INTERFACIAL BONDING MECHANISM OFALUMINIUM AND … · by steel brushes with diameter of 0.1mm~...

Advanced Composites Letters, Vol. 27, Iss. 2, 2018 71

1. INTRODUCTIONLaminated metal composite material is the material made from more than one layers of the same or dif-ferent materials bondedtogether which produce ex-cellent properties of all component metals, such as light weight, high thermal conductivity, good corro-sion resistance and high strength.Laminated metal composite material was widelyused in automobile, household electrical appliances, military equipment, cooling tower of power plant and other fields[1-4]. The interface bonding state is one of most important factor in restricting the performance of laminated metal composite[5-7], and the study of the bonding mechanism of the material interface has become a hot topic in the field of high performance materials research. Diffusion theory, mechanical interlocking theory, film theory, dislocation theory, energy theory, recrystallization theory and other theories about the bonding mechanism are proposed[5, 6, 8, 9]. How-ever, the literatures on system research of laminated metal composite mechanism is still rare.

Experiments on the laminated Al-alloy 4A60 and 08Al steel plate compounded by cold roll bond-

Letter

ing were conducted, including the following three stages: surface pre-treatment, cold roll bonding and diffusion annealing treatment[10-12], the states of bonding interface are observed and tested and the bonding mechanism is studied systematically.

2 MATERIALS AND EXPERIMENTS2.1 ExperimentMaterialsMild steel (08Al) and aluminium alloy (4A60) with the specifications given in Table 1 and Table 2 were used in experiments. Sheets at 150mm long * 30mm wide * 2mm thick were cut from an annealed plate parallel to the original rolling direction.

INTERFACIAL BONDING MECHANISM OFALUMINIUM AND STEEL COM-POSITES

Xian YANG1,2, Hao WENG3, Chao-lan TANG1*

1.Guangdong University of Technology, Guangzhou 510006, China2. Guangdong Provincial Key Laboratory of Innovation Method and Decision Management System, Guangzhou

510006, China3.Dongguan Institute of Advanced Technology, Dongguan 523808, China

*Author to whom correspondence should be addressed: E-mail: [email protected]

Received 10 January 2018; accepted 5 April 2018

ABSTRACTThe research on singular material is gradually converted to composite material which serves to rectify weak-nesses possessed by each constituent when it exists alone. Experiments on Al-alloy 4A60 and 08Al steel plate compounded by cold roll bonding were conducted to analyze the bonding mechanism of the interface during the composite process of laminated metal. SEM, EDS, and laser confocal microscope were used to observe the interface and section of composites while the bonding strength was tested by universal tensile machine. The result showed that bonded metal’s surface microtopography, reduction and diffusion annealing were the most critical influences on the bonding of composites, and the roll bonding mechanism of 4A60/08Al composite was divided into three stages: 1) Physical contact. Two component layers were mechanically occluded by the rolling pressure, the bonding strength was low; 2) Metallic bonding. The oxide layer and the hardened layer covered on the metal surface break which made the two component fresh metals to full contact, chemical ac-tion happened and metallic bonding formed when the interatomic distance reached a certain stage, the bond-ing strength increased; 3) Metallurgical bonding. In the subsequent annealing treatment, the bonding strength significantly increased because the diffusion of metal atoms at the interface.Keywords:Composite Material, Interface Bonding State, Bonding Mechanism, Interface Microtopography, Cold Roll Bonding

72 Advanced Composites Letters, Vol. 27, Iss. 2, 2018

Xian YANG, Hao WENG, Chao-lan TANG

2.2 Experimental Process

Fig.1: The process of Al/St cold roll bonding

Fig.1 shows the process of this experiment including surface pre-treatment, cold roll bonding and diffu-sion annealing. Firstly, measure the surface rough-ness and microtopography on confocal laser scan-ning microscope OSL4000; Secondly, observe the interface microtopography and analysis the inter-face composition through SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Spec-trometer). In the end, according to the standard GB/T 6396-2008, making samples of composite board as Fig2,the bonding strength of composite board is evaluated by tensile shear testing in universal tensile machine, the tensile direction is RD (Rolling Direc-tion), and the stretching speed is 2mm/min[1, 7].

