This EArly-concept Grants for Exploratory Research (EAGER) grant provides funding for the development of a new friction-stir riveting technique for joining hard-to-weld materials such as Mg and Al alloys, and dissimilar metals. In this method, a solid rivet is used as a friction-stir welding head for joining two pieces of materials which can be either sheets or bulk metals. An action of combined rotation and pressing pushes the rivet into the metals. After a predetermined depth is reached, the action stops and the rivet is left in the metals, forming a desirable mechanical joint. A solid bonding ring is also created at the faying interface. The locking mechanism, defect formation, and relationship between influential physical attributes and important strength measures of a joint will be investigated. A direct comparison between this and other joining methods such as self-piercing riveting and resistance spot welding will be conducted to validate this new joining method.
The successful completion of this research will pave the way for a future proposal for a larger scale, in-depth study and optimization of this joining method. The basic understanding of the effects of materials including the base metal(s) and rivet, and design and process variables obtained will provide critical information for further development of this technique. A relationship linking the quality of a joint to easily measurable geometric attributes of the joint will be developed for quality inspection, and certain critical process parameters such as the profiles of torque and thrust during riveting may be used as a means for real time quality control. The ultimate goal of this research is to develop a robust, economical and industrial joining process to enable the use of advanced materials in sheet metal industry for energy saving and emission reduction.
Driven by the ever-increasing demands for weight reduction of vehicles to reduce emission and improve fuel economy, new advanced materials such as aluminum and magnesium alloys are being used in greater amounts. As projected by the industry's guideline Magnesium Vision 2020, the average magnesium content of a vehicle could increase from less than 20 lbs currently to as much as 350 lbs by 2020, substantially reducing a vehicle’s weight and ultimately improving its fuel efficiency. However, the use of magnesium alloys for sheet metal manufacturing poses a significant challenge to both forming and joining processes, as existing equipment and knowledge cannot be directly applied. Because of the metallurgical characteristics of magnesium, resistance spot welding is not feasible in large scale manufacturing. Friction-stir welding and mechanical fastening have been considered as viable alternatives to welding as they involve very little or no metallurgical changes in the joining processes. However, the stringent process requirements and the low ductility of magnesium alloys make friction welding and self-piercing riveting inapplicable to the sheet materials of light metals. The objective of this research is to explore the feasibility of a hybrid friction-stir riveting process for joining difficult to weld materials such as magnesium alloys, and dissimilar metal combinations such as aluminum to magnesium stack-ups. A new mechanical joining technique was developed, taking the advantages of friction-stir welding and conventional mechanical riveting. It involves spinning and pressing a solid rivet into two sheets of same or dissimilar metals. The embedded rivet provides a mechanical interlock between the sheets, augmented by a ‘welded’ portion of the sheets resulted from mechanical mixing, and a small amount of solid bonding at the faying interface. The quality of a friction-stir riveted joint is directly determined by its structural characteristics. Specifically, they are the end of the interface of the sheets at the mixed zone indicating the thicknesses of attachment of the top and bottom sheets, respectively, to the joint; the size of the mixed zone or the diameter of the cohesive part of the joint; and the gap at the end of the rivet-top sheet interface, as a measure of the thermal effect of the riveting process. A balanced combination of these parameters is needed to fully utilize the sheets in bearing loads. The friction-stir riveting process has been applied to three material combinations: Al-Al, Mg-Mg, and Al-Mg. Through a number of iterations of design and process improvement, an optimal combination of rivet, spindle speed, feed rate, and feed depth was developed on Al-Al joints. When applied to an Mg-Mg stack-up, the optimal process combination for Al-Al also created joints of good micro-structural characteristics. The stirred Mg material tends to be oxidized, and small particles of steel were chopped off from the steel rivet, darkening the color of the severely displaced Mg in the mixed zone. The Al-Mg stack-ups were tested with two scenarios: Al (top) – Mg (bottom) and Mg (top) – Al (bottom). For such dissimilar material combinations it was observed that Al takes a larger proportion of the mixed zone than Mg, and the stirred Mg is pushed away from the mixed zone by Al. In general, Al is not as easy as Mg to be softened during friction-stir riveting. Tensile-shear testing of the friction-stir riveted joints shows similar strength level of such joints to that of self-piercing riveted joints on Al-Al stack-ups. A test on a riveted Mg-Mg joint shows slightly lower strength than its counterpart of Al-Al joints, and no direct comparison was made with self-piercing riveting process as it cannot be applied to Mg without heating the Mg sheets by an external heat source. The friction-stir riveting technique appears to overcome the dificulties encountered in other joining techniques for light metals and dissimilar metals.
