Microstructural Investigation of a Friction-Welded 316L Stainless Steel with Ultrafine-Grained Structure Obtained by Hydrostatic Extrusion

Abstract

The paper presents the microstructural investigation of a friction-welded joint made of 316L stainless steel with an ultrafine-grained structure obtained by hydrostatic extrusion (HE). Such a plastically deformed material is characterized by a metastable state of energy equilibrium, increasing, among others, its sensitivity to high temperatures. This feature makes it difficult to weld ultra-fine-grained metals without losing their high mechanical properties. The use of high-speed friction welding and a friction time of <1 s reduced the scale of the weakening of the friction joint in relation to result obtained in conventional rotary friction welding. The study of changes in the microstructure of individual zones of the friction joint was carried out on an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) analysis system. The correlation between the microstructure and hardness of the friction joint is also presented. The heat released during the high-speed friction welding initiated the process of dynamic recrystallization (DRX) of single grains in the heat-affected zone (HAZ). The additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly deformed microstructure with numerous sub-grains. The zones with a microstructure other than the base material were characterized by lower hardness. Due to the complexity of the microstructure and its multifactorial impact on the properties of the friction-welded joint, strength should be the criterion for assessing the properties of the joint.

The Influence of Tool Shape and Process Parameters on the Mechanical Properties of AW-3004 Aluminium Alloy Friction Stir Welded Joints

“…Especially in the case of the CR7, the brittleness might be caused by relatively low heat generation in the relevant joint, so the materials could not be stirred properly. Low heat input was studied not only in FSW but also in underwater welding as well as traditional friction welding, and in each process, it was found to cause the brittle structure of the joint produced [3,29,[58][59][60]. The results reveal that rotational speed may be a more important parameter than welding speed in improving welding conditions and, in addition, joint properties.…”

Influence of Strain Rates during Severe Plastic Strain Processes on Microstructural and Mechanical Evolution in Pure Zinc

“…The effectiveness of the use of the hydrostatic extrusion process as a method leading to the fragmentation of the metals and metal alloys’ microstructure was repeatedly presented by the authors in the literature on the subject for a wide range of materials. This applies to very flexible materials, such as aluminum alloys or copper alloys, but also to hard-deformable materials, such as titanium or austenitic steels [ 16 , 17 , 18 , 19 , 20 , 21 ]. The hydrostatic extrusion process was also used by many authors in zinc alloys with the addition of magnesium where, at a cumulative strain of ε cum ~ 3.55 very high mechanical properties were observed at the level of ultimate tensile strength UTS = 515 MPa and yield strength YS = 375 MPa, significantly exceeding the values reported in the literature [ 22 ].…”

Assessment of Corrosive Behaviour and Microstructure Characterization of Hybrid Friction Stir Welded Martensitic Stainless Steel

“…In this current scenario, due to the requirement of high strength and eco-friendly joining methods, there has been a growing interest in new metal joining methods like welding [5]. For example, Friction Stir Welding (FSW), brought up by The Welding Institute (TWI), the U.K., is a solid-state welding technique that eradicates the welding defects such as gas porosity, cracking and distortion which happens in conventional fusion welding [6−8].…”