3 RESULTS AND DISCUSSION3.1 Surface Pre-treatment

Fig.2: The original surface

Fig.3: The treated surface

In normal condition, the original metal surface is covered with contaminant layers composed of

oxides, water vapors, grease and dirt as shown in Fig.2, these contaminant layers hamper the metal bonding severely[13]. Use the chemical treatment including acid pickling, acetone degreasing and mechanical polishing with wire brush in the surface pre-treatment stage to remove the contaminant lay-ers covered on the metal surface. Among them, the mechanical polishing removed contaminant layers to form a brittle hardening layer on the metal surface and make the surface uneven, which improved the metal bonding (as shown in Fig.3) [1, 14]. For reox-idation protection, the treated specimens were roll bonded immediately after surface pre-treatment.

The effect of surface microtopography on bonding was investigated from two aspects, one is differ-ent roughness obtained by polishing steel surface with wire brush of different diameter, the other is surface scratch direction obtained by changing the direction of polishing. Fig.4 shows the different sur-face microtopography grinded by steel brushes with diameter of 0.1mm / 0.6mm / 0.8mm, and the sur-face roughness are 1.348um / 3.104um / 2.276um (Fig.4).

Fig.5(a) shows the different surface Sa(um) grinded by steel brushes with diameter of 0.1mm~ 0.9mm, the dash lines show the trends between surface roughness and diameter of steel brushes. As shown in Fig.5(b), the bonding strength of the composite board increases with the increase of surface rough-ness of the steel (the dash lines show the trends be-tween shear strength and surface roughness). Thus, the uneven microtopography on the steel surface is beneficial to bonding by piercing the oxide film on Al-alloy surface during the CRB process. The more roughness the steel surface is, the easier to pierce the surface of aluminum foil, resulting in forming more green bonding, and the bonding strength increases.

Fig.6 shows the steel surface microtopography pol-ished by steel brushes with 0.5mm diameter in dif-

Fig.1 shows the process of this experiment including surface pre-treatment, cold roll bonding and diffusion annealing. Firstly, measure the surface roughness and microtopography on confocal laser scanning microscope OSL4000; Secondly, observe the interface microtopography and analysis the interface composition through SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Spectrometer). In the end, according to the standard GB/T 6396-2008, making samples of composite board as Fig2,the bonding strength of composite board is evaluated by tensile shear testing in universal tensile machine, the tensile direction is RD (Rolling Direction), and the stretching speed is 2mm/min[1, 7].

3 RESULTS AND DISCUSSION 3.1 Surface Pre-treatment

Fig.2: The original surface Fig.3: The treated surface In normal condition, the original metal surface is covered with contaminant layers composed of oxides, water vapors, grease and dirt as shown in Fig.2, these contaminant layers hamper the metal bonding severely[13]. Use the chemical treatment including acid pickling, acetone degreasing and mechanical polishing with wire brush in the surface pre-treatment stage to remove the contaminant layers covered on the metal surface. Among them, the mechanical polishing removed contaminant layers to form a brittle hardening layer on the metal surface and make the surface uneven, which improved the metal bonding (as shown in Fig .3) [1, 14]. For reoxidation protection, the treated specimens were roll bonded immediately after surface pre-treatment. The effect of surface microtopography on bonding was investigated from two aspects, one is different roughness obtained by polishing steel surface with wire brush of different diameter, the other is surface scratch direction obtained by changing the direction of polishing. Fig .4 shows the different surface microtopography grinded by steel brushes with diameter of 0.1mm / 0.6mm / 0.8mm, and the surface roughness are 1.348um / 3.104um / 2.276um (Fig.4).

Fig.4:The surface microtopography of steel plate after polishing by different diameter steel brushes (0.1mm / 0.6mm / 0.8mm, confocal laser scanning microscope, ISO 25178 standard, measurement area:

0.1mm

(a)

0.6mm

(b)

0.8mm

(c)





0.1mm

(a)

0.6mm

(b)

0.8mm

(c)





0.1mm

(a)

0.6mm

(b)

0.8mm

(c)

Fig.4:The surface microtopography of steel plate after polishing by different diameter steel brushes (0.1mm / 0.6mm / 0.8mm, confocal laser scanning microscope, ISO 25178 standard, measurement area: 640*640um)


Interfacial Bonding Mechanism of Aluminium and Steel Composites

ferent scratch direction. From Fig.7, the best bond-ing strength is the scratch parallel to the rolling direction, because the metal flows along the rolling direction during the rolling bonding, and the scratch on the steel surface is also in the rolling direction, according to the principle of minimum resistance, it is easier for soft aluminium to squeeze into the cracks and form interlock on steel surface.

