Elizabeth V. Stephens

Effects of Failure Modes on Strength of Aluminum Resistance Spot Welds

This paper examines the effects of failure modes on the static strength and total energy absorption of aluminum spot-welded samples using experimental, statistical, and analytical approaches. The main failure modes for aluminum spot welds are nugget pullout and interfacial fracture. Two populations of aluminum spot welds were studied. Within each population, coupon configurations of lap shear, cross tension and coach peel were considered. Thirty replicate static strength tests were performed for each coupon configuration. The resulted peak load and energy absorption level associated with each failure mode was studied using statistical models. Next, an analytical model was developed to determine the failure mode of an aluminum resistance spot weld based on stress analysis. It is found that weld size, sheet thickness, and level of weld porosity and defects are the main factors determining the cross tension failure mode for an aluminum spot weld. The peak load and energy absorption levels for the cross tension and coach peel samples tested are found not to be very sensitive to the failure modes under static loading.

Effects of fusion zone size on failure modes and performance of advanced high strength steel spot welds

This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS). DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pull-out failure mode are developed for both DP800 and TRIP800 using the limit load based analytical model and the micro-hardness measurements of the weld cross sections. Static weld strength tests using cross tension samples were performed on the joint populations with controlled fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied using statistical data analysis tools. The results in this study show that the conventional weld size of 4 t can not produce nugget pullout mode for both the DP800 and TRIP800 materials. The results also suggest that performance based spot weld acceptance criteria should be developed for different AHSS spot welds.

Forming Limits of Weld Metal in Aluminum Alloys and Advanced High-Strength Steels

This work characterizes the mechanical properties of DP600 laser welded TWBs (1 mm-1.5 mm) near and in the weld, as well as their limits of formability. The approach uses simple uniaxial experiments to measure the variability in the forming limits of the weld region, and uses a theoretical forming limit diagram calculation to establish a probabilistic distribution of weld region imperfection using an M-K method approach

Evaluation of the Mechanical Performance of Self-Piercing Rivets in Friction Stir Welded Structures

This paper presents the coupon performance data of friction stir welded tailor welded blanks (TWBs) joined to a monolithic aluminum sheet by self-piercing rivets (SPRs). Uniaxial tensile tests were performed to characterize the joint strength and the total energy absorption capability of the TWB/monolithic joint assemblies. Cyclic fatigue tests were also conducted to characterize the fatigue behavior and failure mechanisms of the jointed assemblies. It was found that the static and fatigue strength of the TWB/monolithic assembly was approximately 30 percent less in all loading configurations tested in comparison to a common monolithic sheet SPR assembly. The total energy absorbed by the TWB/monolithic sheet assemblies was also found to be 30 percent less than the monolithic sheet assemblies in cross tension loading. In lap shear loading, the total energy absorbed was comparable.

Mechanical Characterization and Corrosion Testing of X608 Al Alloy

This paper describes the mechanical characterization and corrosion testing of X608 Al alloy that is being considered for A-pillar covers for heavy-duty truck applications. Recently, PNNL developed a thermo-mechanical process to stamp A-pillar covers at room temperature using this alloy, and the full-size prototype was successfully stamped by a tier-1 supplier. This study was conducted to obtain additional important information related to the newly developed forming process, and to further improve its mechanical properties. The solutionization temperature, pre-strain and paint-bake heat-treatment were found to influence the alloy’s fabricability and mechanical properties. Natural aging effect on the formability was investigated by limiting dome height (LDH) tests. Preliminary corrosion experiments showed that the employed thermo-mechanical treatments did not significantly affect the corrosion behavior of Al X608.

An Experimental and Modeling Investigation on High-Rate Formability of Aluminum

This work describes the integrated experimental and modeling effort at PNNL to enhance the room-temperature formability of aluminum alloys by taking advantage of formability improvements generally associated with high-strain-rate forming. Al alloy AA5182-O sheets were deformed in near plane-strain conditions at strain-rates exceeding 1000 /s using the electrohydraulic forming (EHF) technique, and at quasi-static strain-rates via a bulge test. A novel capability, combining highspeed imaging with digital image correlation technique, was developed to quantify the deformation history during high-rate forming. Sheet deformation under high rates was modeled in Abaqus and validated with experimentally determined deformation data. The experimental results show a ~2.5x increase in formability at high rates, relative to quasi-static rates, under a proportional loading path that was verified by the experimental data. The model shows good correlation with the experimentally determined strain path. It is anticipated that such integrated experimental and modeling work will enable room-temperature forming of Al and industrial implementation of high-rate forming processes.

