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Corrosion tendency is one of the major inhibitors for increased use of magnesium alloys in automotive structural applications. Moreover, systematic or standardized methods for evaluation of both general and galvanic corrosion of magnesium alloys, either as individual components or eventually as entire subassemblies, remains elusive, and receives little attention from professional and standardization bodies. This work reports outcomes from an effort underway within the U.S. Automotive Materials Partnership - ‘USAMP’ (Chrysler, Ford and GM) directed toward enabling technologies and knowledge base for the design and fabrication of magnesium-intensive subassemblies intended for automotive “front end” applications. In particular, subassemblies consisting of three different grades of magnesium (die cast, sheet and extrusion) and receiving a typical corrosion protective coating were subjected to cyclic corrosion tests as employed by each OEM in the consortium.

This paper will present the results of 3-D finite element analyses of single lap adhesive joints between magnesium and three other automotive materials, namely steel, aluminum and SRIM composites. The modulus of magnesium is lower than that of either steel or aluminum, but is higher than that of SRIM. Thus, this study aims at determining the effect of the difference in substrate modulus on the deformation, stress and strain distributions and maximum stresses in adhesive joints of magnesium with the other three materials. In addition, the effect of adhesive modulus is also explored.

Spot friction welding shows advantages over resistance spot welding for joining light alloys for automotive applications. In this research, fatigue behaviors of spot friction welded joints in lap shear specimens of AM-60 magnesium alloy and AA 5754 aluminum alloy were investigated. Static and fatigue tests were conducted with Mg-Mg, Al-Al and Al-Mg specimens. Fatigue S-N curves were obtained for all these specimens using load-controlled fatigue tests. Finite element analysis was conducted to investigate the stress distribution and the location of maximum stresses in spot friction welded joints in Mg-Mg specimens.

This study reports the performance of three different automotive magnesium substrate materials (AM60B diecastings, AZ31-H24 sheet, and AM30 extrusions), each bonded to a common aluminum reference material with two different toughened adhesives. The magnesium substrates were pretreated with six different commercial pretreatments both with and without a final fused-powder polymeric topcoat. These samples were then evaluated by comparing initial lap-shear strength to the lap-shear strength after cyclic-corrosion testing. Additionally, use of a scribe through the polymer primer permitted assessment of: 1) distance of corrosion undercutting from the scribe (filiform), and 2) percent corrosion over the area of the coupon. The results showed that the performance of each magnesium pretreatment varied on cast AM60B, sheet AZ31-H24, and extruded AM30 substrates.

This study utilized Rutherford Backscattering Spectroscopy (RBS) and Auger Electron Spectroscopy (AES) to determine the thickness and composition of corrosion films formed on magnesium alloys exposed to experimental engine coolants intended for use with creep-resistant magnesium alloy engine blocks. Knowledge on the nature of the film formed may produce a better understanding of corrosion inhibition and protection requirements for these materials. This paper will first briefly review the RBS and AES techniques as applied to the characterization of surface films on metals. It will then present results from experiments conducted with pure magnesium and two different magnesium alloys in two engine coolants.

Spot friction welding is considered a cost-effective method for joining lightweight automotive alloys, such as magnesium and aluminum alloys. An experimental study was conducted to investigate the strength of spot friction welded joints of magnesium to magnesium, aluminum to aluminum, magnesium to aluminum and aluminum to magnesium. The joint structures and failure modes were also studied.

In this investigation, spot friction welds in aluminum 6111-T4 lap-shear specimens were tested under both quasi-static and dynamic loading conditions. Micrographs of the spot friction welds after testing were examined to understand the failure modes of spot friction welds in lap-shear specimens under different loading conditions. The micrographs indicate that the spot friction welds produced by this particular set of welding parameters failed in interfacial failure mode under both quasi-static and dynamic loading conditions. The load and displacement histories for lap-shear specimens were obtained under quasi-static and dynamic loading conditions at three different impact velocities. The failure loads of spot friction welds in lap-shear specimens under dynamic loading conditions are about 7% larger than those under quasi-static loading conditions.

An electrochemical testing protocol for assessing the intrinsic corrosion-resistance of creep-resistant magnesium alloys in aqueous environments, and effects of passivating surface films anticipated to develop in the presence of engine coolants is under development. This work reports progress in assessing the relative corrosion resistance of the base metals (AMC-SC1, MRI-202S, MRI-230D, AM50 and 99.98% Mg) in a common test environment, based on a near-neutral pH buffered saline solution, found to yield particularly stable values for the open-circuit or corrosion potential. This approach was found to provide a platform for the eventual assessment of the durability of certain passivating layers expected to develop during exposure of the magnesium alloys to aqueous coolants.

An experimental study was conducted to investigate the potential of spot joining aluminum with nylon using frictional heat. The process utilizes the heat generated by friction between a rotating tool surface and the aluminum sheet surface to melt nylon locally in the joining area and create a mechanical interlock between the aluminum and nylon sheets. Lap shear joint specimens were prepared using this process to investigate the effect of several parameters such as tool geometry, tool RPM, tool hold time, tool plunge depth and tool feed rate. Tensile tests were conducted to evaluate the joint strength and to investigate the failure mechanisms of the joint. Furthermore, the effects of cleaning the aluminum surface and baking of the nylon on the joint strength were also studied in this paper. Finally, friction heat generated joints were compared with adhesively bonded joints between aluminum and nylon.