3.2 Cold Roll BondingDuring the CRB process, the treated steel and Al-al-loy are staked together and roll bonded at ambient

temperature by high pressure and surface expan-sion in single-pass[2][15].Fig.8 shows the SEM im-age of the cross section of Al-alloy/steel composite roll bonded by 40% reduction[2, 5, 6, 16]. At the beginning of the cold roll bonding process, peaks on the steel surface pierced into the opposing softer Al-alloy layer under the action of press force, green bonding formed between two component layers[4, 10, 17]. With the increase of press and rolling force, plastic deformation took place. Due to the poor plasticity, the work hardening layer covered on steel surface failed to extend along with base metal but

640*640um)

(a) (b)

Fig.5: (a)brushes diameter and roughness, b)roughness and average sheer strength (reduction:

50%) Fig.5(a) shows the different surface Sa(um) grinded by steel brushes with diameter of 0.1mm~ 0.9mm, the dash lines show the trends between surface roughness and diameter of steel brushes. As shown in Fig.5(b), the bonding strength of the composite board increases with the increase of surface roughness of the steel (the dash lines show the trends between shear strength and surface roughness). Thus, the uneven microtopography on the steel surface is beneficial to bonding by piercing the oxide film on Al-alloy surface during the CRB process. The more roughness the steel surface is, the easier to pierce the surface of aluminum foil, resulting in forming more green bonding, and the bonding strength increases.

(a) 90°cross direction (b) Perpendicular to the rolling direction (c) Parallel to the rolling

direction Fig.6:The surface microtopography with different scratch direction polished by steel brushes of 0.5mm

diameter (confocal laser scanning microscope, ISO 25178 standard, measurement area: 640*640um) Fig.6 shows the steel surface microtopography polished by steel brushes with 0.5mm diameter in different scratch direction. From Fig.7, the best bonding strength is the scratch parallel to the rolling direction, because the metal flows along the rolling direction during the rolling bonding, and the scratch on the steel surface is also in the rolling direction, according to the principle of minimum resistance, it is easier for soft aluminium to squeeze into the cracks and form interlock on steel surface.

R

TD

Micracks

TD

RD

Microcracks

TD

RD

Microcracks

Fig.5: (a)brushes diameter and roughness, b)roughness and average sheer strength (reduction: 50%)

640*640um)

(a) (b)

Fig.5: (a)brushes diameter and roughness, b)roughness and average sheer strength (reduction:

50%) Fig.5(a) shows the different surface Sa(um) grinded by steel brushes with diameter of 0.1mm~ 0.9mm, the dash lines show the trends between surface roughness and diameter of steel brushes. As shown in Fig.5(b), the bonding strength of the composite board increases with the increase of surface roughness of the steel (the dash lines show the trends between shear strength and surface roughness). Thus, the uneven microtopography on the steel surface is beneficial to bonding by piercing the oxide film on Al-alloy surface during the CRB process. The more roughness the steel surface is, the easier to pierce the surface of aluminum foil, resulting in forming more green bonding, and the bonding strength increases.

(a) 90°cross direction (b) Perpendicular to the rolling direction (c) Parallel to the rolling

direction Fig.6:The surface microtopography with different scratch direction polished by steel brushes of 0.5mm

diameter (confocal laser scanning microscope, ISO 25178 standard, measurement area: 640*640um) Fig.6 shows the steel surface microtopography polished by steel brushes with 0.5mm diameter in different scratch direction. From Fig.7, the best bonding strength is the scratch parallel to the rolling direction, because the metal flows along the rolling direction during the rolling bonding, and the scratch on the steel surface is also in the rolling direction, according to the principle of minimum resistance, it is easier for soft aluminium to squeeze into the cracks and form interlock on steel surface.

