Advanced Manufacturing Process Accumulative Roll Bonding of Aluminium 2011 Table of Contents I. Introduction3 II. Roll Bonding4 II. 1. Surface Preparation4 II. 2. Bonding Mechanism5 II. 3. Roll Bonded Materials and Applications6 III. Accumulative Roll Bonding7 III. 1. Introduction to Severe Plastic Deformation (SPD) Processes7 III. 2. Accumulative Roll Bonding (ARB) Process8 III. 3. Accumulative Roll Bonded Materials10 III. 4. Material Structure after Accumulative Roll Bonding11 III. 5. Mechanical Properties after Accumulative Roll Bonding13 III. 5. 1. Strength and ductility13 III. 5. 2. Hardness15 III. 6.
Applications of Accumulative Roll Bonding Process16 III. 6. 1. Manufacturing of a Cu/Al2O3 composite16 III. 6. 2. Manufacturing of nanostructure Al/SiCp Composite17 IV. Conclusion18 References19 I. Introduction Research in severe plastic deformation (SPD) processes has increased a great deal over the last ten to fifteen years. Accumulative roll bonding is a radical, new (1998) process with limitless capabilities. Rolling is a process which has been used for a long time in almost all the steel mills around the world. The purpose of roll bonding is to combine two materials through rolling in order to achieve the desired mechanical properties.
This report intends to explore the evolution of the mechanical properties such as ductility, yield strength, ultimate tensile strength and hardness through grain refinement at the microstructural level. The mechanism of surface deformation, grain boundary deformation and reduction of work hardening is further analysed. This report also makes an effort to understand the accumulative roll bonding process as applied to aluminium and its alloys. II. Roll Bonding Roll bonding or also called roll welding is a cold welding process which is performed by the application of pressure on long pieces / strips through a pair of rolls.
The process can be performed by application of heat or otherwise (Kalkpakjian et al. 2009, p760). To perform roll bonding, the material should satisfy certain characteristics which are ductility and crystallographic structures. Firstly, in roll bonding, at least one or both of the materials should have superior ductile properties (Kalpakjian et al. 2011, pp819-820). The reason for this is that if the material is not ductile, the failure would occur at a sooner stage of the plastic deformation and therefore, the bonding would not occur.
Secondly, with respect to crystallographic structures, for the roll bonding of two different materials, the difference in the atomic radius should be less than 15% and the two metals should have the same crystal structures (Kalpakjian et al. 2011, p124). Figure 1. roll bonding principle (Li et al. 2008, p2) II. 1. Surface Preparation Before roll bonding, the work surfaces should be prepared. This is performed by degreasing, wire brushing and wiping in order to remove oxide layer (Kalpakjian et al. 2011, p820). In addition, it is also done by other chemical and mechanical treatment (Danesh et al. 008, p2003). Indeed, it is really important to remove this oxide smudge firstly to avoid any corrosion problem which would occur during the part’s lifetime and secondly, to increase the bond strength between the two materials (Movahedi et al. 2007, p417). II. 2. Bonding Mechanism More specifically, in roll bonding, the establishment of the atomic bond happens between decontaminated areas. During the rolling operation, the surface layers which are in contact with each other break-up and the underlying layers of the components extrude through the cracked layers.
The bond strength between the two materials is dictated by parameters such as material thickness, surface preparation method, surface roughness, time taken between surface preparation and rolling, rolling temperature, reduction of thickness during rolling, rate of deformation and heat treatment after bonding (Movahedi et al. 2007, pp417-418). The geometry of the deformation zone, the bonding length and the mean contact pressure also play a big factor (Danesh et al. 2008, p2003). Many authors have developed theoretical models in order to predict the weld efficiency.
