The metal forming processes of 6061 and 7075 aluminium alloys by forging at elevated temperatures was analysed experimentally. In forging operation, folding, under filling, broken streamline, fracture, coarse grain, and other micro and macro flaws are simply caused due to the deformation properties of aluminium alloys, including slight forge able temperature part, rapid heat depletion to dies, strong bonding, maximum strain sensitivity and high flow resistance. Experimental analysis for different specimens were carried out and compared in terms of forging load to deformation by using pneumatic hammer and hydraulic press and finally after the study it was found out that resistance to deformation rises with the decrease in forging temperature and rise in strain rate in the case of both 6061 and 7075 aluminium alloys and flow stress decreases for both the alloys (Al 6061 & Al 7075) after 50% deformation with showing maximum forge ability (minimum flow stress) under following conditions with minimum reduction of 30% and forging temperature of 450?.

Figure 3: Standard work piece for upsetting

Sample preparation

Aluminium 6061 and 7075 alloys were supplied by Bharat aerospace metals India private limited, Mumbai as represented in (Figure 2). The specimen was considered in a pliable form and placed in a furnace at 529.66°C for one and half hour. Then, it’s dropped into a quench tank to rapidly cool, with this step taking no more than six seconds. From there, it’s formed or straightened and then allowed to set for a designated amount of time. After that, the aluminium part can undergo an aging heat treatment, where it is placed in 176.66? to 260? for 12-24 hours. Aluminium Alloys 6061 and 7075 are used for the experimental work having chemical compositions represented in (Table 1) and the dimensions of the aluminium alloys 7075 and 6061 specimens are presented in (Table 2). The wrought aluminium alloys are cut into 13 pieces/rod for a length of 65 mm per piece as shown in the (Figure 3) and it is machined to length of 60 mm having L/D ratio 1.5. As L/D ratio is limited to 1.5 because of the limitation in hydraulic press that the distance between the two dies is 75 mm only, so we can do upset operation for a maximum length of 70 mm.

Forging equipment

The general arrangement of the pneumatic hammer is shown in (Figure 4). Stainless steel test bearing shaft is held by a pair of bearings. Load is transmitted to the test bearing via three elements in series such as load cell, tie rod and sub-assembly incorporating tension performance nuts, self-straightening ball bearings and two decoupling springs. These springs, with a stiffness much lower than that of the test bearings, the weight of the pneumatic hammer used for the tests was 250 Kg. The tie rod is spirally detected by an innately compensated guide air bearing housed in another cross beam. When it comes to the point of hydraulic press it was weighed about 150 tonnes and is based on the motion of a hydraulic piston guided in a cylinder. Hydraulic presses are essentially load-restricted machines, their capacity for supporting out a forming process is limited by the high obtainable load. Ring stopper and loading of work piece on the stopper is shown the (Figures 5 and 6). Aluminium alloy 6061 is forged under cold condition to the maximum capacity of the hydraulic press till it breaks as shown in (Figure 7), but it is not failed under maximum load rather than deformed to 70% due to low hardness and high ductility. Aluminium alloy 7075 is forged under cold condition in hydraulic Press till it breaks, but it is shear failed at a deformation of 25% due to high hardness and low ductility.

So to decrease the hardness and increase the ductility for 7075 alloy, both the alloys were given annealing treatment as per the cycle.

Metallographic study

The samples for Microstructural observation has been cut from the deformed portion. The samples are grinded & polished using emery papers of grades 120, 220, 320, 400 and 600. After paper polishing the samples are polished in fine cloth using alumina powder with water. Finally, samples are polished by diamond paste. Samples have been polished so that face can be seen like mirror. If there are any scratches are observed they will be removed by polishing them on 600 emery paper and finished touch on cloth polishing. After completion of cloth polishing the samples are etched with keller’s reagent (distilled water of 190 ml, Nitric Acid of 5 ml, HCL of 3 ml, HF of 2 ml) for 10-30 seconds and wash in warm water. The microstructures of polished samples were observed using the optical microscope at a different magnification like 100X and 400X and the photographs are obtained by camera which is fitted with optical microscope.

Forging of aluminium alloy 6061

Forging of aluminium alloy 6061 in hydraulic press: Aluminium alloy 6061 was deformed at different temperatures (Room Temp, 400? & 450?) and were deformed at different degree of deformations (30%, 40%, 50% & 60%). The pressure required for deformation was measured from the dial indicator fitted in the Hydraulic Press and then it was converted into load. Flow stress was calculated from load values. Flow stress calculation for 30% deformation at room temperature is shown below and flow stresses for remaining deformations were calculated similarly.

