No 1 (2023)

One of the important parameters ensuring the production of a welded joint without continuity defects during friction stir welding is the provision of the required temperature in the metal bonding zone. Significant difficulties arise when determining experimentally the temperature directly in the stir zone of metals using thermocouples. In this regard, the application of numerical methods describing the distribution of temperature fields during friction stir welding is relevant. In the work, numerical modeling of temperature fields during friction stir welding was used, which was based on the finite element method using Abaqus/Explicit software. Modeling was carried out taking into account the coupled Euler – Lagrange approach, the Johnson – Cook plasticity model, and the Coulomb friction law. Using the finite element method, the models of a part, substrate, and tool were constructed taking into account their thermophysical properties. To reduce the computation time, an approach based on the metal mass scaling by recalculating the density of the metal and its thermal properties was used. The authors matched coefficients of scaling of the material mass and heat capacity for the selected welding mode parameters. To evaluate the validity of the results of numerical modeling of temperature fields during friction stir welding, the experimental research of the temperature fields using thermocouples was carried out. The paper shows the possibility of numerical modeling of temperature fields during friction stir welding with the help of the coupled Euler – Lagrange approach and Abaqus/Explicit software. Due to the application of the approach associated with material mass scaling, the calculation time is reduced by more than 10 times.

There is a strong belief that hydrogen absorbed by magnesium alloys during corrosion can cause their stress corrosion cracking. One of the characteristic markers indicating the involvement of diffusible hydrogen into the fracture mechanism of metals is the negative strain rate dependence of the embrittlement degree. Recent studies show that the loss of ductility of the ZK60 alloy specimens subjected to a short-term (1.5 h) pre-exposure in a corrosive medium actually decreases with the increasing strain rate. However, after the removal of corrosion products from the surface of the specimens, the strain rate dependence of the ductility loss becomes positive, which indicates the absence of hydrogen in the bulk of the metal. At short-term exposure in a corrosive environment, the deep penetration of hydrogen into a metal could be limited due to the insufficient time for hydrogen diffusion. The paper studies the mechanical behavior of the ZK60 alloy subjected to a longer (12 h) pre-exposure in a corrosive medium followed by tensile testing in air at various strain rates. The authors consider the effect of strain rate, long-term pre-exposure in a corrosive medium, and subsequent removal of corrosion products on the strength, ductility, stages of work hardening, and localized deformation, as well as on the state of the side and fracture surfaces of specimens. It is established that the ductility loss of the specimens pre-exposed in a corrosive medium for 12 h decreases with the increasing strain rate, regardless of whether the corrosion products have been removed from their surface or not. It is shown that in this case, the negative strain rate dependence of the ductility loss is associated not with hydrogen dissolved in the bulk of a metal but with the presence of severe corrosion damage of the specimens’ surface. An explanation for the effect of corrosion damage on the mechanical properties and their strain rate sensitivity is proposed. 

Rotary friction welding (RFW) is used in the production of drill pipes for solid mineral prospecting. The need for the creation of the lightened drill strings for high-speed diamond drilling of ultradeep wells dictates the necessity of a greater focus on the study of a weld zone and setting the RFW technological parameters. This paper presents the results of experimental studies of a welded joint of a drill pipe of the H standard size according to ISO 10097, made of the 30ХГСА (pipe body) and 40ХМФА (tool joint) steels under the cyclic loads. The authors evaluated the influence of the force applied to the workpieces in the process of friction of the contacting surfaces (force during heating), and postweld tempering at a temperature of 550 °С on the cyclic life of welded joints, under the conditions of alternate tension-compression at the cycle amplitude stress of ±420 MPa. The study determined that with an increase in the force during heating, the microstructure changes occur in the zone of thermomechanical influence, contributing to an increase in the fatigue strength of welded joints. The authors identified the negative effect of postweld tempering on the fatigue strength of welded joints, which is expressed in the decrease in the number of cycles before failure by 15–40 %, depending on the magnitude of the force during heating. The optimal RFW mode of the specified combination of steels is determined, which provides the largest number of cycles before failure: the force during heating (at friction) F_h=120 kN, forging force F_for=160 kN, rotational frequency during heating n=800 Rpm, and upset during heating l=8 mm. A series of fatigue tests have been carried out at various values of the cycle amplitude stress of the welded joint produced at the optimal mode and the 30ХГСА steel base metal; limited endurance curves have been plotted. It is shown that the differences in the limited endurance curves of the pipe body material (30ХГСА steel) and the welded joint are insignificant. The obtained results are supplemented by the microhardness measurement data and fractographs of fractured samples, revealing the mechanism of crack propagation under the cyclic loads.