R

TD

Micracks

TD

RD

Microcracks

TD

RD

Microcracks

(a) 90°cross direction (b) Perpendicular to the rolling direction (c) Parallel to the rolling directionFig.6:The surface microtopography with different scratch direction polished by steel brushes of 0.5mm diameter

(confocal laser scanning microscope, ISO 25178 standard, measurement area: 640*640um)

Fig.7: Results of different scratch direction(reduction rate:55%)Fig.8: Cross section of Al-Steel under

40% reduction 3.2 Cold Roll Bonding During the CRB process, the treated steel and Al-alloy are staked together and roll bonded at ambient temperature by high pressure and surface expansion in single-pass[2][15].Fig.8 shows the SEM image of the cross section of Al-alloy/steel composite roll bonded by 40% reduction[2,

5, 6, 16]. At the beginning of the cold roll bonding process, peaks on the steel surface pierced into the opposing softer Al-alloy layer under the action of press force, green bonding formed between two component layers[4, 10, 17]. With the increase of press and rolling force, plastic deformation took place. Due to the poor plasticity, the work hardening layer covered on steel surface failed to extend along with base metal but easy to break up and form a lot of cracks distributed diffusely on the interface of Al-alloy and steel (as shown in Fig.8). Among the cracks, soft aluminium squeezed in and physically contact with the interior virgin steel, the attraction force of atom between Al and Fe is formed, it was a weak VDW (Van der Waals) force and increased with the decrease of the interatomic distance[19] (Fig.9). Therefore, as shown in Fig.10, when the reduction rate is less than certain value (15%), Al-alloy and steel fail to bond, only when the reduction rate is more than 15%, the atoms of Al and Fe form physical contact, Al-alloy and steel start to bond. 15% is so-called the minimum reduction necessary for roll bonding of Al-alloy and steel.

Fig.9: Atomic distance and atomic Fig.10:The relationship between reduction and shear strength The VDW force formed by the physical contact which is too weak to get high bonding strength of composite. Atoms on the metal surface are activated when their atomic energy reach a

Steel

Al

Interface

Reduction 40% Fig.7: Results of different scratch direction(reduction rate:55%)Fig.8: Cross section of Al-Steel under




Steel

Al

Interface

Reduction 40%

Fig.7: Results of different scratch direction(reduction rate:55%)

Fig.8: Cross section of Al-Steel under 40% reduction



easy to break up and form a lot of cracks distributed diffusely on the interface of Al-alloy and steel (as shown in Fig.8). Among the cracks, soft aluminium squeezed in and physically contact with the interior virgin steel, the attraction force of atom between Al and Fe is formed, it was a weak VDW (Van der Waals) force and increased with the decrease of the interatomic distance[19] (Fig.9). Therefore, as shown in Fig.10, when the reduction rate is less than certain value (15%), Al-alloy and steel fail to bond, only when the reduction rate is more than 15%, the atoms of Al and Fe form physical contact, Al-alloy and steel start to bond. 15% is so-called the mini-mum reduction necessary for roll bonding of Al-al-loy and steel. The VDW force formed by the physical contact which is too weak to get high bonding strength of composite. Atoms on the metal surface are activated when their atomic energy reach a certain minimum level that overcome the potential energy barrier BE (as shown in Fig.9). In this case, atomic outer layer electronic clouds of Al and Fe interact mutually and the electrostatic force occurs between negative free electron and positive metal ion which form a metal-lic bonding. The atomic energy of Al and Fe come from respective plastic deformation, and increase with the increasing of plastic deformation, therefore, plastic deformation and atomic activation process of the harder steel layer become a necessary condition for the formation of a stable bonding at the interface of aluminium and steel. With the further increase

of reduction and rolling force, the interatomic dis-tance between two component layers continues to decrease, when it decreases to a certain value ro (as shown in Fig.9), the gravitational potential energy and the repulsion potential energy achieves a dy-namical balance. A stronger electrostatic interac-tion forms into metallic bonding between Al and Fe because the energy of atomic outer layer electronic clouds increases sharply, as shown in Fig.10, the bonding strength increases with the increasing of reduction after 15% (The dash lines show the trends between shear strength and reduction rate).

Fig.11 shows the SEM images of steel bonding sur-face after cold rolling bonding under different re-ductions (15% / 41% / 51%), which confirms the dy-namic roll bonding process. Small cracks distribute along the rolling direction begin to form on the steel surface under the minimum reduction 15%, with the increasing of reduction, cracks propagating, the soft aluminium squeeze into cracks and form the green bonding. In the crack area, the original Al and Fe contact physically, with the increasing of reduction, the interatomic distance of Al and Fe decreases to form weak VDW force and then strong metallic bonding. The area of cracks becomes larger as the reduction grows, and the fresher of metal surface, the greater of the bonding strength (as shown in Fig.10). When the reduction reached to 40%, cracks tend to be stable, the bonding strength is also gradually sta-bilized, so 40% is the threshold reduction necessary for obtaining stable bonding of Al and steel.

