Influence of Post Weld Heat Treatment on the Grain Size, and Mechanical Properties of the Alloy-800H Rotary Friction Weld Joints

This study evaluated the microstructure, grain size, and mechanical properties of the alloy 800H rotary friction welds in as-welded and post-weld heat-treated conditions. The standards for the alloy 800H not only specify the composition and mechanical properties but also the minimum grain sizes. This is because these alloys are mostly used in creep resisting applications. The dynamic recrystallization of the highly strained and plasticized material during friction welding resulted in the fine grain structure (20 ± 2 µm) in the weld zone. However, a small increase in grain size was observed in the heat-affected zone of the weldment with a slight decrease in hardness compared to the base metal. Post-weld solution heat treatment (PWHT) of the friction weld joints increased the grain size (42 ± 4 µm) in the weld zone. Both as-welded and post-weld solution heat-treated friction weld joints failed in the heat-affected zone during the room temperature tensile testing and showed a lower yield strength and ultimate tensile strength than the base metal. A fracture analysis of the failed tensile samples revealed ductile fracture features. However, in high-temperature tensile testing, post-weld solution heat-treated joints exhibited superior elongation and strength compared to the as-welded joints due to the increase in the grain size of the weld metal. It was demonstrated in this study that the minimum grain size requirement of the alloy 800H friction weld joints could be successfully met by PWHT with improved strength and elongation, especially at high temperatures.

Cold Welding in Hold Down Points of Space Mechanisms Due to Fretting When Omitting Grease

Cold welding refers to an effect related to space (vacuum). The heavy vibrations during a launch subject interfaces (hold down points) to oscillating motions which may lead to formation of a kind of “friction weld”. If so, these mechanisms may get stuck, and deployment will be hindered. This may endanger the functionality of the mission (instruments) or even the whole spacecraft (if solar panels do not open). Several studies have been done to characterize material combinations (including coatings) for their ability to cold welding in space. Meanwhile, also during launch grease free contacts are demanded. If grease hat to be omitted, the risk of cold welding under fretting was found to increase (when testing in high vacuum). To rate this risk under launch conditions, the test method was recently extended for testing under launch conditions. The new tests procedure consists of fretting applied in the sequence in air, low vacuum and high vacuum. The paper shall present first results gained with this new method of testing in launch conditions and compare them to previous studies done in vacuum. Following the need of space industry on mechanisms for launch and in-orbit life, a first set of combinations of materials and coatings were selected for this new test sequence where fretting is now applied in a sequence of air, low vacuum and high vacuum. Under this sequence, the measured levels of adhesion and it’s evolvement was found to differ strongly from tests done formerly. The paper outlines these first results and compares them to existing data.

Conduction Heat Assisted Friction Element Welding

Increasing awareness of global warming and strict government regulations have required the automotive industry to pursue lightweighting as an avenue towards increased vehicle efficiency. Lightweight designs typically rely heavily on multi-material use, which enables selective strengthening of critical areas without additional, unnecessary mass. Joining these materials during manufacturing has proven to be a challenging endeavor. Friction element welding (FEW) is one process that is capable of joining aluminum to steel. This two-sided joining technique utilizes a fastener to secure the aluminum sheet by creating a friction weld with the steel sheet. While this process is extremely robust for most materials, the FEW process can result in the extrusion of material from underneath the head of the fastener, termed chipping, which leads to corrosion and aesthetic issues. This behavior is typically seen in high strength aluminum alloys, such as 7075. A solution to chipping is implemented herein, which utilizes a modified downholder to conductively heat the aluminum sheet prior to the FEW process. This heating method was explored experimentally and through various numerical analyses. This method was found to be a viable option for relieving chipping. While the process time was only increased by a maximum of 2.5 seconds, faster, more localized heating should be targeted for future work.

Tensile strength and thermal cycle analysis of AA6061 friction weld joints with different diameters and various friction times

The paper reports measurement of tensile strength and the thermal cycle of AA6061 aluminum alloy circular bar friction weld with different diameters and various friction times. A continuous drive friction welding (CDFW) of AA6061 was conducted to weld the AA6061 circular bar with different diameters of 30 mm for the rotating part and 15 mm for the stationary part. The CDFW process was carried out with the revolution speed of 1,600 rpm, the initial compressive force of 2.8 kN during the friction stage for various friction times of 10, 12, and 14 seconds, and an upset force of 28 kN for 60 seconds. The flash temperature was measured using a digital infrared thermometer gun. Computer simulation using the finite element method was also done by coupling transient thermal and static structural methods. The flash temperature becomes higher along with increasing friction time based on the digital infrared thermometer gun measurement and finite element analysis. The results of tensile strength testing show that the specimen with a friction time of 12 seconds has the highest tensile strength. Based on the hardness testing result, it is found that the specimen with a friction time of 10 seconds has higher hardness, but it has an incomplete joint flash so that the tensile strength is lower than that of the specimen with a friction time of 12 seconds. Besides, the hardness of the specimen with a friction time of 12 seconds is higher than that of the specimen with a friction time of 14 seconds. The flash size becomes bigger along with the increase of the friction time based on the macrostructure observation on the longitudinal section of the CDFW specimen. It is confirmed by the temperature measurement and finite element analysis that the specimen with a friction time of 12 seconds has heat input to form the CDFW joint that has a maximum tensile strength in the range of this study

A COMPARATIVE STUDY ON SINGLE AND DOUBLE JOINTED CU-AL BIMETALLIC STRIPS USING FRICTION STIR SPOT WELDING PROCESS

Welding is one of the best and quick process to join metals. In modern times, we concern more about environmental hazards due to hazardous gases liberated during conventional welding processes. Fiction Stir welding is a simple process in which a solid metal joint is produced by the heat of friction. In a similar technique, we can make spot welds of similar or dissimilar metallic joints by using Friction Stir Spot Welding. Since there is an absence of any liberation of poisonous gases, this method is safe to the environment and user. In this study, we made a dissimilar lap joint of Copper and Aluminium strips using an H13 steel tool. The pin profile of the tool is based on the thickness of the plate/weld. Weld based process parameters such as tool rotational speed, Dwell time and plunge depth. A lap joint with a desirable overlapping length between friction weld spots is compared and analyzed for mechanical bonding strength for both single and double joints. The results show a considerable increase in tensile strength for double-jointed specimen compared to a single-joint.