Flow stress for 30% deformation at room temperature:

(Table 3) shows the variation of load and flow stress for Al 6061 alloy deformed at different amount of deformations and at different temperatures in hydraulic press. From the data analysis as shown in (Figure 8), it is clear that the force required to deform the aluminium alloy 6061 increases with increase in rate of deformation. For any particular deformation rate, the force required for forging decreases with increase in forging temperature.

For all of specimens, after yielding the flow stress reduces monotonically with various softening rates. Flow stress decreases with increase in temperature. In cold forging, the flow stress curve is continuously decreasing, but in hot forging flow stress increases up to 50% deformation and then decreases due to dynamic softening. (Table 4) shows that the effect of deformation on mechanical property (hardness) deformed at various temperatures and different deformations for Al 6061 in hydraulic press. Data analysed in the graph as represented in (Figure 9) shows that the hardness value is continuously increasing with respect to deformation rate due to strain induced hardening for higher percentage of deformation. Hardness values for 6061 alloy are decreased with increasing temperature, but at room temperature the values are less when compared with the deformation at 400?.

Forging of aluminium alloy 6061 in pneumatic hammer: It is unable to find out the load required for forging, as there is no dial indicator fitted to the Pneumatic Hammer. Hence, in this case, only effect of deformation on mechanical property was studied. Shows that the effect of deformation on mechanical property (Hardness) deformed at various temperatures and different deformations for Al 6061 in pneumatic hammer (Table 5) (Figure 10).

Forging of aluminium alloy 7075

Forging of aluminium alloy 7075 in hydraulic press: Aluminium alloy 7075 was deformed at different temperatures (Room Temp, 400? & 450?) and were deformed at different degree of deformations (30%, 40%, 50% & 60%). The pressure required for deformation was measured from the dial indicator fitted in the hydraulic press and then it was converted into load. (Table 6) shows the variation of load and flow stress for Al 7075 alloy deformed at different amount of deformations and at different temperatures in hydraulic press. From the data analysis as represented in (Figure 11), it is clear that the force required to deform the Aluminium alloy 7075 increases with increase in rate of deformation. For any particular deformation rate, the force required for forging decreases with increase in forging temperature. For all of specimens, after initial yielding, the flow stress decreases monotonically with different softening rates. Flow stress decreases with increase in temperature. In cold forging, the flow stress curve is continuously decreasing, but in hot forging flow stress increases up to 50% deformation and then decreases due to dynamic softening. (Table 7) shows that the effect of deformation on mechanical property (hardness) deformed at various temperatures and different deformations for Al 6061 in hydraulic press. Data analysed by graphs shown in the (Figure 12) above state that the hardness value is continuously increasing with respect to deformation rate due to strain induced hardening for higher percentage of deformation. Hardness values for 7075 alloy are decreased with increasing temperature, but at room temperature the hardness values are similar when compared with the deformation at 450°C. Flow stress values of Al 7075 alloy at different temperatures are represented in the (Table 8).

Figure 4: Pneumatic hammer and hydraulic press.

Figure 5: Ring stopper and loading of work piece

Figure 6: Work Piece material with stoppers in hydraulic Press.

Figure 7: 7075 Al alloy work sample failed due to shear (it was upset at RT in HP).

Figure 8: Deformation (%) vs Flow Stress (MPa) for Al 6061 in HP.

Figure 9: Deformation (%) vs Hardness (BHN) for Al 6061 in HP.

Figure 10: Deformation (%) vs Hardness (BHN) for Al 6061 in PH.

Figure 11: Deformation (%) vs Flow Stress (MPa) for Al 7075 in HP.

Figure 12: Deformation (%) vs Hardness (BHN) for Al 7075 in HP.

Figure 13: Deformation (%) vs Flow Stress (MPa) for Al 7075 in PH.

Figure 14: Deformation (%) vs Hardness (BHN) for Al 7075 in PH.

                                                                                                          Initial Al 7075 at 400X                Initial Al 6061 at 400X

Figure 15: Initial microstructure of Aluminium 7075 and 6061 at 400^oC

Al 7075 at 400X                               Al 6061 at 400X

Figure 16: Aluminium alloys deformed to 50% in Hydraulic Press at 450 ^oC

Al 7075 at 400X                                 Al 6061 at 400X

Figure 17: Aluminium alloys deformed to 50% in Hydraulic Press at Room temperature.

Al 7075 at 400X                                            Al 6061 at 400X

Figure 18: Aluminium alloys deformed to 50% in Pneumatic Hammer at 400^oC.

Al 7075 at 400X                                                       Al 6061 at 400X

Figure 19: Aluminium alloys deformed to 50% in [pneumatic hammer at room temperature.

