Joining of dissimilar metals is a process that is of interest in many fields, especially the automotive industry where lightweighting of the body structure is important. However, creating strong joints between dissimilar metals can be challenging. Friction bit joining (FBJ) is a solid-state method that uses a consumable bit to create a strong joint between dissimilar metals such as aluminum and steel. The purpose of this research is to gain understanding of how adjusting FBJ parameters affects the heating and pressure at the weld interface using a modeling approach, in order to better understand the bonding process. The questions guiding this research are: (1) What is the effect of spindle speed, plunge rate, and plunge depth on joint strength? (2) Can the proposed model be developed with enough fidelity to correlate the effect of these parameters on joint strength, within 10%? (3) What is the effect of the simulated vertical load profile on heating at the interface? (4) Does the load profile/heating relationship correlate to experimental joint strength to within 10%? A design of experiments approach found that the effect of spindle speed on joint strength is significant. Plunge rate did not have a significant effect, but the interaction between plunge rate and spindle speed was significant. A model was created, and multiple simulations were run to study these interactions. Initial simulations were run based on the input parameters used for the experiments. The simulation data was used to run a full second order regression was run which found that spindle speed had a significant effect on the experimental Z load. The data also revealed that spindle speed and plunge rate have a strong correlation between bonded area and temperature. Simulated versus experimental Z loads have a good correlation. Experimental bonded area had a slight correlation to joint strength trending in the correct direction. The shape of the simulated cross section did not fully match the experimental cross sections but was reasonable. Simulated bonded area and experimental bonded area also have a positive correlation. Despite some weaknesses, the current model does appear to be predictive enough that it can provide insight into other FBJ design configurations and material combinations in terms of temperature profiles and welding loads.

Friction Bit Joining (FBJ) is a new technology that allows lightweight metals to be joined to advanced high-strength steels (AHSS). Joining of dissimilar metals is especially beneficial to the automotive industry because of the desire to use materials such as aluminum and AHSS in order to reduce weight and increase fuel efficiency. In this study, FBJ was used to join 7075 aluminum and DP980 ultra-high-strength steel. FBJ is a two-stage process using a consumable bit. In the first stage, the bit cuts through the top material (aluminum), and in the second stage the bit is friction welded to the base material (steel). The purpose of the research was to examine the impact a solid head bit design would have on joint strength, manufacturability, and ease of automation. The solid head design was driven externally. This design was compared to a previous internally driven head design. Joint strength was assessed according to an automotive standard established by Honda. Joints were mechanically tested in lap-shear tension, cross-tension, and peel configurations. Joints were also fatigue tested, cycling between loads of 100 N and 750 N. The failure modes that joints could experience during testing include: head, nugget, material, or interfacial failure. All tested specimens in this research experienced interfacial failure. Welds were also created and examined under a microscope in order to validate a simulation model of the FBJ process. The simulation model predicted a similar weld shape and bond length with 5 percent accuracy. Joints made with external bits demonstrated comparable joint strength to internal bits in lap-shear tension and cross-tension testing. Only external bits were tested after lap-shear tension, because it was determined that external bits would perform comparably to internal bits. Joints made with external bits also exceeded the standard for failure during fatigue testing. Peel tested specimens did not meet the required strength for the automotive standard. Examining specimens under a microscope revealed micro-cracks in the weld. These defects have been shown to decrease joint strength. Joint strength, especially during peel testing, could be increased by reducing the presence of micro-cracks. The external bit design is an improvement from the internal bits for manufacturability and ability to be automated, because of the less-expensive processes used to form the bit heads and the design that lends to ease of alignment.

Friction Bit Joining (FBJ) is a recently developed spot joining technology capable of joining dissimilar metals. A consumable bit cuts through the upper layer of metal to be joined, then friction welds to the lower layer. The bit then snaps off, leaving a flange. This research focuses on FBJ using DP980 or DP590 steel as the lower layer, AZ31 magnesium alloy as the top layer, and 4140 or 4130 steel as the bit material. In order to determine optimal settings for the magnesium/steel joints, experimentation was performed using a purpose-built computer controlled welding machine, varying factors such as rotational speeds, plunge speed, cutting and welding depths, and dwell times. It was determined that, when using 1.6 mm thick coupons, maximum joint strengths would be obtained at a 2.03 mm cutting depth, 3.30 mm welding depth, and 2500 RPM welding speed. At these levels, the weld is stronger than the magnesium alloy, resulting in failure in the AZ31 rather than in the FBJ joint in lap shear testing.

Friction Bit Joining (FBJ) is a method used to join lightweight metals to advanced high-strength steels (AHSS). The automotive industry is experiencing pressure to improve fuel efficiency in their vehicles. The use of AHSS and aluminum will reduce vehicle weight which will assist in reducing fuel consumption. Previous research achieved joint strengths well above that which was required in three out of the four standard joint strength tests using DP980 AHSS and 7075 aluminum. The joints were mechanically tested and passed the lap-shear tension, cross-tension, and fatigue cycling tests. The t-peel test configuration never passed the minimum requirements. The purpose of continuing research was to increase the joint strength using FBJ to join the aluminum and AHSS the automotive industry desires to use specifically in the t-peel test. In this study FBJ was used to join 7085 aluminum and GADP1180 AHSS. The galvanic coating on the AHSS and its increased strength with the different aluminum alloy required that all the tests be re-evaluated and proven to pass the standard tests. FBJ is a two-step process that uses a consumable bit. In the first step the welding machine spins the bit to cut through the aluminum, and the second step applies pressure to the bit as it comes in contact with the AHSS to create a friction weld.

