Accumulative Roll Bonding of AA6005 and AA1060 Metal Strip: Study on Microstructure, Mechanical Properties and Evaluation of Minimum Bonding Criteria

Transcription

1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Accumulative Roll Bonding of AA6005 and AA1060 Metal Strip: Study on Microstructure, Mechanical Properties and Evaluation of Minimum Bonding Criteria Suprim Sardar a,1*, Atanu Mandal b,2, Surjya Kanta Pal b,3, Shiv Brat Singh a,4 a Department of Metallurgical & Materials Engineering, IIT Kharagpur, Pin b Department of Mechanical Engineering, IIT Kharagpur, Pin * suprim.sardar@gmail.com, 2 atanuju88@gmail.com, 3 skpal@mech.iitkgp.ernet.in, 4 sbs22@metal.iitkgp.ernet.in Abstract The accumulative roll bonding (ARB) is a plastic deformation process, in which micro-grains and sometimes nanograins are produced. The Severe Plastic Deformation (SPD) processes are widely proposed methods to produce materials with ultra-fine grained (UFG) microstructure. Among various SPD techniques, ARB process is widely used due to its ability to produce UFGs throughout the bulk material. In the present study an ARB process is carried out for the study of efficient bonding characteristics of AA6005 and AA1060 metal strips. Results reveal significant grain refinement along with improvement in ultimate tensile strength. It has been observed that, if the experimental temperature and corresponding pressure achieve their respective threshold values, then the roll bonding or the solid state welding can be successfully joined together. Keywords: ARB, AA6005/AA1060 composite, Grain refinement, Roll bonding 1. Introduction The roll bonding (RB) and ARB are the solid-state welding processes, applied to bond similar or dissimilar metals by rolling; these processes are easy and low cost fabrication methods for mass production of sheet metals. It is possible to manufacture ultra-fine grained (UFG) products by applying the roll bonding process. The other SPD processes, such as, Equal Channel Angular Pressing, High Pressure Torsion, Cyclic Extrusion Compression, Continuous Confined Strip Shearing and Mechanical Milling processes have the following disadvantages. Firstly, the required forming dies are quite expensive; also the required forces are extremely high. Secondly, the above mentioned processes are found to be inappropriate to produce bulk materials. As compared to the above techniques, ARB has no such shortcomings. The ARB process was initially suggested by Saito et al. (1998) to produce ultrafine grained bulk aluminum sheets. Subsequently, the ARB process was revised by Tsuji et al. (1999) through producing UFG bulk sheet of interstitial free (IF) steel, which had the average grain size less than 1µm. Hongzhi et al. (2004) stated the development of efficient joining of Al 6111 alloy which exhibits high shear strength compared to the parent alloy. The four vital controlling factors of the roll bonding process were reheating temperature, entry temperature, percentage of reduction and the rolling speed. Among the above mentioned four factors, rolling temperature was found to be the significant factor influencing rolling bonding. Saito et al. (1999) have conducted the same experiment with AA5083, IF steel and AA1100. The results indicate that the strength of AA1100 increased from 84 MPa to 304 MPa after 8 cycles, and the elongation was decreased from 42 % to 8 %. The strength of AA5083 increased from 319 MPa to 551 MPa after 7 cycles and elongation decreased from 25 % to 6 %, but for IF steel the strength was raised from 274 MPa to 751 MPa after 5 cycles, and the elongation decreased from 57 % to 6 %. Tsuji and his associates (1999) mentioned that the true strain reaches 4.0 at 200 C and average grain size decreases to 280 nm. Eizadjou et al. (2008) experimentally investigated the bond strength by peeling of AA1100. AlsoAA1100 strips were improved its strength with increase of total thickness reduction and temperature. Finally the peel strength reached the value of base metal strength. The bond strength was increased by increasing the rolling temperature at constant reduction in the percentage of thickness of two Al 1100 and Al 1100 bi-layered strips. Manesh et al. (2003) applied large plastic deformation on two types of metals like aluminium and steel to produce the cold roll joining. The work hardened sheet was annealed and as a result higher ductility was achieved. It was recommended that the heat treatment of the sheets were 610-1

