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Multiple principal element alloys (MPEA) represent a class of materials consisting of at least three alloying elements, each with 5 to 35 atomic %. The MPEAs encompass the so-called high-entropy (HEA) and medium-entropy alloys (MEAs) and are fundamentally different from conventional materials like the Fe-based steel. Within the last 20 years, highly innovative MPEA with individually adjustable properties for industrial applications were identified. However, the focus has been on pure material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding. In that connection, the weldability of MPEAs has received very little attention so far and experience in dissimilar metal welds (DMWs) is lacking so far but. The present study summarizes comprehensive experimental results on the weldability of MPEA-DMWs and their resulting microstructure. For this purpose, two equiatomic MPEAs, CoCrFeMnNi (HEA) and CoCrNi (MEA) in cold-rolled and annealed conditions were joined by solid-state friction stir welding (FSW) to an austenitic stainless steel 316L. The DMWs showed very interesting microstructure features. In addition, the mechanical-technological properties were obtained by instrumented tensile tests, and the local straining was determined in-situ by digital image correlation (DIC). A significant influence of the FSW processing on the mechanical performance was identified,in terms of the formation of FSW-specific defects like whitebands or tunnel defects. The experiments proofed the general (but currently limited) FS weldability of the MPEAs to conventional austenitic steel grade AISI 304. This enables targeted further considerations of these highly innovative MPEAs.
High entropy alloys (HEA) are a new class of materials. In contrast to conventional alloys, HEA are single-phase alloys with at least five alloying elements. HEAs have enormous application potential due to (postulated) excellent structural property combinations from low to high temperatures. For HEA-application as structural materials in real components, a key issue is the suitability for joining processing. Requirements for the reliable and safe joining of these materials are crucial regarding economical component manufacture for future applications. In this context, friction stir welding (FSW) is a promising joining process due to the welding process temperature below the material melting point avoiding major issues, e. g. formation of (hard and brittle) intermetallic phases, which may have detrimental influences on the weld joint properties.
This study presents elementary research about the FSW process influences on a CoCrFeMnNi-HEA with focus on the microstructure and mechanical properties. For that purpose, the FSW joint of the HEA is compared to that of an austenitic stainless steel AISI 304. The microstructures of the welds were investigated and characterized by means of light microscopy, SEM, EBSD and XRD. Hardness and tensile testing were applied to determine influences on the mechanical properties. Generally, a comparable weldability of HEA and AISI 304 in terms of metallurgical characteristics and resulting mechanical properties exhibited. For the weld joints of both materials typical characteristics regarding FSW were observed within the weld metal and thermo-mechanically influenced zone: fine-grained stirred zone with increased hardness and reduced fracture elongation compared with the respective base material.
High entropy alloys (HEA) are a new class of materials. In contrast to conventional alloys, HEA are single-phase alloys with at least five alloying elements. HEAs have enormous application potential due to (postulated) excellent structural property combinations from low to high temperatures. For HEA-application as structural materials in real components, a key issue is the suitability for joining processing. Requirements for the reliable and safe joining of these materials are crucial regarding economical component manufacture for future applications. In this context, friction stir welding (FSW) is a promising joining process due to the welding process temperature below the material melting point avoiding major issues, e. g. formation of (hard and brittle) intermetallic phases, which may have detrimental influences on the weld joint properties.
This study presents elementary research about the FSW process influences on a CoCrFeMnNi-HEA with focus on the microstructure and mechanical properties. For that purpose, the FSW joint of the HEA is compared to that of an austenitic stainless steel AISI 304. The microstructures of the welds were investigated and characterized by means of light microscopy, SEM, EBSD and XRD. Hardness and tensile testing were applied to determine influences on the mechanical properties. Generally, a comparable weldability of HEA and AISI 304 in terms of metallurgical characteristics and resulting mechanical properties exhibited. For the weld joints of both materials typical characteristics regarding FSW were observed within the weld metal and thermo-mechanically influenced zone: fine-grained stirred zone with increased hardness and reduced fracture elongation compared with the respective base material.
This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multiprincipal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong <001> θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123} <634> S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.