Basically, the method consists to estimate the effective load-bearing area of peel test specimens. The theoretical models are as follows: Model of Vaidyanath et al. : ? = SwSm=Rf(2-Rf) Where, Sw is the strength of the weld, Sm the strength of base metal and Rf is the final reduction at the end of the rolling pass (Vaidyanath et al. cited in Madaah-Hosseini 2001, p186). Model of Wrigth et al. : ? = SwSm=H1-1-Rf21-Rt2 Where, H is an empirical hardening factor, Rf is the final reduction at the end of the rolling pass and Rt is the reduction at threshold deformation (Wright et al. ited in Madaah-Hosseini 2001, p186). II. 3. Roll Bonded Materials and Applications Some of the common materials that are used for roll bonding are cupronickel (Cu-Ni) which is used for example in manufacturing the United States quarter cents (Kalpakjian et al. 2011, p820), Copper/Aluminium, Aluminium/Steel, Copper/Copper, Copper/Iron, Copper/Silver, Zinc/Zinc, Aluminium/Aluminium, Lead/Lead, Tin/Tin. The figure below shows the different materials commonly used in roll bonding. In addition, this figure represents the bond strength as function of the deformation reduction (Li et al. 2008, p3). Figure 2. ond strength as function of the deformation reduction (Cold roll bonding process) (Li et al. 2008, p3) Some of the common applications of the different metal composites produced by roll bonding are for example: * Cooking utensils, roof and wall plate, heat exchangers, special engraving plates and electrical components produced in Al/Cu, * Cookware, roast and bowl for induction heater produced in Al/Cu/Fe, * Reflectors in electric heaters, automobile silencers produced in Al/Fe, * Automotive trims produced in Al/Stainless steel, * Automotive exhaust systems produced in Al/Steel/Al, Electrical components produced in Ag/Cu, * Bullet jacket produced in Cu/Fe, * Commutator plate, armature winding wire, cooking utensils produced in Cu/Stainless steel, * Communication cable shields buried in acidic soil produced in Cu/Stainless steel/Cu, * Battery cap produced in Ni/Stainless steel/Cu, * Bipolar electrode in fuel cell produced in Ti/Stainless steel/Ni, * Coin produce in Ag/Cu/Ag or Ni/Cu/Ni, * Printing plate for high-speed wrap-around press produced in Al/Zn (Li et al. 2008, p9). III. Accumulative Roll Bonding III. 1. Introduction to Severe Plastic Deformation (SPD) Processes
Materials with Ultra Fine Grain structure, i. e lesser than 1 micrometer, are supposed to have superior mechanical properties. Such bulky materials are fabricated in a process called as Severe Plastic Deformation. Plastic strain over 4 is applied to the material so that ultra fine grain structure is formed in heavily deformed areas (Tsuji et al. 2003, p338). Some of the successful SPD processes along with accumulative roll bonding are Equal Channel Angular Extrusion (ECAE) and High Pressure Torsion (HPT). In ECAE, the shear deformation is achieved by applying force on the material which is placed in an angular die.
The ECAE process can be further classified as continuous and batch ECAE process. In the HPT process circular or disc shaped components can be deformed by application of high pressure through torsion. This process is limited to small and thin components (Tsuji et al. 2003, p339). Some other SPD processes include Cyclic Extrusion Compression, Continuous Cyclic Bending and Repetitive Corrugation and Straightening. The CEC process involves both extrusion and compression, where the earlier is used for decrease the diameter of the components and the latter is used for increasing the diameter of the same.
This is carried out repetitively to achieve the Ultra fine grains. The CCB process involves bending the component repetitively in both directions to develop high strain in the material. The RCS process is still in development and is mostly similar to the CCB process. The difference being the strain per pass, which is higher in the RCS process, which is attained by reducing the corner radii of die and tools (Tsuji et al. 2003, pp339-340). III. 2. Accumulative Roll Bonding (ARB) Process ARB is the only SPD process which involves rolling for deformation, as it is the most feasible way for continuous production.
This process was invented by Saito et al. in 1998 to achieve UFG in metallic materials by sever plastic deformation. Figure 3. Schematic illustration showing the principle of the accumulative roll-bonding (ARB) process (Tsuji et al. 2003 p340) In an ARB process, the two materials of equal size are surface treated and then stacked upon each other. Then it is subjected to bonding by the rolling process, and the thickness of the material is almost constant. The process is repeated to increase the strain across the surface of the material and thereby achieve UFG.
Very much like the roll bonding, the surfaces of the materials are wire brushed and degreased below the re-crystallisation temperature for oxidation and to increase the bonding between the materials. The table below shows the relation between layers, cycles, bonded boundaries, interval, reduction and equivalent strain. For instance consider a material of 4 layers; this would need to go through 2 cycles and it would have 3 bonded boundaries when stacked upon each other. The interval or the gap between the layers would be 250 micro meter. The thickness is reduced by 75% and the equivalent strain is 1. . According to the studies conducted by Tsuji et al, bonding is not difficult to achieve in ARB. For materials like low carbon steel, bonding can be achieved even at room temperatures; however the surface treatment is very important. Usually the critical reduction is dependent on the materials, but it is necessary to achieve more than 35% in the first pass as it makes the roll force large when compared to conventional rolling. As rolling is not a hydrostatic process high amount of stress is accumulated on the edges, which result in the formation of the cracks.