2 Accumulative Roll Bonding of AA6005 and AA1060 Metal Strip: Study on Microstructure, Mechanical Properties and Evaluation of Minimum difficult due to the development of a brittle intermetallic compound of Al and iron. The formability of the Al claded steel sheet depended on time and temperature of the annealing. The duration of annealing time and heating temperature were the key factors of transformation of bond strength of Al claded steel sheets. The inter-metallic phase thickness strength was increased by increasing the annealing temperature at constant duration of time. Movahedi et al. (2008) found that better influence of the annealing temperature; up to 450 C can improve the joining strength. The formation of brittle Al-Fe inter-metallic film at the bond boundary reduced the bond strength at the annealing temperature of 500 C. Furthermore, increase in the reduction of thickness improved the overall joint strength. Therefore, the above discussed studies have completed the experiments on grain refinement, share strength improvement, peel strength improvement, essential controlling factors of roll bonding, increase ductility and formability by annealing; but there is a lack of research has been identified on minimum bonding criteria for RB and ARB. As a result, the present study gave the priority to find out threshold parameters for roll bonding. 2. Experimental procedure In order to address the aforementioned aspects, a detailed two-stage experiment is conducted. The first component of the experiment aimed at developing the minimum and the essential bonding criteria. The grain refinement and the strengthening were demonstrated in second part. 2.1 Selection of parent metals and temperatures The aluminium-magnesium-silicon alloy AA6005 was chosen for their comparative better structural properties than other aluminium alloys. The commercially pure aluminium i.e. AA1060 is soft, ductile and it has the property of excellent workability, after annealing. For this reason AA1060 was picked up also for the present study. The chemical composition of AA6005 and AA1060 has been shown in Table 1. The experiment is conducted between the temperatures 200 C and 400 C. The lowermost and topmost temperatures were below and upper to the recryatsllization temperature for both types of aluminium respectively. First of all, the experimental specimens were prepared with fully annealing of parent metals by heating them at a temperature of 380 C. They were held for a duration of 1800 second in the furnace. Then the heated parent metals were cooled in controlled atmosphere inside the furnace along second. 2.2 Initial roll bonding for parent metal In the present work, ARB processing was done in the temperature range from 200ºC to 400ºC with AA6005 and AA1060 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 and below of the recrystalization 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 of AA6005 and AA1060 were stacked one upon another. Then the rolling process was done. 2.3 Thickness reduction for fixed temperature The initial dimensions were 20 mm width and 150 mm length for each sample of aluminium sheets. The roll bonding experiment was started from 200ºC with the percentage of thickness reduction starting from 1% to 19%. The two metals bonded at the thickness reduction of 19%. Again the rolling was conducted at a temperature of 250ºC with the percentage of thickness reduction between 1% and 16%. The two metals were stacked together with successful bonding at a thickness reduction of 16%. Similarly for 300ºC, 350ºC and 400ºC the percentage of thickness reductions were started from 1% and stopped at 13%, 10% and 6% respectively, for their successful bonding. Moreover, for temperature 200ºC, 250ºC, 300ºC, 350ºC and 400ºC with correspondingly thickness reductions below of 19%, 16%, 13%, 10% and 6% the bonding was not done fruitfully. 2.4 Accumulative Rolling Bonding In the second part of the experiment, the applied reductions were taken only as 19%, 13% and 6% for their corresponding preheat temperatures. After the first pass was completed, the rolled specimens were cut into two equal halves for subsequent passes. Then the cleaning, degreasing and wire-brushing operations were done before stacking and pre-heating in furnace. The sheets were rolled with mentioned percentage of thickness reductions. Again, the rolled sheets were cut by 50% of its length. This process was repeated Table 1 Chemical composition of Aluminium alloys Al alloys Si Fe Cu Mn Mg Ti Ga V Others Al AA balance AA balance 610-2