Numerical analysis of ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys
(2022)
The ultrasonic vibration enhanced friction stir welding (UVeFSW) process has unique advantages in joining dissimilar Al/Mg alloys. While there are complex coupling mechanisms of multi-fields in the process, it is of great significance to model this process, to reveal the influence mechanism of ultrasonic vibration on the formation of Al/Mg joints.
In this study, the acoustic-plastic constitutive equation was established by considering the influence of both ultrasonic softening and residual hardening on the flow stress at different temperatures and strain rates. And the ultrasonic induced friction reduction (UiFR) effect on friction coefficient in different relative directions at the FSW tool-workpiece interface was quantitatively calculated and analyzed.
The Al/Mg UVeFSW process model was developed through introducing the above acoustic effects into the model of Al/Mg friction stir welding (FSW). The ultrasonic energy is stronger on the aluminum alloy side. In the stirred zone, there is the pattern distribution of ultrasonic sound pressure and energy. The heat generation at the tool-workpiece contact interface and viscous dissipation were reduced after applying ultrasonic vi-bration. Due to the UiFR effect, the projection of friction coefficient and heat flux distributions at the tool-workpiece interface present a "deformed" butterfly shape. The calculated results show that ultrasonic vibration enhanced the material flow and promoted the mixing of dissimilar materials.
Numerical analysis of ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys
(2023)
The ultrasonic vibration enhanced friction stir welding (UVeFSW) process has unique advantages in joining dissimilar Al/Mg alloys. While there are complex coupling mechanisms of multi-fields in the process, it is of great significance to model this process, to reveal the influence mechanism of ultrasonic vibration on the formation of Al/Mg joints.
In this study, the acoustic-plastic constitutive equation was established by considering the influence of both ultrasonic softening and residual hardening on the flow stress at different temperatures and strain rates. And the ultrasonic induced friction reduction (UiFR) effect on friction coefficient in different relative directions at the FSW tool-workpiece interface was quantitatively calculated and analyzed.
The Al/Mg UVeFSW process model was developed through introducing the above acoustic effects into the model of Al/Mg friction stir welding (FSW). The ultrasonic energy is stronger on the aluminum alloy side. In the stirred zone, there is the pattern distribution of ultrasonic sound pressure and energy. The heat generation at the tool-workpiece contact interface and viscous dissipation were reduced after applying ultrasonic vibra-tion. Due to the UiFR effect, the projection of friction coefficient and heat flux distributions at the tool-workpiece interface present a "deformed" butterfly shape. The calculated results show that ultrasonic vibra-tion enhanced the material flow and promoted the mixing of dissimilar materials.
Understanding the void defect formation mechanism and simultaneous predicting the tool service life in friction stir welding are critical for optimizing the welding parameters. However, the void defect formation mechanism in friction stir welding is not yet elucidated. In this study, a novel integrated thermal-fluid-structure coupling model of the friction stir welding process was proposed for simultaneous prediction of the weld formation and tool service life. A new non-uniform distribution model of the tool-workpiece contact pressure was proposed to describe the interaction between the tool and the workpiece. The void defect formation mechanism was quantitatively studied using the proposed integrated thermal-fluid-structure coupling model. The results show that the plastic material flows in the horizontal direction and can completely fill the cavity behind the tool for the welding condition of forming a sound weld. While the tool-workpiece contact interfacial frictional shear stress in the rear of the tool is decreased significantly which leads to a severe decrease in the plastic material flow velocity. Therefore, after bypassing the tool from the retreating side, the plastic material at the bottom of the weld stagnates, and void defect forms in the middle and lower part of the weld at the advancing side. The difference between the maximum and the minimum tool-workpiece contact pressure could serve as a numerical criterion to predict void defects. A sound joint is formed when the difference is lower than the critical value of 15 MPa, while a void defect is formed in the weld if it is higher than this critical value. The maximum equivalent stress acting on the tool is located at the pin root with severe stress concentration at a high welding speed. The front of the tool is subjected to tensile stress while its rear is subjected to compressive stress, therefore the tool is apt to fracture at its root under an inappropriate welding condition. The average normal stress of the tool varies periodically with its period consistent with the rotation period of the tool. The service life of the tool is decreased with the increase in welding speed and the decrease in rotation speed. The model is validated by experimental results.