It has been found that cracks occur at higher cycles. This leads to frequent failure which is the main limitation of the ARB process. However by following certain simple processes cracking can be avoided. According to Tsuji et al, pure aluminium and iron having dimensions of 1X50X300 can be fabricated with simple infrastructure and most importantly cracking can be avoided (Tsuji et al. 2003, pp340-341). III. 3. Accumulative Roll Bonded Materials ARB can be largely applied to most of the ductile materials and their alloys. The table below illustrates some of the materials used in Accumulative roll bonding.
The ARB process applied to different materials can be illustrated from the data available in the above table. For further understanding, let us take the instance of 100-Al (99% Aluminium). After 8 accumulative roll bonding cycles performed at room temperature, the microstructure of aluminium changes to pancake UFG. In addition, the grain size also changes to 0. 21? m and the tensile strength increases from 80 MPa (Aalco 2011) to 310 MPa. Similarly, when ARB is applied to 5083-Al (Al-405Mg+0. 57Mn), after 7 cycles performed at 100°C, the microstructure changes to ultrafine lamellae, the grain size changes to 0. 8? m and the tensile strength increases from 300 MPa (Aalco 2011) to 530MPa. Based on the table and mechanical properties of these materials before accumulative roll bonding, it can be easily see that accumulative roll bonding refines the grain to ultra fine and most crucially enhances the strength of the material (Tsuji et al. 2003, p342). III. 4. Material Structure after Accumulative Roll Bonding It has been seen in the previous part of the report that the grain is refined after accumulative roll bonding. It can be further justified with the study conducted by Elseaidy et al. and Pirgazi et al..
Pirgazi et al. ‘s study of accumulative roll bonding when performed on AA1100 shows the change of grain size at different stages of the ARB process. The figure below illustrates this evolution. Figure 4. Evolution of the grain size After ARB (Pirgazi et al. 2008, p2845) Figure 5. Evolution of aspect ration and subgrain size after ARB (pirgazi et al. 2008 p2845) It can be seen from the figure 4 that the length of the grain changes from just over 3? m after two cycles to just over 1? m after ten cycles. Similarly, the thickness decreases steadily from just over 1? m at two cycles to approximately 0. ? m at ten cycles. The grain size changes drastically from 3 ? m at two cycles to approximately 0. 7? m after ten cycles (Pirgazi et al. 2008, p2845). The major dimension out of the minor dimension is described as the aspect ratio. It can be seen from the figure 5 that the aspect ratio decreases from 0. 35? m at two cycles to just over 0. 2? m after five cycles. Then, there is a sharp increase to 0. 4? m at eight cycles and it increases further to 0. 44? m at ten cycles. In addition, sub-grain is part of the grain that is slightly disoriented from other part of the grain.
It has been found that the higher density of sub-grain increases the yield stress of the material. Sub-grain size decreases linearly from 0. 6? m at two cycles to 0. 4? m at six cycles and then steadies at 0. 35? m at ten cycles (Pirgazi et al. 2008, p2845). Elseaidy et al. ‘s study of accumulative roll bonding when performed on AA6061 shows the change of grain size at different stages of the ARB process. AA6061 contains aluminium, magnesium and silicon in higher percentages. It also has other alloying elements like copper, chromium, iron, tin and zinc in lower proportions.
The figure below illustrates the evolution of the grain size of AA6061 after accumulative roll bonding. Figure 6. SEM micrographs of the AA6061 ARB specimen after a) two, b) four, c) six, d) eight cycles in the rolling direction (Elseaidy 2007, p5) When AA6061 was observed under the scanning electron microscope, it can be seen that the microstructure of the ARB processed material at one cycle has larger grain in the direction of rolling with clear grain boundaries. The figure 6 illustrates the change of grain size from 45? m before accumulative roll bonding to 3? after eight ARB cycles (Elseaidy et al. 2007, p3). Hence, it can be concluded that the accumulative roll bonding process has an effect of grain refinement after each cycle. As per Hall Petch formula ? y=? 0+kyD where, ? y is the yield stress, ? 0 is a material constant for the starting stress of dislocation moment, ky is strengthening coefficient and D is the grain diameter. Thus, it is evident from this formula that the yield stress increases when the grain size decreases. III. 5. Mechanical Properties after Accumulative Roll Bonding III. 5. . Strength and ductility Figure 7. Tensile properties of the AA6061 ARB processed sheets at 500°C (Elseaidy 2007, p7) As per Elseaidy et al. , the ARB process of the AA6061 was performed at 500°C. It can be seen from the figure 7 that mechanical properties such as ultimate strength, yield strength and total elongation change with respect to the number of ARB cycles. More specifically, the yield strength of the material increases from around 60MPa to 120MPa after one ARB cycle. It continues to increase gradually till 250MPa after eight cycles.
