Third-generation advanced high-strength steels (3G AHSS) are being developed to assist in vehicle light weighting so that fuel efficiency may be improved without sacrificing passenger safety. 3G-AHSS have received significant interest from the automotive industry as a critical candidate for their unique combination of high strength and ductility. However, due to selective oxidation of the principal alloying elements such as Mn, Si, Al, and Cr at the steel surface during the annealing stage prior to immersion in the galvanizing Zn(Al, Fe) bath, the process of continuous hot-dip galvanizing of these steel is challenging. This thesis determined the influence of annealing process parameters such as oxygen partial pressure and annealing time, on the selective oxidation and reactive wetting of an Fe-0.15C-5.56Mn-1.17Si-1Al (wt%) prototype 3G AHSS during intercritical annealing as well as continuous galvanizing. Simulated annealing and galvanizing were conducted on the prototype Fe-0.15C-5.56Mn-0117Si-1Al (wt%) 3G steel; Intercritical annealing heat treatments were carried out at 690˚C in a N2-5 vol pct H2 process atmosphere under dew points of 223 K (–50 °C), 243 (–30 °C) and 268 K (–5 °C). MnO was the major oxide formed at the outmost layer of the external oxides on all annealed samples. The experimental parameters, on the other hand, had a substantial impact on the morphology, distribution, thickness, and surface oxide coverage. The greatest Mn surface concentration as well as maximum surface oxide coverage and thickness was obtained by annealing the panels under the 223 K (–50 °C) and 243 (–30 °C) dp process atmospheres. The oxides formed under these process atmospheres largely comprised coarse, compact, and continuous film nodules. In contrast, MnO nodules formed under the 268 K (–5 °C) dewpoint process, exhibited wider spacing between finer and thinner nodules, which was consistent with the internal oxidation mode, while under 223 K (–50 °C) dp process atmosphere, generally external oxidation took place. Poor reactive wetting was obtained for the panels annealed under the 223 K (–50 °C) dp process atmosphere for both the 60 s and 120 s holding times as well as the 243 K (–30 °C) dp process atmosphere for 120 s. This was attributed to the formation of a thick, compact oxide layer on the steel surface, which acted as a barrier between the substrate and Zn bath, preventing Fe dissolution from the substrate surface for the formation of the desired Fe2Al5Znx interfacial layer. However, a well-developed interfacial Fe-Al intermetallic layer was formed under the 268 K (–5 °C) and 243 (–30 °C) dp process atmospheres for intercritical annealing times of 60 s, which is indicative of a good reactive wetting since the thinner and nodule-like oxides on the steel surface after annealing encourage the reactive wetting. External oxides morphology plays a dominant role in facilitating the contact between Zn-alloy bath and the substrate via different mechanisms such as aluminothermic reduction which occurred for the sample annealed under the 268 K (–5 °C) dp process atmosphere. / Thesis / Master of Applied Science (MASc)

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Friction Bit Joining (FBJ) is a new technology that can be used to join dissimilar materials together. This ability makes it a good candidate for creating light weight structures for the automotive industry by combining lightweight materials such as aluminum to stronger materials like advanced high-strength steels. The automotive industry and many other industries have great interest in reducing structure weight to increase fuel efficiency. The purpose of this research is to make FBJ of GADP 1180 to AA 7085-T76 a production ready process by (1) better understanding the effects of process parameters, bit design and tool design on joint strength and reliability especially as they relate to different joint configurations; (2) determining if consecutive FBJ joints on a part will be additive in strength; (3) improving surface finish for better coating adhesion so that joints can be made to withstand extended corrosion testing; and (4) determining the failure modes and fatigue life of joint components at high and low load amplitudes. No universal parameter set for optimizing peak load for T-peel, cross tension, and lap-shear tension configurations were found. Due to the extreme load conditions of T-peel and the smaller margin of safety it is better to optimize for T-peel. However, strength and reliability were still improved across the board. Cutting features and tapered shanks were found to not always be necessary. Removing cutting features from the bit design increased peak weld cycle loads, but a stiffer machine can overcome this. Consecutive FBJ joints on a part are mostly additive in nature. When the weakest joint fails, its load is distributed to the remaining joints and will limit the peak load of the whole part. If all joints are "good" then the peak load will be approximately additive. Most of the stress is localized on the side of the bit opposite of the pulling direction. Failure modes in lap-shear tend to change from weld nugget pullouts in single weld specimens to aluminum material failures in multi-weld specimens. This is because of the added stiffness that additional material and welds provide to resist coupons bending and creating a peeling action. Surface finish was improved by development of a floating carbide cutting system which cut aluminum flash as it was generated around the head of the bit. A new internal drive design provided the ability to drive bits flush with the aluminum top layer if desired with minimal reductions in strength. Flush bits provided benefits in safety, cosmetics, and coating adhesion.