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India (e) (d) (c) (f) (a) (b) Figure 1 Schematic diagram of various steps of accumulative Roll-Bonding (a) Straight sheet metal, (b) Cutting of the sheet in to equal length, (c) Roughing and degreasing (d) Stacking of two strips, (e) Preheating in furnace, (f) Roll-bonding up to third cycle of its rotation. The schematic representation of ARB process is shown in Figure 1. The alloy forged steel cylindrical roll of ϕ320 mm diameter, 300 mm length was used for this experiment and the linear surface velocity of roll was 10 m/minute, in non lubrication condition. 2.5 Microstructure and Strength The metallographic samples were prepared by cutting, cold mounting, grinding, polishing and etching process for optical microscopy. The microstructural image was taken with the aid of an optical microscope Leica DM6000M setup. The mean grain size, G was measured with intercept method following the number of intersections of a fixed test line with the grain boundaries following standard ASTM procedure. In addition to this, the tensile-test was carried out in 100kN electro-mechanical controlled Universal Testing Machine (INSTRON 8862). The specimen was prepared as per ASTM specifications, and loaded at a cross head velocity of 0.1 mm/minute. The specimen was finally failed after necking and crack generating; then the load versus displacement was recorded for tensile strength and percentage of elongation were evaluated. 3. Results and discussion In the present observation, the average grain size of fully annealed parent metals of AA6005 and AA1060 were recorded as 18 µm and 32 µm respectively. After the third cycle, at 200 C preheat temperature and 19% thickness reduction of successful bonding, the average grain size of AA6005 and AA1060 were measured as 9 µm and 11 µm respectively. The optical micrographs of annealed parent metals and accumulative roll bonded metals for 19% reduction at 200 C are presented in Figure 2. Moreover, the prominent joining lines are indicated using white arrows in the optical image of 100 magnification, in Figure 3, after the third cycle of ARB stripes. It is observed that, the ultimate strength and percentage of elongation for annealed AA6005 are 170 MPa and 19%, and for annealed AA1060 are 82 MPa and 23% respectively. In Figure 4, individual parent metal strengths are indicated by point a for AA6005 and b for AA1060 are combined into 210 MPa, for first cycle single pass in roll bonded condition for preheat temperature as 200 C with 19% thickness reduction. The strength reaches its maximum value 271 MPa for 200 C in multilayered AA6005/AA1060 composite after the third pass. The average strength values are 202 MPa, 228 MPa and 245 MPa for first, second and third cycle respectively for the temperature of 300 C and 13% thickness reduction combination is indicated by marked line. The lowest average strength is 199 MPa for the last combination of temperature 400 C and 6% of reduction, but it increases with increasing number of cycles, as 219 MPa for second cycle and 236 MPa for third cycle, can be identified with marked line in Figure 4. 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. The average elongation of parent metals are designated with c for AA1060 and d for AA6005 are transformed to 11%, 9% and 8% elongation for first, second and third cycle of ARB, marked with can be evidenced in Figure 5. The percentage of elongation is decreasing with cycle by cycle, which is identified with other two temperatures, in the same figure. But it is increasing with increasing preheat temperature for a fixed number of cycle

4 Accumulative Roll Bonding of AA6005 and AA1060 Metal Strip: Study on Microstructure, Mechanical Properties and Evaluation of Minimum a b c d Figure 2 Optical micrograph of parent and ARB metals for 19% reduction and 200 C preheat temperature (a) Fully annealed parent metal AA1060, (b) Fully annealed parent metal AA6005, (c) AA1060 after 3 rd cycle and (d) AA6005 after 3 rd cycle. It has clearly been observed that successful bonding between two metal strips can be achieved with the parametric combination of reduction of thickness in Table 2 Average strength and average percentage of elongation after ARB of multi layered AA6005/AA1060 composite 1 st cycle 2 nd cycle 3 rd cycle UTS (MPa) Elongation % UTS (MPa) Elongation % UTS (MPa) Elongation % Figure 3 Optical micrograph of roll bonded sheet after third pass 200⁰C 19% reduction 300⁰C 13% reduction 400⁰C 6% reduction

