graded age treatments, with the last aging step having a greater temperature and longer duration. The data above suggest that heat treatment has a major influence on the hardness of matrix alloy.

6.2. Wear behaviour of alloy/composite due to reinforcements and varying temperature

Figures 11, 12, 13, 14, 15 illustrate the change in wear rate with sliding distance and temperature for alloys and composites subjected to pre-aging and three-step T6 treatment. At a specific load and temperature wear rate is observed to be decreasing as the sliding distance increases. At a given normal load as the temperature raises the mass loss and hence the average wear rate also increases for both the alloy and composite. Higher load and temperature always lead to higher average wear rate. Composite specimens are shown to have lower mass loss and wear rates than alloy specimens for same sliding distance and temperature. Also by comparison, the average wear rate is lower for composite and has increased with an increase in the magnitude of normal load due to higher contact pressure. The contact pressure is lower and hence the wear rate is lower under a lower load, 20 N between the disc and pin. Rate of wear for Al7075 at load 20 N is higher than for composites because to its softer character and dislocation networks (Siddesh Kumar et al., 2020). Figure 15 depicts the rate of wear for both alloy and composite is inversely related to hardness but directly proportional to working temperature. The introduction of hard ceramic natured GP and Si₃N₄ particles in Al7075 matrix enhances the composites resistance to wear. The influence of temperature at increased temperatures causes the formation of a tribo-layer; however, when the temperature with applied load rises, the generated tribo-layer vanishes, resulting in a transformation in wear mode from moderate to severe. The matrix material softens at high temperatures, resulting in a faster rate of material loss from the pin material and its attachment to the counter surface, resulting in an enhanced wear rate of the alloy (Ayyanar et al., 2020).

Figure 11. Change of wear rate with sliding distance of A_T and C_T specimen at 200 rpm and 35°C for 20, 30, 40 and 50 N load for (a) alloy and (b) composite.

Figure 12. Change of wear rate with sliding distance of A_T and C_T specimen at 200 rpm and 70°C for 20, 30, 40 and 50 N load for (a) alloy and (b) composite.

Figure 13. Change of wear rate with sliding distance of A_T and C_T specimen at 200 rpm and 140°C for 20, 30, 40 and 50 N load for (a) alloy and (b) composite.

Figure 14. Change of wear rate with sliding distance of A_T and C_T specimen at 200 rpm and 210°C for 20, 30, 40 and 50 N load for (a) alloy and (b) composite.

Figure 15. Change of avg. wear rate of A_T and C_T specimen at 200 rpm, temperature 35, 70, 140 and 210°C for 20, 30, 40 and 50 N load.

Due to tiny dispersal of reinforced particles, the existence of hard GP and Si₃N₄ particles functions as load bearing elements, protecting the pin surface from direct contact with the counter surface and improving composite resistance to wear. With continuous sliding action in high conditions, the matrix material softens, causing the bonding strength to relax; as a result, reinforcements shear across the counter surface. These sheared particles react with the environment, debris, and counter surface, forming a thin protective layer over the sliding surface and reducing composite wear rate. Aside from exceeding 140°C, spalling out of the produced tribolayer causes an increase in wear rate. Under the same heat treatment circumstances, the composite has superior wear resistance than alloy [43].

Figure 16 depicts a scanning electron micrograph of composite wear tested at 210°C working temperature, 50 N load, and 200 rpm disc speed. The photos clearly show the multiple wear scratches suggesting abrasive wear. The composites worn surfaces have fine grooves and minor plastic deformation at the groove edges. The severe wear occurred at fewer points on the surface, resulting in increased delamination. Considerable surface damage and higher-scale material transfer to the counter face were implicated. The existence of isolated oxide deposits and their broken remains is also shown on the surface. Figure 17 depicts SEM-EDAX of the oxide deposit appeared in composite under the operating temperature of 210°C; 50 N load and 200 rpm disc speed. The chemical elements of oxide are as listed.

Figure 16. SEM micrographs of worn out surface of the composite at operating temperature of 210°C; 50 N load and 200 rpm disc speed.

Figure 17. SEM-EDAX of the oxide deposit formed in composite under the operating temperature of 210°C; 50 N load and 200 rpm disc speed.

6.3. Friction characteristics of alloy and composite

Figure 18 shows the friction behaviour of alloy and composite with respect to the operating temperature under a specific normal load. As can be observed the average COF is higher for alloy as compare to the composite. The addition of thermally stable GP particles and Si₃N₄ particles resulted in a considerable differential in thermal expansion coefficient among the matrix and these reinforcements, resulting in greater matrix stability at elevated temperature settings. Because of the considerable thermal disagreement between the matrix and the reinforcements, a rise in temperature causes higher magnitude interface stresses during sliding action. Due to higher friction, reinforced particles are dragged out of the matrix material and create a crack because of increase in interface bonding strength among matrix and reinforcements.

Figure 18. Variation of avg. COF of A_T and C_T specimen at 200 rpm, temperature 35, 70, 140 and 210°C for 20, 30, 40 and 50 N load.

The similar type of behaviour is also noticed by (Loganathan et al., 2021) in their work. Higher magnitude of load has resulted in higher COF for both alloy and composite. Also there is an increase in COF noticed for increase in the operating temperature.

Figures 19 and 20 show the variation of COF captured for operation of composite at 50 and 20 N respectively, both operating at 210°C and 200 rpm disc speed.

Figure 19. Variation of COF captured for operation of composite at 210°C, 50 N and 200 rpm disc speed.

Figure 20. Variation of COF captured for operation of composite at 210°C, 20 N and 200 rpm disc speed.

7. Conclusions

The Al7075 matrix composites consisting of granite powder particles and silicon nitride particles were stir casted successfully with uniform distribution of reinforcements. The combined pre-aging and 3-step T6 treatment has led to an increase in the composite hardness by nearly 28% in comparison to the alloy peak hardened at 100°C. Increased hardness is always the consequence of graded aging procedures, with the last aging phase having a higher temperature and a longer duration. According to the statistics shown above, heat treatment has a substantial impact on the hardness of matrix alloy. The wear test findings showed that the average wear rate of alloy was higher than that of composite for each temperature and normal load. Hardness and wear rate have an inverse relationship. These sheared reinforced particles in composite react with the environment, debris, and counter surface, forming a thin protective layer over the sliding surface and reducing composite wear rate. Wear rate was found to vary in direct relation to the temperature. Also the co-efficient of friction showed an increasing trend with increase in temperature. It is found to be noticeably high for alloy than the composite for all temperature and normal load.

Disclosure statement

No potential conflict of interest was reported by the authors.