The effects of lubricant on lap shear strength of Spot Friction Welded (SFW) joints made of 6111-T4 alloys were studied. Taguchi L8 design of experiment methodology was used to determine the lubricant effects. The results showed that the lap shear strength increased by 9.9% when the lubricant was present at the top surface compared to that of the baseline (no lubricant) whereas the lap shear strength reduced by 10.2% and 10.9% when the lubricant was present in the middle and at the bottom surfaces compared to that of the baseline (no lubricant), respectively. The microstructure analysis showed a zigzag interface at the joint between the upper and the lower sheet metal for the baseline specimen, the specimens with the lubricant at the top and at the bottom. However, a straight line interface is exhibited at the joint between the upper and the lower sheet for the specimen with the lubricant in the middle. The weld nugget sizes of the lap shear tested specimens were measured.

Fatigue failures of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets under cyclic loading conditions are investigated in this paper. The paths of fatigue cracks near the spot friction welds are first discussed. A fatigue crack growth model based on the Paris law for crack propagation and the global and local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of these spot friction welds. The global stress intensity factors and the local stress intensity factors based on the recent published works for resistance spot welds in lap-shear specimens are used to estimate the local stress intensity factors for kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.

Over the past five years, the US Automotive Materials Partnership (USAMP) has brought together representatives from DaimlerChrysler, General Motors, Ford Motor Company and over 40 other participant companies from the Mg casting industry to create and test a low-cost, Mg-alloy engine that would achieve a 15 - 20 % Mg component weight savings with no compromise in performance or durability. The block, oil pan, and front cover were redesigned to take advantage of the properties of both high-pressure die cast (HPDC) and sand cast Mg creep- resistant alloys. This paper describes the alloy selection process and the casting and testing of these new Mg-variant components. This paper will also examine the lessons learned and implications of this pre-competitive technology for future applications.

This paper presents a comparison of aqueous corrosion rates in 5% NaCl solution for eight experimental creep-resistant magnesium alloys considered for automotive powertrain applications, as well as three reference alloys (pure magnesium, AM50B and AZ91D). The corrosion rates were measured using the techniques of titration, weight loss, hydrogen evolution, and DC polarization. The corrosion rates measured by these techniques are compared with each other as well as with those obtained with salt-spray testing using ASTM B117. The advantages and disadvantages of the various corrosion measurement techniques are discussed.

In this paper, fracture and fatigue mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens are investigated based on experimental observations. Optical and scanning electron micrographs of these spot friction welds before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show the fracture and fatigue mechanisms of spot friction welds under quasi-static and cyclic loading conditions. The experimental observations indicate that the fracture mechanisms depend on the microstructure and geometry of welds under quasi-static loading conditions. Under cyclic loading conditions, the fatigue mechanisms depend not only on the microstructure and geometry of welds but also on the load amplitudes.

An exploratory study was conducted to investigate the feasibility of friction stir spot welding advanced high-strength steel sheet metals. The fixed pin approach was used to weld 600MPa dual phase steel and 1310MPa martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over one hundred welding trials without noticeable degradation and wear. Solid-state metallurgical bonding was produced with welding time in the range of 2 to 3 seconds, although the bonding ligament width was relatively small. The microstructures and hardness variations in the weld regions are discussed. The results from tensile-shear and cross-tensile tests are also presented.

Surfaces of A356 castings were treated by friction stir processing to reduce porosity and to create more uniform distributions of second-phase particles. Dendritic microstructures were eliminated in stir zones. The ultimate tensile strength, ductility, and fatigue life of the cast A356 was increased by friction stir processing. Tensile specimens of cast and friction stir processed metal were also given a T7 heat treatment. Higher tensile strengths and ductilities were also measured for these friction stir processed specimens.

The fatigue lives of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets are investigated here. The paths of fatigue cracks near spot friction welds are first discussed. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of spot friction welds. The global and local stress intensity factors based on a recent work of Wang and Pan for resistance spot welds in lap-shear specimens are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.

This paper presents the results of fatigue tests conducted on tailor-welded aluminum blanks consisting of 1.66 mm thick and 1.06 mm thick AA 5754-O sheets. The method of joining the sheets was friction-stir welding. The primary purpose of this study was to determine the effect of tensile pre-strains on the fatigue performance of the welded joint. The welded specimens as well as unwelded 1.06-mm thick specimens were subjected to tensile pre-strains of 60 and 80% of their respective uniform strains before the fatigue tests. Fatigue S-N data of all these specimens were compared with similar data for unstrained specimens. Microscopic examinations were conducted to understand the failure modes.

Microstructures and failure mechanisms of spot friction welds (SFW) in aluminum 5754 lap-shear specimens were investigated. In order to study the effect of tool geometry on the joint strength of spot friction welds, a concave tool and a flat tool were used. In order to understand the effect of tool penetration depth on the joint strength, spot friction welds were prepared with two different penetration depths for each tool. The results indicated that the concave tool produced slightly higher joint strength than the flat tool. The joint strength did not change for the two depths for the flat tool whereas the joint strength slightly increases as the penetration depth increases for the concave tool. The experimental results show that the failure mechanism is necking and shearing for the spot friction welds made by both tools. The failure was initiated and fractured through the upper sheet under the shoulder indentation near the crack tip.

This paper describes the results of dynamic denting experiments conducted on AA5754 and AA6061 alloys. Dynamic denting tests were performed using a drop weight impact machine. The drop height was varied from 38 mm to 914 mm to generate impact velocities ranging from 53.4 m/min to 254 m/min. The dent depth created at different drop heights was related to the input impact energy and peak load observed in the tests. The effects of sheet thickness and yield strength were explored.