Table 1: Composition of Aluminium alloys.

Table 4: Variation of Hardness for Al 6061 alloy deformed in HP.

Table 7: Variation of Hardness for Al 7075 alloy deformed in HP.

Table 9: Load required for forging Al 7075 in PH at a strain rate of 10 s^-1.

Table 10: Variation of load and flow stress Al 7075 alloy deformed in PH.

Table 11: Variation of Hardness for Al 7075 alloy deformed in PH.

Table 13: Comparison of Flow Stress values for Al 6061 and Al 7075.

Table 14: Comparison of mechanical property (hardness) for Al 6061 and Al 7075.

Forging of aluminium alloy 7075 in pneumatic hammer: It is unable to find out the load required for forging, as there is no dial indicator fitted to the pneumatic hammer. Hence the load required for forging in pneumatic hammer is calculated by using the flow stress values which are taken from the data book for a strain rate of 10s^-1 [35], which were presented in the (Table 9).

Load is calculated as follows:

Load = k × ? × A        (2)

Where,

? = Flow stress, MPa.

A = Area of cross-section at a given deformation, mm².

k = constant and it depends on complexity of the job.

Generally, k =1.2 for simple upsetting operation.

(Table 10) indicates the variation of load and flow stress for Al 7075 alloy deformed at different amount of deformations and at different temperatures in Pneumatic Hammer. From the data analysis from the (Figure 13), it is clear that the force required to deform the Aluminium alloy 7075 increases with increase in rate of deformation. For any particular deformation rate, the force required for forging decreases with increase in forging temperature. After initial yielding, the flow stress decreases monotonically with different softening rates. Flow stress decreases with increase in temperature. In hot forging flow stress increases up to 40% deformation and then decreases due to dynamic softening. (Table 11) shows that the effect of deformation on mechanical property (hardness) deformed at various temperatures and different deformations for Al 6061 in pneumatic hammer. Data analysed by graphs shown in the (Figure 14), show that the hardness value is continuously increasing with respect to deformation rate due to strain induced hardening for higher percentage of deformation. Hardness values for 7075 alloy are decreased with increasing temperature. Flow stress values for different deformations and strain rates at 400oC by using both hydraulic press and pneumatic hammer is represented in the (Table 12) below. Comparison of flow stress and hardness values for both Al 6061 and Al 7075 was performed and shown in the (Tables 13 and 14). Force required for forging 7075 aluminium alloy at room temperature is high when compared to forging at higher temperatures, as it is work hardened due to cold forging. The flow stress and the form of the flow curves is discreetly dependent on the strain rate and temperature. For controlled temperature, flow stress normally rises with the strain rate which is due to the rise in the dislocation solidity and the dislocation expansion rate. Flow softening appears more clearly with the temperature decreasing [15].

Microstructure analysis

The optical microstructures shown in the (Figures 15-19) indicates that the grains of billets compressed become more and more refined with strain rate increasing as well as the degree of dynamic softening and work-hardening higher.

Microstructures of forged work samples

In the current research forging behaviour of aluminium alloys 7075 and 6061 has been studied by using two different forging equipment’s such as hydraulic press and pneumatic hammer to analyse the consequences of strain rate and forging temperature on flow stress. Experimental study was carried out to compare and validate the results and the following conclusions have been drawn:

• The stress value reduces due to the rise in temperature which will result in producing lower stress during flow process. But when it comes to the point of constant and steady temperature the flow stress rises with an increase in strain rate.
• It is noticed from the flow stress deformation graphs that the resistance to deformation rises with reduction in the forging temperature and rise in strain rate. Since that high temperatures and low strain rates provide an extended time for energy extraction and collection.
• The outcomes indicate that the temperature differences have an outstanding consequence on the dynamic softening rate. It is specified that the considerable dynamic softening rate which is associated with dynamic recrystallization guides the flow stress value to decrease gradually.
• The flow stress and shape of the flow curves are sensitively dependent on temperature and strain rate.
• Al 6061 and Al 7075 alloys showed maximum forgeability (minimum flow stress) under following conditions with minimum reduction of 30%, forging temperature of 450? with forging equipment such as hydraulic press
• Forge ability of Al 6061 is higher when compared to that of Al 7075 alloy, as the minimum flow stress for Al 6061 is 81.88 MPa and minimum flow stress for Al 7075 is 109.18 MPa.
• It is noted that the flow stress decreases for both the alloys (Al 6061 & Al 7075) after 50% deformation.
• Even though aluminium has FCC structure, it was noticed that the flow stress at room temperature is high. So the flow stress is reduced by annealing treatment.

• Conflicts of Interest

  The authors declare no conflict of interest.

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