Wear and Microstructure Characteristics of Friction Stir Processed Al6063/B 4 C+SiO 2 Composites
3. RESULTS AND DISCUSSION
Fig. 3 Shows the appearance of the SHC fabricated through FSP. It reveals that surface of the specimen is free from defects.
Fig. 3. Appearance of FSP Specimens 3.1 Microstructural Observation
Fig. 4 shows the SEM micrographs of the stir zone after FSP.FSP is an effective grain refinement technique which is obtained by dynamic recrystallisation [29]. The square pin tool plunges on to the substrate generates frictional heating between the contact surface of tool shoulder and work piece followed by dynamic stirring action of the tool tends to flow the material from advancing side to the retreating side [17].
Fig. 4. Microstructure observation of three pass FSP stir zone. (a) 80% Sio2 + 20% B4C (b) 60%
Sio2 + 40% B4C (c) 50% Sio2 + 50% B4C (d) 40% Sio2 + 60% B4C (e) 30% Sio2 + 70% B4C; (f) 20%
Sio2 + 80% B4C
The average grain size 8 μm of all the FSP samples obtained with different weight ratio of reinforcement is observed through optical Microscope image analyzing software. Some of the researchers reveal that agglomeration of ceramic particles in single FSP pass [13] which can be rectify by two or three passes FSP sequentially. It is observed that B4C and SiO2 particles in aluminium matrix are uniformly dispersed and ultrafine grain structure is obtained through three pass FSP in the stir zone. Due to these fine grain attainment features, the mechanical properties were enhanced.
FSP Zone
Base metal
3.2 Wear Characterization
Dry sliding wear behaviour is characterized by pin –on –disc tribometer apparatus as shown in Fig. 5.
The specimen was prepared as per ASTM G99 (10mm square with a height of 30mm) and testing parameters involved for base metal and SHC are specified in the Table 3. The effectiveness of Friction stir processing technique on specific wear rate was calculated using the equation (1).
Specific wear rate (SWR) = ∆
ρ × × (1)
∆m = mass loss in (g) = Weight before wear –Weight after wear ρ - Density of the material (g/mm3).
F - Applied Force (N).
D - Sliding distance (m)
Table 3. Parameters involved for Wear test analysis
Load (N) Sliding Speed(rpm) Sliding Distance(m)
10 200 1000
20 30
Fig. 5. Pin on disc tribometer apparatus
The specific wear rate of Al6063 and SHC with different weight ratio of B4C and SiO2 produced by FSP has been plotted as shown in Fig. 6. It is observed that irrespective of loads, the specific wear rate decreases in the FSPed specimen compared to base metal (Al6063). At a load of 10N, specific wear rate decreases with increasing the relative content of B4C upto 50%, on further increasing the B4C to 70%, 6.46% increase in specific wear rate is obtained when compared to that of base metal.
When the load of 20N is applied, specific wear rate decreases with increasing the relative content of B4C upto 50%, on further increasing the B4C to 70%, 1.14% increase in specific wear rate is obtained when compared to that of base metal.
Disc
Pin
Fig. 6. Specific wear rate on different weight ratio of reinforcements
Fig. 7. SEM Images of the Worn out surfaces of the SHC (a) 80% SiO2 + 20% B4C (b). 50% SiO2 + 50%B4C; (c) 30% SiO2 + 70% B4C (d) 20% SiO2 + 80% B4C at a load of 20N (e) 20% SiO2 + 80%
B4C at a load of 30N
Specific wear rate initially decreases and then increases due to gradual break up of oxide layer on the softer material which results in higher specific wear rate. Then, further increase of B4C to 80%, decrease in specific wear rate is obtained due to the renovation of oxide layer formation. At a load of 30N, specific wear rate decreases on increasing the B4C to 70%, then increases on increasing the B4C to 80% because the reinforced particles gets embedded into the softer surface tends to decrease the specific wear rate ,finally the particles of reinforcements are lifted out of the surface apparently tends to form delamination. SHC obtained through FSP, which has 20% Sio2 and 80% B4C possesses superior wear resistance at a load of 10 and 20 N, similarly SHC having 30% Sio2 and 70% B4C exhibits excellent wear resistance at a load of 30N.
0 9E-05 0.00018 0.00027 0.00036 0.00045 0.00054 0.00063 0.00072
Al6063 80% Sio2 and 20%
B4C
60% Sio2 and 40%
B4C
50% Sio2 and 50%
B4C
40% Sio2 and 60%
B4C
30% Sio2 and 70%
B4C
20% Sio2 and 80%
B4C Specific wear rate in mm3/N-m
Different weight ratio of Reinforcements
10 N 20 N 30 N
c
Fig. 8. EDS Analysis of worn out sample 20% SiO2 + 80% B4C
Fig. 9. EDS Analysis of worn out sample of 80% SiO2 + 20% B4C
Fig. 7 shows the worn out surface micrographs of Al6063 reinforced B4C and SiO2 composites by scanning electron microscope. Fig. 7 (a) at a load of 30N, of 80 % SiO2 and 20 % B4C composite reveals that scratching is obtained in the form of small grooves parallel to the sliding direction. The Reinforced particles gets embedded in the matrix material at a uniform rate indicates the mild abrasive wear occurs. Fig. 7 (b) shows that the abrasive particles of Sio2 gets embedded into the softer material surface forming the aluminium oxide layer(Al2O3) which is confirmed with EDS analysis Fig. 8. Fig. 7(c) shows that the material is removed during wear as patches because the frictional heating occurs upon increasing the load as well as increasing the reinforcement of B4C at few regions.
Also Surface oxide flim that confirm the wear mode is a combination of adhesive and abrasive wear mechanism. Fig. 9 shows that the content of oxygen is employed more Fig. 7 (d) and (e) reveal that the oxide flim was consequently reduced during wear so that larger pits and local delaminating were observed.
Fig. 8 portrays the EDS analysis of 20% SiO2 + 80% B4C worn out samples. Fig. 9 portrays the EDS analysis of 80% SiO2 + 20% B4C worn out sample.