Richard W. Davies

Weld metal ductility in aluminum tailor welded blanks

The objective of the research described in this article was to characterize and numerically describe the ductility of weld material in aluminum tailor welded blanks under uniaxial tension conditions. Aluminum tailor welded blanks consist of multiple thickness and alloy sheet materials welded together into a single, variable thickness blank. To evaluate the mechanical properties of the weld material in these tailor welded blanks, a series of tensile specimens containing varying ratios of weld and monolithic material in the gage area of the specimen were tested. These experimental results show that increasing the amount of weld in the cross-sectional area of the specimen decreases the ductility of the specimen and that the weld characteristics have a pronounced impact on ductility. Using the experimental results and classical tensile instability and necking models, a numerical model was developed to describe the ductility of the weld metal. The model involves basic material properties and an initial imperfection level in both the weld and monolithic materials. The specimens studied were produced from 1- to 2-mm AA5182-O aluminum alloy sheet material welded into blanks using an autogenous gas tungsten arc welding process.

Modeling of Stone-impact Resistance of Monolithic Glass Ply Using Continuum Damage Mechanics

The stone-impact resistance of a monolithic glass ply is studied using a combined experimental and computational approach. Instrumented stone-impact tests are first carried out in a controlled environment. Explicit finite element analyses are then used to simulate the interactions of the indentor and the glass layer during the impact event, and a continuum damage mechanics (CDM) model is used to describe the constitutive behavior of glass. The experimentally measured strain histories for low-velocity impact serves as validation of the modeling procedures. Next, stair-stepping impact experiments are performed with two indenter sizes on two glass ply thicknesses, and the test results are used to calibrate the critical stress parameters used in the CDM constitutive model. The purpose of this study is to establish the modeling procedures and the CDM critical stress parameters under impact loading conditions. The modeling procedures and the CDM model will be used in our future studies to predict through-thickness damage evolution patterns for different laminated windshield designs in automotive applications.

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.

Anisotropic Yield Locus Evolution During Cold Pilgering of Titanium Alloy Tubing

The cold pilger metal forming technique is known to produce round titanium alloy tubing with mechanical properties that may be significantly anisotropic. These mechanical properties are of interest to both the manufacturers and consumers for defining initial manufacturing limitations and defining the final product design limitations. This study focuses on experimentally characterizing the yield locus development of Ti-3Al-2.5V seamless tubing during cold pilgering and a subsequent thermal stress relieving process. The materials are experimentally characterized using a biaxial testing apparatus, which subjects the specimen tubes to combined axial load and internal pressure. The Hill yield criterion is subsequently fit to the experimental results producing continuous yield loci. Each specimen is also experimentally characterized using X-ray diffraction to gain insight into the material textures that accompany the macroscopic properties. All work is focused on one particular pilger pass at two different production rates. A second experimental variable is introduced to the study by using two significantly different input materials, as characterized by X-ray diffraction. This study also investigates the nature of the plastic deformation of the tubing developed during cold pilgering via finite element analysis and discusses the relationship between the finite element predictions and the mechanical anisotropy.

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.

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.

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