Bonding Characteristics of Al and Al-Alloy Strips: Processed by Severe Plastic Deformation

In this paper, the study of efficient bonding characteristics of AA6014 and AA1050 metal strips is carried out for Accumulative Roll Bonding (ARB) process. Among various Severe Plastic Deformation (SPD) techniques, ARB process is widely used due to its ability to produce ultra-fine grained ( UFG) or nanostructured bulk material. It has been observed that, if the experimental pressure and corresponding temperature achieve their respective threshold values, then the bonding can be successfully joined as compared to the solid state welding. KeywordsComposite Metal Sheet, Ultra-Fine Grained ( UFG), Nano-Structured Material, Severe Plastic Deformation, Accumulative Roll Bonding


II. EXPERIMENTAL PROCEDURE
In order to address the facts as mentioned earlier, a comprehensive experimental study is conducted with two core points, essential bonding criteria, and grain refinement with strengthening sheet metals. Annealing, the commercially pure aluminum i.e. AA1050 is soft, ductile and has the property of excellent workability and for this reason, AA1050 has selected for the present study. The aluminum-alloy i.e. AA6014 has chosen for its better structural properties from the other aluminum alloys. The chemical composition of AA1050 and AA6014 has been shown in Table 1, [15], 19]. The experiment is carried out at the temperatures 150°C and 500°C. The minimum and maximum temperatures were below and upper to the re-crystallization temperature for both types of aluminum metal strips respectively. First of all, the samples of metal strips have prepared for annealing at a temperature of 400°C; they have held for the duration of half an hour in the furnace. Then the heated samples of metal strips were cooled in t h e controlled furnace for 24 hours. In the present work, roll bonding (RB) process was used for the temperature range of 200ºC to 400ºC with AA6014 and AA1050 sheet metals with starting thickness of 0.3 mm and 0.5 mm, respectively. In order to perform the successful bonding of joining, the metal samples were preheated above as well as below the recrystallization temperature, for separate cases. The cleaning, degreasing, and wire-brushing operations were done to those surfaces of sheets to prepare them for rolling. Following the surface preparation, two metal strips having AA6014 and AA1050 have used for stacking one upon another. Then the rolling process was done. 1) Thickness reduction for fixed temperature: The underlying measurements were 20 mm width and 150 mm length for every specimen of aluminum sheets. The roll bonding analysis has begun from 200ºC with the rate of thickness reduction starting from 1% to 20%. The two metals join at the thickness reduction of 20%. Again the rolling was conducted at a temperature of 250ºC with the rate of thickness reduction between 1% and 15%. Likewise, for 300ºC, 350ºC, and 400ºC the rate of thickness reduction started every time from 1% and ended at 15%, 10% and 5% individually, for their adequate bonding. Also, for temperatures 200ºC, 250ºC, 300ºC, 350ºC and 400ºC with corresponding thickness reductions below of 20%, 15%, 10% and 5% the bonding was not done productively.
2) Accumulative Roll Bonding: In the second part of the investigation, the functional reductions have taken just like 20%, 15% and 5% for their comparing preheat temperatures. After the first pass has ended, the rolled samples have cut into two equivalent parts for subsequent passes. At that point, the operations of cleaning, degreasing and wire-brushing were performed before stacking and pre-heating. The sheets have rolled with specified rate of thickness reduction. Again, the rolled sheets have cut by 50% of its length. This procedure has repeated. In the second part of this experiment, the u s e d reductions have 20%, 15% and 5% for their corresponding preheat temperatures. When the first pass was successful, the rolled specimens have cut into two halves for subsequent passes. After this the operations of cleaning, degreasing, and wire-brushing were done before stacking and pre-heating in the furnace. The sheets have rolled considering the mentioned percentage of thickness reductions. Again, the rolled sheets have cut by 50% of its length. This process has repeated up to the third cycle of its rotation. The schematic representation of ARB procedure has illustrated in Fig. 1, [7]. The alloy steel roller having 320 mm diameter, 300 mm length has used for this investigation and the linear surface velocity of the roll was 10 m/minute, in dry condition.
