Additive Technologies, Advanced Joining Technology and Study of Weld Joints

1. Introduction

Since the beginning of the third millennium, welding has remained one of the leading technological processes for the creation of the material basis of modern civilization.

Welding is used to achieve permanent joints between a wide range of metallic, non-metallic and composite structural materials in the conditions of the Earth’s atmosphere, the world’s oceans, and outer space. The use of light alloys, polymeric materials, and composites in modern structures and products is constantly increasing, but steel remains the main structural material.

Welding processes proceed according to complex physical and chemical laws at high temperatures, and the combination of various factors and phenomena determines the quality of welded joints. To improve the operational reliability of welded structures, new technologies for controlling the properties of welded joints are constantly being developed. In addition, new equipment, new additive technologies using welding equipment and welding methods, and the number of methods to create hybrid techniques by combining welding technologies with other types of production is increasing.

2. Contributions

Metallic materials play a vital role in the economic life of modern societies; contributions are sought on fresh developments that enhance our understanding of the fundamental aspects related to the relationships between processing, properties and microstructure.

The effect of side limiters on the formation of the structure and hardness of AISI 308LSi stainless-steel workpieces obtained by multilayer build-up welding in an argon environment has been studied [1]. The studies were carried out on deposited specimens using graphite limiters, copper limiters and without limiters. As a result of numerical simulations, it was found that the lowest temperatures of metal specimens are observed when using copper limiters, and the highest when using graphite limiters in comparison with the temperatures of the specimen received without limiters. With the use of graphite limiters, most of the specimen’s metal is in the temperature range of austenite formation (45%) and a more uniform growth of structural elements is observed, without sharp transitions between the deposited layers, in contrast to the other two types of specimens. The technology of electric arc multilayer build-up welding with the use of shaping graphite blocks makes it possible to produce a workpiece with a uniform structure and properties. This suggests a promising direction in electric arc additive manufacturing.

In [2], the influence of consumable electrode welding mode parameters in shielding gas on the deformation of AISI 316 austenitic steel samples was studied; additionally, the influence of the welding current and purging gas consumption on the samples’ ability to perceive the force of cold cupping was studied. The conducted metallographic studies confirmed an increase in the homogeneity of the dendritic structure in the weld zone due to the redistribution of heat input, as well as the absence of uneven grains and a decrease in the spread of grain sizes, which were in the range of 0.068–0.045 mm. The study resulted in determining the optimal range of technological parameters for the manufacture of flexible expansion elements to ensure their high operational properties.

One of the promising ways to obtain bimetallic products is the Wire Arc Additive Manufacturing (WAAM) technology. One study, ref. [3], investigated the influence of WAAM parameters and the subsequent heat treatment on the composition, structure, and physical and mechanical properties of the bimetallic composite “ER70S-6-R309LSI”. Spectral, metallographic, and X-ray diffraction studies were carried out, as were mechanical tests of the samples obtained under various WAAM modes. To improve the composites’ properties, various types of heat treatments were applied. It is shown that the WAAM modes, the building strategy, and heat treatment determine the structure of layers and transition zones, as well as the mechanical characteristics of the composite.

In one study, ref. [4], a fiber laser at a wavelength of 1070 nm with different beam shapes was used to weld thin 316 L stainless-steel foils. The weld geometry, microstructure, lap shear strength, and crystallographic grain structure of the micro-joints were analyzed and correlated with the beam shape and welding speed. This study proved, for the first time, that a spot-wobble laser beam could achieve better mechanical properties and microstructural characteristics than a doughnut beam during the high-power laser welding of thin-foil stainless-steel plates.

High-entropy alloy samples of non-equiatomic composition (33.4 Al, 8.3 Cr, 17.1 Fe, 5.4 Co, 35.7 Ni, at. %), fabricated by WAAM, were used as study objects [5]. The modification of the high-entropy alloy surface layer was carried out by a complex method combining the deposition of (B + Cr) film samples on the surface and irradiation with a pulsed electron beam in an argon medium. This made it possible to increase microhardness and wear resistance; in addition, the friction coefficient of the high-entropy alloy surface layer was reduced by 1.3 times due to the decrease in average grain size, the formation of particles of borides and oxyborides of complex elemental composition, and the introduction of boron atoms into the crystal lattice of high-entropy alloy.

One paper, ref. [6], presents the results of applying wire-feed electron beam additive manufacturing technology to produce bimetallic samples of CuCr1 copper alloy and Udimet 500 nickel-based superalloy. Different printing strategies were used to obtain samples with a defect-free structure and high-quality mechanical properties in the transition zone, which was not inferior to the strength of copper alloy. Two types of samples were fabricated with a sharp and smooth CuCr1/Udimet 500 interface. The printing strategies of type 1 and 2 samples differed in the combination and arrangement of nickel and copper alloy layers. Structural studies on the transition zone revealed mechanical mixtures of initial copper and nickel alloy components and solid solutions based on nickel, copper, and chromium.

Another paper, ref. [7], presents the results of an evaluation of the mechanical characteristics of samples of multi-metal “copper-steel” structures fabricated by the additive double wire electron beam method. The global and local mechanical characteristics were evaluated by using uniaxial tensile tests and the full-field two-dimensional digital image correlation (DIC) method. It was found that shear lines were formed in copper before propagating to steel. Electron microscopy proves that uniformly distributed iron particles can always be found in “Fe–Cu” and “Cu–Fe” interfaces.

The authors of [8] reviewed and showed that, compared to traditional manufacturing technologies, SLM has the advantages of being more time-efficient; cost-effective; and capable of producing components with superior mechanical, tribological, and corrosion properties. Much of the existing literature highlights the influence of SLM on softer materials such as aluminum or magnesium due to their thermal expansion coefficients, as compared to that observed in materials such as steel. By understanding the trends of laser energy density (LED), scanning patterns, and building directions for these properties, a comprehensive understanding of SLM steel can be obtained.

The authors of [9] believe that their study will provide industrial applications for the fabrication of multi-layer structures. The study aimed to optimize gas metal arc welding (GMAW)-based WAAM variables of travel speed (TS), wire feed speed (WFS), and voltage (V) for the bead geometries of bead width (BW) and bead height (BH) on an SS 316L substrate. Single-layer depositions were made using a metallic wire of SS 316L by following the experimental matrix of the Box–Behnken design (BBD) technique. Multivariable regression equations were generated for design variables and responses, and ANOVA was used to investigate the feasibility of the obtained regression equations.

In paper [10], a significant amount of basic research was conducted on the corner-constrained and unconstrained zones of 4043 aluminum alloy made using CMT+P. The results showed that there were cellular crystals at the top, columnar dendritic crystals in the middle and bottom, and smaller equiaxed crystals in the bottom center. The grain size in the corner-constrained zone was larger than that in the unconstrained zone, and the grain size increased by about 88.34%.