Fig.7: Results of different scratch direction(reduction rate:55%)Fig.8: Cross section of Al-Steel under




Steel

Al

Interface

Reduction 40% Fig.7: Results of different scratch direction(reduction rate:55%)Fig.8: Cross section of Al-Steel under




Steel

Al

Interface

Reduction 40%

Fig.9: Atomic distance and atomic Fig.10:The relationship between reduction and shear strength

certain minimum level that overcome the potential energy barrier BE (as shown in Fig .9). In this case, atomic outer layer electronic clouds of Al and Fe interact mutually and the electrostatic force occurs between negative free electron and positive metal ion which form a metallic bonding. The atomic energy of Al and Fe come from respective plastic deformation, and increase with the increasing of plastic deformation, therefore, plastic deformation and atomic activation process of the harder steel layer become a necessary condition for the formation of a stable bonding at the interface of aluminium and steel. With the further increase of reduction and rolling force, the interatomic distance between two component layers continues to decrease, when it decreases to a certain value ro (as shown in Fig.9), the gravitational potential energy and the repulsion potential energy achieves a dynamical balance. A stronger electrostatic interaction forms into metallic bonding between Al and Fe because the energy of atomic outer layer electronic clouds increases sharply, as shown in Fig .10, the bonding strength increases with the increasing of reduction after 15% (The dash lines show the trends between shear strength and reduction rate). Fig.11 shows the SEM images of steel bonding surface after cold rolling bonding under different reductions (15% / 41% / 51%), which confirms the dynamic roll bonding process. Small cracks distribute along the rolling direction begin to form on the steel surface under the minimum reduction 15%, with the increasing of reduction, cracks propagating, the soft aluminium squeeze into cracks and form the green bonding. In the crack area, the original Al and Fe contact physically, with the increasing of reduction, the interatomic distance of Al and Fe decreases to form weak VDW force and then strong metallic bonding. The area of cracks becomes larger as the reduction grows, and the fresher of metal surface, the greater of the bonding strength (as shown in Fig.10). When the reduction reached to 40%, cracks tend to be stable, the bonding strength is also gradually stabilized, so 40% is the threshold reduction necessary for obtaining stable bonding of Al and steel.

Fig.11:The SEM images of steel surface after cold rolling bonding under different reductions (15% / 41%

/ 51%)

As shown inFig.12, bonding also existin non-crack areas. “TLK” surface structure model [20] holds on the view that the metal crystal surface is constituted by some platforms of low-index crystal planes and a certain density of monatomic steps. These steps contain a certain density of kink, which plays an important role in formation of surface defects or volume defects. As the rising of temperature, the number of kinking will increase, that is, the concave-convex of surface will intensify. When the plastic deformation occurs, these concave-convex will contact

Fig.11:The SEM images of steel surface after cold rolling bonding under different reductions (15% / 41% / 51%)


Interfacial Bonding Mechanism of Aluminium and Steel Composites

As shown inFig.12, bonding also existin non-crack areas. “TLK” surface structure model [20] holds on the view that the metal crystal surface is constituted by some platforms of low-index crystal planes and a certain density of monatomic steps. These steps con-tain a certain density of kink, which plays an impor-tant role in formation of surface defects or volume defects. As the rising of temperature, the number of kinking will increase, that is, the concave-convex of surface will intensify. When the plastic deforma-tion occurs, these concave-convex will contact with each other to deform and rub, generate deformation and friction heat raising the temperature of the metal surface, which promotes the bonding of metals[21]. After cold rolling, due to the difference in linear ex-pansion coefficient of aluminium and steel, and the partial restore of elastic plastic deformation, static friction happens at the contact surface, it may pro-duce deadlock each other so as to make steel and aluminum bonding more firmly[22]. Thus, it can be seen that the intermetallic contact area relates to the property of the material and the rolling force, this stage is a beginning to produce a process for elec-tronic exchanging between steel and aluminum, but it belongs to a kind state of machinery bite-clamp of physical contact between aluminium and steel[23].