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.

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With the desire to lighten the frame while keeping or increasing the strength, Advanced High-Strength Steels (AHSS) have been developed for use in the automotive industry. AHSS meet many vehicle functional requirements because of their excellent strength and acceptable ductility. But joining AHSS is a challenge, because weldability is lower than that of mild steels. Friction stir spot welding (FSSW) is a solid state joining process that can provide a solution to the weldability issues in AHSS, but FSSW has not been studied in great detail for this application. In this work, Si3N4 tools were used for FSSW experiments on DP 980 steel with 1.2mm thickness. Joint strength was measured by lap shear tension testing, while thermocouples were used for the temperature measurements. A finite element model was developed in order to predict material flow and temperatures associated with FSSW. Since a 3D model of the process is very time consuming, a novel 2D model was developed for this study. An updated Lagrangian scheme was employed to predict the flow of sheet material, subjected to the boundary conditions of the fixed backing plate and descending rotating tool. Heat generation by friction was computed by including the rotational velocity component from the tool in the thermal boundary conditions. Material flow was calculated from a velocity field while an isotropic, viscoplastic Norton-Hoff law was used to compute the material flow stress as a function of temperature, strain and strain rate. Shear stress at the tool/sheet interface was computed using the viscoplastic friction law. The model predicted welding temperatures to within 4% of the experiments. The welding loads were significantly over predicted. Comparison with a 3D model of FSSW showed that frictional heating and the proportion of total heat generated by friction were similar. The position of the joint interface was reasonably well predicted compared to experiment.

Friction Bit Joining (FBJ) is a new technology that can be used to join dissimilar materials together. This ability makes it a good candidate for creating lightweight structures for the automotive industry by combining lightweight materials such as aluminum to stronger materials like advanced high-strength steels. The automotive industry is putting significant effort into interest in reducing vehicle structure weight to increase fuel efficiency and reduce greenhouse gas emissions. Joining of dissimilar materials is a challenge they face in the light weighting the body of the vehicle. The purpose of the current research is to employ FBJ in the joining of a very challenging material combination: GA DP 1180 to AA 7085-T71. In accomplishing this purpose, the goal is to move FBJ toward a more production ready process by better understanding the effects of tooling, bit design, and process parameters on joint strength and reliability as they relate to load profiles captured during the joining process. It was found that the joint strength variation was influenced strongly by the hardness and the geometric consistency of the consumable bits. Bit hardness below 45 HRC led to joint strength that was less than the required specification (5kN in lap shear tension, and 1.5kN in cross-tension and T-peel). Variation in bit height and diameter also led to excessive scatter in joint strength values, where it was not possible to meet the standard for 10 consecutive specimens (for each of the three tests). Implementation of high-speed data acquisition (1000Hz) enabled the capture of load curve profiles generated during FBJ. Load curve profiles were correlated with destructive testing results to discover the impact of process parameter combinations. Analysis of load curve profiles led to improvements in parameter selections of spindle speeds (revolutions per minute) and spindle feed-rates (inches per minute). Process parameters of 5000 RPM and 15 IPM reduced variation in load-curve profiles and destructive testing. Satisfactory joint strength was achieved in lap shear tension, cross-tension, and T-peel testing configurations with values of 10.1 kN, 4.1 kN, and 1.8 kN, respectively. The presence of wet adhesive had little impact on joint performance. Finally, the analysis of a load-curve profiles resulted in a criterion that allowed for distinguishing "good" welds from "bad" ones, where a threshold load of 6kN, or higher, during the dwell phase of welding was required in order to meet joint strength standards.

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