3) Microstructure and Strength: The metallographic tests were set up for optical microscopy by cutting, cold mounting, grinding, cleaning and etching processes. The help of an optical magnifying instrument Leica DM6000M setup has taken for finding the macrostructural image. The mean grain size has measured with intercept technique taking after the quantity of convergences of an altered test line with the grain limits taking after standard ASTM methodology. The bending test has completed in 100kN electro-mechanical controlled Universal Testing Machine (INSTRON 8862). The sample was set up according to ASTM particulars and loaded at a crosshead speed of 0.1 mm/minute. The samples have eventually fizzled after necking and break creating; then the heap versus removal was recorded for rigidity and rate of extensions were assessed. III. RESULT AND DISCUSSION It has observed that, for annealed AA6014 the ultimate strength and percentage of elongation are 172MPa and 20%, and for annealed AA1050 t h e s e v a l u e s are 85 MPa and 22% respectively. Consequently, the strength is increasing with increasing number of cycles of ARB for the three sets of temperature with thickness reduction. The results are presented in Table 2. In Fig.4, individual parent metal strengths are indicated by AA6014 and AA1050 a n d t h e n t h e s e have combined into 215 MPa, for the first cycle single pass in roll bonded condition for preheating temperature as 200°C with 20% reduction of thickness. After the third pass, the strength becomes as the maximum value of 272 MPa for 200°C in multilayered AA6014/AA1050 composite. The values of average strengths are 205 MPa, 230 MPa, and 245 MPa for first, second and third cycle respectively for the temperature of 300°C and 15% thickness reduction combination is indicated by the marked line. It has apparently been observed and shown in Fig.2 and Fig. Three that successful bonding between two metal strips can be achieved with the parametric combination of reduction of thickness in the average elongation of parent metals are designated with AA1050, AA6014 and are transformed to 12%, 10% and 9% elongation for t h e first, second and third cycle of ARB, respectively, as shown in Fig. 5. The percentage of elongation is decreasing with cycle by cycle, which has identified with other two temperatures, in the same figure. But it is increasing with increasing preheat temperature for a fixed number of cycle. The lowest average strength is 200 MPa for the last combination of temperature 400°C and 5% of reduction, but it increases with increasing number of cycles, as 200 MPa for the second cycle and 235 MPa for the third cycle, can be identified with the marked line as shown in Fig.4. Consequently, the strength is increasing with increasing number of cycles of ARB for each pass and preheats temperature of five restrictive conditions. The threshold parameters of bonded samples have accordingly indicated in Table 3. The dot marks having such parameters canton join the AA6014 and AA1050 metallic strip successfully, while the left side of dot marks, the strong bond has not achieved. The outcome reveals that low preheat temperature with high reduction rate and high preheat temperature with low reduction rate combinations can create sufficient bonding. It is also established that the combination of low preheat temperature with a low percentage of thickness reduction is not capable of resulting in successful bonding. To fabricate successful bonding, minimum energy is essential to overcome the initial energy obstruction. This minimum necessary energy can be supplied by rising rolling temperature or by external force followed by general conventional mechanical processes like pressing, hammering, rolling or forging. The percentage of thickness reduction is similar to that of the applied forces, in rolling. The combination of applied force and the preheat temperature contribute the required energy for the successful bonding. In this study, the key factors like pre-heat temperature a n d threshold percentage of thickness reduction govern the successful bonding.

III. CONCLUSION
In the current study, it has been seen that after the third cycle at 200°C preheat temperature with 20% thickness reduction, the average grain size transforms to 8 µ m and 10 µ m for AA6014 and AA1050, respectively; whereas, the mean grain size of fully annealed parent metals are 20 µ m and 30 µ m, respectively. For that reason, it should illustrate the repercussion that grain refinement takes place in ARB process. By the grain refinement, the strength attains the greatest value as 272 MPa which is 1.6 and 3.4 times better than the strength of parent metals AA6014 and AA1050, respectively. In the same way, ductility reduces with increasing of cycle numbers. The current study also investigates the best parameters of minimum joining. For that reason, the preheat temperature and equivalent thickness reduction have created a substantial effect on solid-state bonding in the area of hot rolling which contributes the threshold parameters for bonding of AA6014 and AA1050 strips.
ACKNOWLEDGEMENT The Principal Author is very thankful to Prof. P. B. Sharma, Vice Chancellor Amity University Haryana, for his kind support and motivation.