Similarly, the ultimate tensile strength increases from 110MPa to 130 MPa after one cycle of ARB and continues to do so to just over 250 MPa after eight cycles. The most important aspect to observe in this graph is that the difference between ultimate tensile strength and the yield strength is very small after 8 cycles. Therefore, It can also be seen that there is a decrease in the work hardening area. In addition, the total elongation decreases sharply from 30% to 10% and increases steady to about 14% after eight cycles (2007, p7). Figure 8.
Tensile properties of the AA1100 ARB processed sheets at 500°C (Elseaidy 2007, p8) A similar change in ultimate tensile strength, yield strength and total elongation can be observed at various cycles of ARB process when performed on AA1100 at 500°C as shown on the figure 8 (Elseaidy et al. 2007, p8). From the earlier conclusions, it was learnt that with increasing the number of ARB cycles, the grain size decreases, and with decreasing the grain size, strength increases. III. 5. 2. Hardness Figure 9. Evolution of the Vickers hardness in ARB process (Hoppel et al. 2004, p220)
Based on the study of Hoppel et al. it can be observed on the figure 9 that the Vickers hardness increases very drastically from 20HV to 55HV after just one cycle but the rest of the cycle fail to show measurable increase in Vickers hardness and the value settles at 60HV after eight ARB cycles. From this graph, it can be concluded that with decreasing grain size, the hardness increases (2004, p220). To conclude, it is observed that the accumulative roll bonding process applied on AA1100 and AA6061 enhances mechanical properties such as yield strength, ultimate tensile strength and hardness.
It can also be seen that the ductility decreases with increasing ARB cycles. III. 6. Applications of Accumulative Roll Bonding Process (c) Figure [ 10 ]. Evolution of tensile strength, elongation and hardness of copper and copper/alumina in ARB process (toroghinejad 2010 pp7433-7434) III. 6. 1. Manufacturing of a Cu/Al2O3 composite Toroghinejad et al. studied on the application of the accumulative roll bonding process to Cu/Al2O3. Copper is widely known for its high thermal and electrical conductivity. However, its mechanical properties are very inferior.
For applications such as resistant welding electrodes, lead frames, accelerators and electrical connectors, it is imperative that the material has both high conductivity and high strength. This can be achieved by metal matrix composite of Cu-Al2O3. As copper already has good conductivity, there would be a need to increase the mechanical properties of the metal matrix composite (2010, p7430). Perhaps, ARB is the best process to achieve this. The figure 10 (a) shows the difference in increase of the tensile strength between plain copper and copper/alumina composite.
It is evident that the composite displays a higher tensile strength after each cycle when compared to plain copper. The figure 10 (b) illustrates interestingly the evolution of elongation of plain copper and copper/alumina. More specifically, the elongation displayed by plain copper after nine cycles is over 6% whereas the elongation of a copper/alumina composite decreases from 1. 6% at one cycle to just over 1% after five cycles and increases to slightly under 5% at nine stages. The figure 10 (c) shows the increase of hardness over different accumulative roll bonding cycles of copper/alumina when compared to plain copper.
Both this material start off with a hardness value of 50HV. there is a dramatic increase in hardness after just one cycle to over 120 HV in both the materials. It can be seen on further cycles that the hardness value of copper/alumina increases further to just below 160 HV and 160 HV at the end of seven and nine cycles respectively. But the hardness value in copper does not increase significantly and settles to a value of just over 130 HV at the end of nine cycles (Toroghinejad et al. 2010, pp7433-7434).
Toroghinejad et al have concluded in their study that the ARB process when applied to copper/alumina composite can give high strength, due to its finely dispersed grain. They also found out that there was an homogeneous distribution and strong bonding between the particles and the matrix without any porosity. Furthermore, Cu-Al composite attained a higher tensile strength and higher hardness than annealed and ARBed copper (2010, pp7434-7435). III. 6. 2. Manufacturing of nanostructure Al/SiCp Composite
Alizadeh and Paydar conducted a study on the application of the accumulative roll bonding process on AA1050 aluminium strips along with silicon carbide in a powdery form. They found out that the ultra fine grain of the aluminium carbide composite improve significantly. Likewise, the tensile strength increased by over four times by the end of the eight cycles. A commonality was found with other material in terms of dramatic variation in hardness and strength after one cycle (2010, p235). The fabrication process used by Alizadeh and Paydar to produce the Al/SiCp composite is shown on the figure below.
