| Abstract Scope |
For many years, the defense industry has made large investments into making their armored combat vehicles as light as possible without compromising the security of the operators. These heavy vehicles are less agile and more expensive to operate and transport. Friction Stir Welding (FSW) offers a lightweight solution that promotes the weight reduction of these armored vehicles. Traditional arc welding processes create weak areas, such as the heat affected zone (HAZ), that need additional armored material that could now be eliminated if FSW were used. As a solid-state process, FSW offers an extremely low heat input to the welded joints which results in good mechanical performance from the reduction of the HAZ and the dynamic recrystallization that occurs within the stir zone. Advancements have allowed for research to be conducted with the control of temperature and force during welding. This work has explored the FSW process on Rolled Homogenous Armor (RHA) steel through comparing parameters, mechanical properties and defect formation while also investigating the toughness of the joints at various weld temperatures.
The work carried out for this project involved making many welds on ¼” thick RHA steel butt joints utilizing a Bond RM-15 Friction Stir Machine which has the capability to produce welds utilizing multiple different control methodologies such as force and position. The welds were made utilizing a PCBN FSW-tool manufactured by MegaStir with a thermocouple inserted to track the tool temperature during welding. Utilizing this machine, baseline parameters were established utilizing force control welding. A forging force was selected by analyzing the hardness of various welds done at varying forces. Other parameters such as welding speed and spindle speed were also established. This was done through cross sectioning each weld and performing basic metallography to explore the effects each parameter had on the mechanical properties of the joint along with the potential to form defects within the joint. These parameters also resulted in varying temperatures within the joint which were compared in how they affected tool wear and the mechanical properties of the various joints. Once the baseline parameters were established, a weld was made and a cross section was taken to preform a hardness map of the joint and surrounding material. This joint also had charpy impact tests conducted at room temperature at various locations within the weld such as the stir zone, HAZ, and base metal.
When the finding the optimal parameters for FSW of RHA steel. A forging force of 35 kN was settled on for the entirety of the force control welds as this provided the hardness within the joint. It was seen that as RPM increased so did the temperature of the tool during welding. This led to making 4 different welds at varying rpm and travel speeds to gauge the effect the temperature had on the surrounding area. It was seen that the higher temperatures resulted in low transverse forces but high temperatures which could lead to over tempering of the surrounding martensite that is present from the manufacturing of the material. The colder welds had a higher transverse force but the HAZ of the weld experienced less tempering thus producing a slightly harder HAZ compared to the warmer welds. These welds also showcased how going above 100 mm/min in travel speed could cause void defects because adequate stirring of the plasticized metal could not be achieved. From these four welds a set of baseline parameters were found, a travel speed of 100 mm/min and a spindle speed of 200 RPM. The weld produced with these parameters had a tool temperature of roughly 770 degrees Celsius, which provided appropriate transverse forces on the tool while not over tempering the surrounding base metal at a large rate. Hardness maps were produced from the baseline weld which exhibited a hard stir zone due to the dynamic recrystallization and the untempered martensite present. The HAZ, however, had a large amount of over tempered martensite there which caused a drop in hardness for that area compared to the rest of the joint and surrounding base metal. Charpy test samples of these weld have also been analyzed and the stir exhibited moderate toughness due to the dynamic recrystallization that occurred during the friction stir welding process created finer grains allowing for higher toughness but the martensite that is formed there has not been tempered and would cause the toughness of the area to drop. The surrounding area around the weld exhibited good toughness due to the martensite being tempered during the welding process.
Looking at the results obtained from the work successful defect free welds were able to be made utilizing a PCBN tool with the force control method at 35kN utilizing a spindle speed of 200 RPM and a travel speed of 100 mm/min. Looking at the mechanical property of these joints there is a very hard stir zone due to the untempered martensite and dynamic recrystallization. The HAZ of the weld experienced a drop in hardness due to the over tempering of the martensite from the heat generated during the welding process. The toughness of the stir zone is moderate due to the untempered martensite being brittle but the dynamic recrystallization creating a fine grain structure which offers good toughness. However, the HAZ experiences good toughness since the martensite in this area has been over tempered.
Keywords: Friction Stir Welding, Armor Steel, RHA steel, PCBN Tool, Force Control |