Fig.12: Interface schematic diagram

3.3 Diffusion AnnealingDiffusion annealing after CRB can eliminate the defects (holes, dislocation and oxide inclusions) at the interface and the internal residual stress in met-al, improve the forming performance of composite. Fig.13 shows SEM images of the cross section at the interface of composite before and after diffusion annealing. It shows that there are some small gaps at the interface before diffusion annealing (Fig.13(a)), but these gaps disappear and the interface becomes closed and straight after diffusion annealing because of heating (Fig.13(b)). Furthermore, heated at a cer-tain temperature for a period of time, Fe and Al at-oms on both sides of the interface diffuse mutually due to thermal activation. A certain depth of diffu-sion layer is formed at the interface, which changes

with each other to deform and rub, generate deformation and friction heat raising the temperature of the metal surface, which promotes the bonding of metals [21]. After cold rolling, due to the difference in linear expansion coefficient of aluminium and steel, and the partial restore of elastic plastic deformation, static friction happens at the contact surface, it may produce deadlock each other so as to make steel and aluminum bonding more firmly [22]. Thus, it can be seen that the intermetallic contact area relates to the property of the material and the rolling force, this stage is a beginning to produce a process for electronic exchanging between steel and aluminum, but it belongs to a kind state of machinery bite-clamp of physical contact between aluminium and steel[23].


(a) (b)

Fig. 13:a) after cold roll bonding, b)after diffusion annealing

3.3 Diffusion Annealing Diffusion annealing after CRB can eliminate the defects (holes, dislocation and oxide inclusions) at the interface and the internal residual stress in metal, improve the forming performance of composite. Fig.13 shows SEM images of the cross section at the interface of composite before and after diffusion annealing. It shows that there are some small gaps at the interface before diffusion annealing (Fig.13(a)), but these gaps disappear and the interface becomes closed and straight after diffusion annealing because of heating (Fig.13(b)). Furthermore, heated at a certain temperature for a period of time, Fe and Al atoms on both sides of the interface diffuse mutually due to thermal activation. A certain depth of diffusion layer is formed at the interface, which changes the lattice of atoms at the interface to co-crystal. The green bonding of interface is changed to metallurgical bonding with higher bonding strength[24, 25].

(a) (b)Fig. 13:a) after cold roll bonding, b)after diffusion an-

nealing

with each other to deform and rub, generate deformation and friction heat raising the temperature of the metal surface, which promotes the bonding of metals [21]. After cold rolling, due to the difference in linear expansion coefficient of aluminium and steel, and the partial restore of elastic plastic deformation, static friction happens at the contact surface, it may produce deadlock each other so as to make steel and aluminum bonding more firmly [22]. Thus, it can be seen that the intermetallic contact area relates to the property of the material and the rolling force, this stage is a beginning to produce a process for electronic exchanging between steel and aluminum, but it belongs to a kind state of machinery bite-clamp of physical contact between aluminium and steel[23].


(a) (b)

Fig. 13:a) after cold roll bonding, b)after diffusion annealing

3.3 Diffusion Annealing Diffusion annealing after CRB can eliminate the defects (holes, dislocation and oxide inclusions) at the interface and the internal residual stress in metal, improve the forming performance of composite. Fig.13 shows SEM images of the cross section at the interface of composite before and after diffusion annealing. It shows that there are some small gaps at the interface before diffusion annealing (Fig.13(a)), but these gaps disappear and the interface becomes closed and straight after diffusion annealing because of heating (Fig.13(b)). Furthermore, heated at a certain temperature for a period of time, Fe and Al atoms on both sides of the interface diffuse mutually due to thermal activation. A certain depth of diffusion layer is formed at the interface, which changes the lattice of atoms at the interface to co-crystal. The green bonding of interface is changed to metallurgical bonding with higher bonding strength[24, 25].

the lattice of atoms at the interface to co-crystal. The green bonding of interface is changed to metallurgi-cal bonding with higher bonding strength[24, 25].

Fig.14 shows the line scan EDS analysis images of the interface, it can be seen that, at the interface, there is a certain degree of diffusion among Fe and Al [19, 20]. After 520ΟC-24h annealing, the shear strength of composite sheet was improved signifi-cantly (as shown in Fig.15). 4. CONCLUSIONSTo sum up, during the CRB process of 4A60 / 08Al, surface microtopography of bonded metal, reduc-tion and annealing parameters are the major fac-tors which influence the bonding strength of 4A60 / 08Al composite. The bonding mechanism is divided into three stages: 1) Physical contact: at the begin of

Fig.14 shows the line scan EDS analysis images of the interface, it can be seen that, at the interface, there is a certain degree of diffusion among Fe and Al [19, 20]. After 520℃-24h annealing, the shear strength of composite sheet was improved significantly (as shown in Fig.15).