In this study, the dissimilar metals 5754 aluminum and DP980 ultra-high strength steel were joined using the friction bit joining (FBJ) process. The friction bits were made using one of three steels: 4140, 4340, or H13. Experiments were performed in lap shear, T-peel, and cross tension configurations, with the 0.070" thick 5754 aluminum alloy as the top layer through which the friction bit cut, and the 0.065" thick DP980 as the bottom layer to which the friction bit welded. All experiments were performed using a computer controlled welding machine that was purpose-built and provided by MegaStir Technologies. Through a series of designed experiments (DOE), weld processing parameters were varied and controlled to determine which parameters had a significant effect on weld strength at a 95% confidence level. The parameters that were varied included spindle rotational speeds, Z-command depths, Z-velocity plunge rates, dwell times, and friction bit geometry. Maximum lap shear weld strengths were calculated to be 1425.4lbf and were to be obtained using a bit tip length at 0.175", tip diameter at 0.245", neck diameter at 0.198", cutting and welding z-velocities at 2.6"/min, cutting and welding RPMs at 550 and 2160 respectively, cutting and welding z-commands at -0.07" and -0.12" respectively, cooling dwell at 500 ms, and welding dwell at 1133.8 ms. These parameters were further refined to reduce the weld creation time to 1.66 seconds. These parameters also worked well in conjunction with an adhesive to form weld bonded samples. The uncured adhesive had no effect on the lap shear strengths of the samples. Using the parameters described above, it was discovered that cross tension and T-peel samples suffered from shearing within the bit that caused the samples to break underneath the flange of the bit during testing. Visual inspection of sectioned welds indicated the presence of cracking and void zones within the bit.

Friction bit joining (FBJ) is a new method that enables lightweight metal to be joined to advanced high-strength steels. Weight reduction through the use of advanced high-strength materials is necessary in the automotive industry, as well as other markets, where weight savings are increasingly emphasized in pursuit of fuel efficiency. The purpose of this research is twofold: (1) to understand the influence that process parameters such as bit design, material type and machine commands have on the consistency and strength of friction bit joints in dissimilar metal alloys; and (2) to pioneer machine and bit configurations that would aid commercial, automated application of the system. Rotary broaching was established as an effective bit production method, pointing towards cold heading and other forming methods in commercial production. Bit hardness equal to the base material was found to be highly critical for strong welds. Bit geometry was found to contribute significantly as well, with weld strength increasing with larger bit shaft diameter. Solid bit heads are also desirable from both a metallurgical and industry standpoint. Cutting features are necessary for flat welds and allow multiple material types to be joined to advanced high-strength steel. Parameters for driving the bit were established and relationships identified. Greater surface area of contact between the bit and the driver was shown to aid in weld consistency. Microstructure changes resulting from the weld process were characterized and showed a transition zone between the bit head and the bit shaft where bit hardness was significantly increased. This zone is frequently the location of fracture modes. Fatigue testing showed the ability of FBJ to resist constant stress cycles, with the joined aluminum failing prior to the FBJ fusion bond in all cases. Corrosion testing established the use of adhesive to be an effective method for reducing galvanic corrosion and also for protecting the weld from oxidation reactions.

Friction bit joining (FBJ) is a new solid-state spot joining technology developed in cooperation between Brigham Young University of Provo Utah, and MegaStir Technologies of West Bountiful Utah. Although capable of joining several different material combinations, this research focuses on the application of FBJ to joining 5754 aluminum to DP 980 steel, two alloys commonly used in automotive applications. The thicknesses of the materials used were 0.070 inches (1.78 mm) and 0.065 inches (1.65 mm), respectively. The FBJ process employs a consumable 4140 steel bit and is carried out on a purpose built research machine. In the first stage of the weld cycle the bit is used to drill through the aluminum top sheet to be joined. After this, spindle speed is increased so that the bit tip effectively forms a friction weld to the steel bottom sheet. Momentary stoppage of the spindle facilitates weld cooling before the spindle is restarted, shearing the bit tip from the bit shank, and retracted. Incorporated into the bit tip geometry is a flange that securely holds the aluminum in place after joint formation is complete. This research consists of several developmental steps since the technology only recently began to be formally studied. Initial joint strengths observed in lapshear tensile testing averaged only 978.5 pounds (4.35 kN), with a relatively high standard deviation for the data set. Final lapshear tensile test results were improved to an average of 1421.8 pounds (6.32 kN), with a significantly lower, and acceptable, standard deviation for the data set. Similar improvements were realized during the development work in cross tension tensile test results, as average strengths increased from 255.8 pounds (1.14 kN) to 566.3 pounds (2.52 kN). Improvements were also observed in the standard deviation values of cross tension data sets from initial evaluation to the final data set presented in this work.