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Figure 4 The average tensile strength of parent metals before rolling and after ARB, versus cycle no. each pass and preheats temperature of five limiting conditions. The threshold parameters of bonded samples are duly indicated (using tick symbol) in Table 3. The tick marks containing such parameters which are able to join the AA6005 and AA1060 metallic strip effectively, whereas left side of tick marks, successful bond was not achieved. The results reveal that low preheat temperature with high reduction rate and high preheat temperature with low reduction rate combinations can produce successful bonding. It is also found that the combination of low preheat temperature with low percentage of thickness reduction was unable to result into successful bonding. Table 3 Effect of preheat temperature on roll joining Limiting Preheat temperature thickness reduction 200 C 250 C 300 C 350 C 400 C 19% 16% 13% 10% 6% To produce successful joining minimum energy is required to overcome the initial energy barrier. This essential minimum energy can be supplied by increasing rolling temperature or by external applied force followed by general traditional mechanical processes like pressing, hammering, rolling or forging. The percentage of thickness reduction is comparable to the applied forces, in rolling. The combination of applied force and the preheat temperature contribute the required energy for the successful bonding. In this analysis, the key factors like pre-heat temperature and Figure 5 The average elongation % of parent metals before rolling and after ARB, versus cycle no. threshold percentage of thickness reduction govern the successful bonding. 4. Conclusion In the present study, it has been seen that after third cycle at 200 C preheat temperature with 19% thickness reduction, the average grain size transforms to 9 µm and 11 µm of AA6005 and AA1060, respectively; whereas, the mean grain size of fully annealed parent metals are 18 µm and 32 µm, respectively. For that reason, it should draw the implication that grain refinement occurred in ARB process. By the grain refinement the strength reaches the maximum value as 271 MPa which is 1.6 and 3.3 times greater than the strength of parent metals AA6005 and AA1060, respectively. Correspondingly, ductility decreases with increasing of cycle numbers. The present research also explores the optimal parameters of minimum joining. Therefore, the preheat temperature and corresponding thickness reduction have found significant effect on solid-state bonding in the area of warm rolling and hot rolling that contribute the threshold parameters for bonding of AA6005 and AA1060 strips. References Eizadjou, M., Manesh, H.D. and Janghorban, K. (2008), Investigation of roll bonding between aluminum alloy strips, Materials and Design, Vol. 29, pp Hongzhi, Y. and Lenard, J.G. (2004), A study of warm and cold roll-bonding of an aluminium alloy, Materials Science and Engineering A, Vol. 385, pp Manesh, H.D. and Taheri, A.K. (2003), Bond strength and formability of an aluminum clad steel sheet, Journal of Alloys and Compounds, Vol. 361, pp Movahedi, M., Madaah-Hosseini, H.R. and Kokabi, A.H. (2008), The influence of roll bonding parameters 610-5

6 Accumulative Roll Bonding of AA6005 and AA1060 Metal Strip: Study on Microstructure, Mechanical Properties and Evaluation of Minimum on the bond strength of Al-3003/Zn soldering sheets, Materials Science and Engineering A, Vol. 487, pp Saito, Y., Tsuji, N., Utsunomiya, H., Sakai, T. and R.G. Hong. (1998), Ultra-Fine Grained Bulk Aluminum Produced By Accumulative Roll-Bonding (ARB) Process, Scripta Materialia, Vol. 39, No. 9, pp Saito, Y, Utsunomiya, H., Tsuji, N. and Sakai, T. (1999), Novel ultra-high straining process for bulk Materials of development of the accumulative rollbonding (ARB) process, Acta Materialia, Vol. 47, No. 2, pp Tsuji, N., Saito, Y., Utsunomiya, H. and Tanigawa, S. (1999), Ultra-fine grained bulk steel produced by Accumulative roll-bonding (ARB) process, Scripta Materialia, Vol. 40, No. 7, pp Tsuji, N., Shiotsuki, K. and Saito, Y. (1999), Superplasticity of UFG Al-Mg alloy produced by ARB, Materials Transactions, The Japan Institute of Metals and Materials, Vol. 40, No. 8, pp