Figure 11. Fabrication process used to produce Al/SiCp composite (a) first step, (b) second step(Alizadeh et al. 2010, p232) IV. Conclusion The Accumulative Roll Bonding (ARB) process as applied to aluminium and its alloys have been discussed in length throughout this report. Rolling is the most feasible process in the production of bulk materials till date. there is enough scope to apply the accumulative roll bonding process to widely used materials like steel. heat treatment has become a quintessential part of the manufacturing process in the current scene, especially where high strengths are required.
The side effects of heat treatment has been well documented; namely residual stresses, deformation, which hampers the accuracy of the manufactured parts. It has been shown that ARB process has a grain refinement effect on the material and therefore increases strength and changes ductility. By using this process, it is potentially possible to completely get rid of the heat treatment processes. It is possible to obtain high mechanical properties materials without adding special alloying elements or performing complex thermomechanical treatments. This process can therefore contribute to satisfy the demand in energy saving.
Unfortunately, the application of the ARB process has not trickled down to the production floors of metal manufacturing units. The quality of the products can be improved by multiple folds in the future upon the application of the ARB to other metals. References Aalco 2011, Aluminium – Specifications, Properties, Classifications and Classes, Supplier Data by Aalco, accessed 14/05/2011, http://www. azom. com/article. aspx? ArticleID=2863 Alizadeh, M ; Paydar, M 2010, ‘Fabrication of nanostructure Al/SiCP composite by accumulative roll-bonding (ARB) process’, JOURNAL OF ALLOYS AND COMPOUNDS, vol. 92, no. 1-2, pp231-235 Danesh Manesh, H, Eizadjou, M, Janghorban, K, Shakur Shahabi, H ; Kazemi Talachi, A 2008, ‘Investigation of structure and mechanical properties of multi-layered Al/Cu composite produced by accumulative roll bonding (ARB) process’, Composites Science and Technology, vol. 68, no. 9, pp2003-2009 Elseaidy, I M, Ibrahim, M M, Ghoneim, M M ; Abd El-Azim, M E 2007, ‘Aluminium Alloys Strengthening by Accumulative Roll-Bonding (ARB) Process’, Transactions, vol. 9, pp1-10 Hoppel, H W, Goken, M ; May, J 2004, ‘Enhanced Strength and Ductility in Ultrafine-Grained Aluminium Produced by Accumulative Roll Bonding’, Advanced Engineering Materials, vol. 6, no. 4, pp219-222 Kalpakjian, S ; Schmid, S R 2009, Manufacturing processes for engineering materials, Dorling Kindersley, India Kalpakjian, S ; Schmid, S R 2011, Manufacturing engineering and technology, Dorling Kindersley, India Li, L, Nagai, K ; Yin, F 2008, ‘Progress in cold roll bonding of metals’, Science and Technology of Advanced Materials, vol. , no. 2, pp1-11 Madaah-hosseini, H ; Kokabi, A 2002, ‘Cold roll bonding of 5754-aluminum strips’, Materials Science and Engineering A, vol. 335, no. 1-2, pp186-190 Movahedi, M, Kokabi, A H ; Madaah-Hosseini, H R 2007, ‘The influence of roll bonding parameters on the bond strength of Al-3003/Zn soldering sheets’, Materials Science and Engineering: A, vol. 487, no. 1-2, pp417-423 Pirgazi, H ; Akbarzadeh, A, 2008, ‘Characterization of nanostructured aluminum sheets processed by accumulative roll bonding’, INTERNATIONAL JOURNAL OF MODERN PHYSICS B, vol. 2, no. 18-19, pp2840-2847 Toroghinejad, M R ; Jamaati, R 2010, ‘Application of ARB process for manufacturing high-strength, finely dispersed and highly uniform Cu/Al2O3 composite’, Materials Science ; Engineering A, vol. 527, no. 27-28, pp7430 Tsuji, N, Saito, Y, Lee, S ; Minamino, Y 2003, ‘ARB (Accumulative Roll-Bonding) and other new Techniques to Produce Bulk Ultrafine Grained Materials’, Advanced Engineering Materials, vol. 5, no. 5, pp338-344