OBRABOTKAMETALLOV Vol. 25 No. 3 2023 technology profiles of contact surfaces, the greater the bond strength. It should be noted separately that the results of measuring H/W ratio were presented only in this work. In [14], the highest bond strength between sheets of stainless martensitic steel 1Cr11Ni2W2MoV was obtained for a contact surface roughness Ra of 0.43 µm among two options of 0.43 and 0.95 µm. As can be seen from the last two works, a decrease in the roughness of the contact surfaces promotes the bonding between materials for certain conditions of accumulative roll bonding. The mechanism of contact interaction between dissimilar materials during plastic deformation To develop the theory of material joint by plastic deformation and create new fundamental models, in the previous work of the author [18], a developed theoretical model of accumulative roll bonding of dissimilar materials was presented. The model assumed the contact of two materials, one of which is harder in relation to the other. Up to a certain limit of effective stress at the contact between materials, a harder material can be considered as ideally rigid. The model was developed under plane strain conditions. The stress analysis was carried out by the slip line method with the appropriate assumptions. The model considered the surface profile of only the hard material since the soft material was actively deformed at the first stages and took the form of the hard material. This assumption is indirectly confirmed in [10], where it was concluded that the influence of the surface roughness of a hard material is greater than that of a soft one. Schematically, the model of bonding is shown in fig. 1 in the form of successive stages of deformation of the subsurface layers of materials: 1 – embedding of asperities of a harder material into a soft material. The soft material is squeezed out from under the asperities of hard material and flows into the cavities of the surface profile of hard material. The deformation zones are not in contact with each other. 2 – filling cavities on the surface of hard material with the soft material. The deformation zones are in contact. Common deformation zone is formed in the center, which is filled from under the neighboring asperities. 3 – the critical stage of filling the cavities on the surface of hard material with the soft one, whose flow is hindered by the influence of neighboring asperities. The unfilled parts of the cavities are residual pores at the interlayer boundary. 4 – propagation of plastic deformation into the deep layers of soft metal due to the damming created at the contact with the hard material. Further filling of the cavities at the surface of hard material, as well as its plastic deformation, is possible only after hardening of the main volume of the soft material. From the point of view of bond formation, the moment and place of fracture of surface oxide films are important. According to the results of the theoretical analysis performed in [18, 19], the areas of the most probable fracture of the surface oxide layers were identified as follows: 1) areas of soft metal under the asperities, characterized by high values of accumulated plastic deformation Λ and low values of the relative average normal stress σ/T, which means the prevailing proportion of compressive stresses. Λ is the degree of shear deformation, σ is the mean stress, and Т is the intensity of shear stresses. 2) areas of soft metal in the center of the free surface, characterized by low values of accumulated plastic deformation Λ and high values of the stress state index σ/T, corresponding to an increased proportion of tensile stresses. Values of the stress-strain state and the volume of unfilled cavities vary in a wide range depending on the profile of hard material surface, expressed as the ratio of the height to the asperity base width H/W [18]. Due to limitations of the theoretical model, it is not possible to establish the location of fracture of oxide films and the subsequent initiation of the formation of bond bridges between pure metals. In addition, it is not known to what extent the developed theoretical model reflects the actual contact interaction of surfaces of dissimilar materials during plastic deformation. In this regard, the aim of this work is to analyze the stress-strain state of dissimilar materials under plastic deformation on a microscale and to establish the location of the onset of fracture of surface oxide
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