Relationships governing plastic deformation in pressure welding dissimilar materials

In pressure welding of metals and also metals to nonmetals, deformation is one of the main technological parameters. Plastic deformation is usually associated with the formation of physical contact, cleaning of the surfaces to remove oxide films (in particular, in the conditions of cold deformation), activation of the contacting surfaces, reflected in the formation of active centres and, finally, the development of volume interaction, associated with the activation of diffusion, and so on, when the structure of the contact zone is characterised by the more or less homogeneous condition, and the mechanical, service and other properties of welded joints are similar to the properties of the parent metal. The process of plastic deformation is one of the forms of relaxation (reduction) of the internal stresses, whose magnitude increases in the course of external welding loading and, more accurately, the form of plastic deformation which may occur by different mechanisms: the slip of individual dislocations, as a result of the collective movement of crystal lattice defects, rotation of the microvolumes, including twinning, etc. When the plastic deformation is inhibited for any reason, the relaxation of internal stresses in the deformed zones already takes place by means of crack formation. Therefore, taking into account the multiplan nature of the effect of deformation on the formation of the welded joint and the properties of the weld, it is important to examine the problem of development of the optimum technological conditions, resulting in the active occurrence of plastic deformation in the welding zone. To solve this problem, it is necessary to obtain the maximum amount of information on the nature of distribution of deformation in the welding zone and the relationships governing the localisation of deformation in the thickness of the welded material, the degree of localisation of deformation, and also the specific mechanism of occurrence of the process in different conditions of pressure welding. In this work, investigations were carried out on pairs of modelling materials, including nickel NP-2, and also nickel-based alloys (KhN77TYu), joints in dissimilar metals and alloys, such as commercial purity aluminium AD1 and hardened aluminium alloys 1201, titanium VT1-0, copper M0, M1, and also steel St3 and stainless steels. The welded joint were produced with different rates of welding deformation ε from 10 to 10–10 s by means of diffusion (DW), impact (IW), magnetic pulse (MPW), explosion welding (EW) and other welding methods. The general nature of distribution of plastic deformation and the degree of localisation of deformation in the contact zone of the welded metals were evaluated by the optical methods on the coordinate grids, with the base 10–20 μm, so that it was possible to evaluate not only the degree of deformation localised in the contact zone but also the distribution of deformation in the longitudinal and transverse directions (compression and tensile deformation). In addition to this, the degree and extent of propagation of deformation were evaluated on the basis of the change of the form of the measured elements of the structure used as reference points. The mechanism of plastic deformation, which is usually associated with the type of the structures, formed in the deformation zone, was analysed on the basis of the results of examination of the fine structure of the welded metals in transmission electron microscope JEM-200 CX (JEOL, Japan), with an accelerating voltage of 200 kV. Foils for the examination of the contact zone in dissimilar welded joints were prepared using the specially developed method of ion thinning in the ionising beam of argon. The results of examination of the special features of plastic deformation of the welded metals at the value of ε equal to 10–10 s, show that in all thermal– deformation conditions plastic deformation is clearly heterogeneous, as confirmed by the layer measurements of deformation in the welding zone in the longitudinal and transverse directions of welding deformation. The same relationship is characteristic of all welded materials, differing in the deformation capacity and phase composition. The distribution of deformation in the welded joint in nickel in welding with the low deformation rate is shown in Fig. 1. As indicated by Fig. 1a, deformation greatly differs in the thickness of the deformed metal, i.e. if the lower curve shows the general (average) level of deformation of the entire welded specimens, the other curves correspond to the variation of deformation at different distances from the interface of the welded pair. The distribution of deformation (compression and tensile deformation) is indicated in greater detail by analysis of the variation of the coordinate grids in the longitudinal and transverse directions of the specimens. The example of distribution of the deformation in the thickness of the Relationships governing plastic deformation in pressure welding dissimilar materials