Study of Different Surface Pre-treatment Methods on Bonding Strength of Multilayer Aluminum Alloys/Steel Clad Material
Abstract
Metallic composite materials, as part of a large group of materials, are well known to design materials properties to the customers’ demands. Due to its unique service performance features in comparison with other methods the cold roll bonding process for producing clad metal material has witnessed a rapid growth and development in recent years. The solid state joining technique in the CRB can be applied to a large number of metals, which may be the same or similar, possessing identical attributes, or different, possessing widely varying mechanical or metallurgical property. Here, bonding is caused by adhesion requiring specially prepared surfaces. However, surface cleanliness is difficult to achieve without a controlled atmosphere. In this work the effect of the surface roughness and the initial thickness of the sheets on the bonding strength of Al/St-clad materials were studied using wire brush and belt grinding to reach surfaces of different but defined roughness on the steel. Different Al thicknesses were used, as well. Highest roughness on the surface was achieved using grinding with a grit size of 40. Al-sheet thickness also showed influence on the green bonding strength with the thicker the Al-sheet the better the green bonding strength was. The adhesion between the clad partners exhibited higher when the steel surface was belt grinded.
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Introduction
1.1 Cold Roll Bonding (CRB)
CRB is a solid phase welding process establishing bonding by joint plastic deformation of different metal partners [1]. Bonding is obtained when the surface expansion exposes the surfaces of the virgin metal or when the pressure reaches a value large enough to extrude the virgin material through the cracks of the fractured layer resulting in the establishment of contact and bonding between opposing virgin surfaces [2]. The schematic illustration of CRB for the production of layered materials is sketched in Fig. 1 [3]. Before roll bonding, the surfaces to be bonded must be properly cleaned, so to remove surface layers, here especially grease and carbon-based components [4]. During bonding, a high reduction in thickness of the materials (up to 40% or more in one rolling pass) has to be achieved [5]. The high reduction generates a great amount of heat and creates virgin surfaces on the materials to be bonded. To increase the bond strength, an annealing treatment is performed after rolling [6, 7].
Conclusion
The bonding strength of cold roll bonded aluminum on steel can be controlled with the combination of surface roughness of the steel and the total deformation. Best results were achieved using belt grinding with 40 grit for steel surface preparation. Wire brushing only let to a low roughness. Due to this higher roughness, a bigger contact area between the steel and aluminum was available and subsequently adhesion could be improved. The adhesion of Al on the steel sheets in cold roll bonded materials was investigated on the shear surface of the test´s samples after shear tensile test using SEM. The Al adhesion to the steel surface was improved with a rougher steel surface and – of course – with increasing the thickness reduction by cold roll bonding.