Fig.14: Interface elements under 520℃-24 h annealing Fig.15:Average shear strengthbefore and after

diffusion annealing

4. CONCLUSIONS

To sum up, during the CRB process of 4A60 / 08Al, surface microtopography of bonded metal, reduction and annealing parameters are the major factors which influence the bonding strength of 4A60 / 08Al composite. The bonding mechanism is divided into three stages: 1) Physical contact: at the begin of rolling, the rolling pressure makes two component layers occluded mechanically, the bonding strength is low; 2) Metallic bonding: as the increasing of reduction, the oxide layer and the hardened layer covered on the metal surface break, which makes full contact between the two fresh component metals, when the interatomic distance reaches some certain degree to make the chemical action happen and form metallic bonding, the bonding strength is increased; 3) Metallurgical bonding: in the subsequent annealing treatment, the bonding strength significantly increase because of diffusion between the two metal atoms at the interface.

References:

1. Tang, C., et al., Surface Treatment with the Cold Roll Bonding Process for an Aluminum Alloy and Mild Steel. Strength of Materials, 2015. 47(1): p. 150-155. 2. Jamaati, R. and M.R. Toroghinejad, Cold roll bonding bond strengths: review. Materials Science and Technology, 2011. 27(7): p. 1101-1108. 3. Hosseini, M. and H. Danesh Manesh, Bond strength optimization of Ti/Cu/Ti clad composites produced by roll-bonding. Materials & Design, 2015. 81: p. 122-132. 4. Chaudhari, G.P. and V. Acoff, Cold roll bonding of multi-layered bi-metal laminate composites. Composites Science and Technology, 2009. 69(10): p. 1667-1675. 5. Maleki, H., et al., Analysis of Bonding Behavior and Critical Reduction of Two-Layer Strips in Clad Cold

Fig.14 shows the line scan EDS analysis images of the interface, it can be seen that, at the interface, there is a certain degree of diffusion among Fe and Al [19, 20]. After 520℃-24h annealing, the shear strength of composite sheet was improved significantly (as shown in Fig.15).

Fig.14: Interface elements under 520℃-24 h annealing Fig.15:Average shear strengthbefore and after

diffusion annealing

4. CONCLUSIONS

To sum up, during the CRB process of 4A60 / 08Al, surface microtopography of bonded metal, reduction and annealing parameters are the major factors which influence the bonding strength of 4A60 / 08Al composite. The bonding mechanism is divided into three stages: 1) Physical contact: at the begin of rolling, the rolling pressure makes two component layers occluded mechanically, the bonding strength is low; 2) Metallic bonding: as the increasing of reduction, the oxide layer and the hardened layer covered on the metal surface break, which makes full contact between the two fresh component metals, when the interatomic distance reaches some certain degree to make the chemical action happen and form metallic bonding, the bonding strength is increased; 3) Metallurgical bonding: in the subsequent annealing treatment, the bonding strength significantly increase because of diffusion between the two metal atoms at the interface.

References:

1. Tang, C., et al., Surface Treatment with the Cold Roll Bonding Process for an Aluminum Alloy and Mild Steel. Strength of Materials, 2015. 47(1): p. 150-155. 2. Jamaati, R. and M.R. Toroghinejad, Cold roll bonding bond strengths: review. Materials Science and Technology, 2011. 27(7): p. 1101-1108. 3. Hosseini, M. and H. Danesh Manesh, Bond strength optimization of Ti/Cu/Ti clad composites produced by roll-bonding. Materials & Design, 2015. 81: p. 122-132. 4. Chaudhari, G.P. and V. Acoff, Cold roll bonding of multi-layered bi-metal laminate composites. Composites Science and Technology, 2009. 69(10): p. 1667-1675. 5. Maleki, H., et al., Analysis of Bonding Behavior and Critical Reduction of Two-Layer Strips in Clad Cold

Fig.15:Average shear strengthbefore and after diffusion annealing

Fig.14: Interface elements under 520OC-24 h annealing

rolling, the rolling pressure makes two component layers occluded mechanically, the bonding strength is low; 2) Metallic bonding: as the increasing of re-duction, the oxide layer and the hardened layer cov-ered on the metal surface break, which makes full contact between the two fresh component metals, when the interatomic distance reaches some certain degree to make the chemical action happen and form metallic bonding, the bonding strength is increased; 3) Metallurgical bonding: in the subsequent anneal-ing treatment, the bonding strength significantly in-crease because of diffusion between the two metal atoms at the interface.

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