Surface Treatments for Improved Performance of Spinel-coated AISI 441 Ferritic Stainless Steel

Ferritic stainless steels are promising candidates for IT-SOFC interconnect applications due to their low cost and resistance to oxidation at SOFC operating temperatures. However, steel candidates face several challenges; including long term oxidation under interconnect exposure conditions, which can lead to increased electrical resistance, surface instability, and poisoning of cathodes due to volatilization of Cr. To potentially extend interconnect lifetime and improve performance, a variety of surface treatments were performed on AISI 441 ferritic stainless steel coupons prior to application of a protective spinel coating. The coated coupons were then subjected to oxidation testing at 800 and 850°C in air, and electrical testing at 800°C in air. While all of the surface-treatments resulted in improved surface stability (i.e., increased spallation resistance) compared to untreated AISI 441, the greatest degree of improvement (through 20,000 hours of testing at 800°C and 14,000 hours of testing at 850°C) was achieved by surface blasting.

An Investigation of Sheet Metal Deformation Behavior During Electro-Hydraulic Forming (EHF)

This work describes recent advances in our understanding of sheet metal behavior during electro-hydraulic forming (EHF) process. Two sets of experiments were performed using AA5182-O Al sheet material. In the first set, 1 mm thick sheet samples were subjected to a single pressure-pulse or two consecutive pressure-pulses with the deformation being carried out under free-forming or inside a conical die. In the second set of experiments employing 2 mm sheet samples, a circular region at the center of the sheet was pre-thinned to 1 mm thickness and the sheet was subjected to a single pressure-pulse under free-forming conditions. The sheet deformation history for both sets of experiments was quantified using a recently developed technique that combines high-speed imaging and the digital image correlation (DIC) techniques. The results from the first set of experiments show that the manner in which the discharge is created can influence the strain-rates and hence, the deformation history experienced by the sheet materials. The results of the multi-pulse experiments demonstrate the applicability of the EHF technique for re-strike operations. The results from the second set of experiments show that the pre-thinned region is analogous to a reduced gauge section with the resulting strain-rate (in the pre-thinned region) exceeding that in the adjacent homogeneous sheet by more than 50%.Copyright

Electro-Hydraulic Forming of Advanced High-Strength Steels: Deformation and Microstructural Characterization

The deformation behavior and texture evolution during forming of an advanced high-strength steel (DP600 grade) were characterized. The deformation history of DP600 during electro-hydraulic forming (EHF) was quantified using a unique experimental capability developed at PNNL. The texture evolution during quasi-static and high-strain-rate deformation was determined using the electron backscatter diffraction (EBSD) technique. The deformation history of EHF formed steel sheets shows an amplification of the strain-rate, relative to free-forming conditions, when the forming was carried out inside a conical-die. This strain-rate amplification was attributed to the focusing action of the conical die. The undeformed DP600 sheet was found to possess a {111} fiber texture in the sheet-normal direction. Quasi-static deformation was found to strengthen the pre-existing texture whereas high-rate forming using EHF had a lesser influence. The results of this work demonstrate the unique capability to correlate deformation history during high-strain-rate metal forming processes with the corresponding microstructural evolution. It is expected that results of this work can help fill-in the gaps in our understanding of high-rate forming processes, leading to development of accurate and validated numerical models.Copyright

Effects of Manufacturing Processes and In-Service mperature Variations on the Properties of TRIP Steels

This paper examines key aspects of the manufacturing process that “Transformation Induced Plasticity” (TRIP) steels would be exposed to, and systematically evaluate how the forming and thermal histories affect final strength and ductility of the material. The paper evaluates in-service temperature variations, such as under hood and hot/cold cyclic conditions, to determine whether these conditions influence final strength, ductility and energy absorption characteristics of several available TRIP steel grades. As part of the manufacturing thermal environment evaluations, stamping process thermal histories are included in the studies. As part of the in-service conditions, different pre-straining levels are also included. Materials from four steel suppliers world wide are examined. The material properties are established over a full range of expected thermal histories and selected loading modes. Establishing these relationships will allow OEM designers to select TRIP steels for proper vehicle applications, and to specify manufacturing process conditions that yield reliable final material property levels.