Bita Ghaffari

Spatially resolved ultrasonic attenuation in resistance spot welds: implications for nondestructive testing.

Spatial variation of ultrasonic attenuation and velocity has been measured in plane parallel specimens extracted from resistance spot welds. In a strong weld, attenuation is larger in the nugget than in the parent material, and the region of increased attenuation is surrounded by a ring of decreased attenuation. In the center of a stick weld, attenuation is even larger than in a strong weld, and the low-attenuation ring is absent. These spatial variations are interpreted in terms of differences in grain size and martensite formation. Measured frequency dependences indicate the presence of an additional attenuation mechanism besides grain scattering. The observed attenuations do not vary as commonly presumed with weld quality, suggesting that the common practice of using ultrasonic attenuation to indicate weld quality is not a reliable methodology.

Microstructure characterization and quasi-static failure behavior of resistance spot welds of AA6111-T4 aluminum alloy

Abstract The microstructure, microhardness and quasi-static failure behavior of resistance spot welds of AA6111-T4 aluminum alloy were experimentally investigated. Optical metallography and high-resolution hardness traverses were utilized to characterize the weld nugget, heat affected zone and base metal. The AA6111 spot welds displayed a softer nugget and hardened heat affected zone, compared with the base metal. The through-thickness hardness of the base metal sheet was not constant and had to be carefully considered to determine the effect of welding on material properties. Quasi-static lap-shear tensile tests were used to determine the failure load and failure mode. All tensile specimens failed through the interfacial fracture. This failure mode is consistent with the observed reduced hardness in the weld nugget.

Comparison of Metallurgical and Ultrasonic Inspections of Galvanized Steel Resistance Spot Welds

Metallurgical examination of galvanized steel resistance spot welds was used to gauge the capabilities of two ultrasonic, non‐destructive, scanning techniques. One method utilized the amplitude of the echo from the weld faying surface, while the other used the spectral content of the echo train to map the fused area. The specimens were subsequently sectioned and etched, to distinguish the fused, zinc‐brazed, and non‐fused areas. The spectral maps better matched the metallurgical maps, while the interface‐amplitude method consistently overestimated the weld size.

Ultrasonic Evaluation of Weld Strength for Aluminum Ultrasonic Spot Welds

The goal of this work is to determine the feasibility of using an ultrasonic, non‐destructive technique for post‐process evaluation of aluminum ultrasonic spot welds. A focused immersion transducer was utilized to obtain a C‐scan of the weld interface, from which a weighted ultrasonic contact area was estimated. Weldments were subsequently tested destructively to determine the weld strength. The square root of the weld contact area displayed a relatively good correlation with weld strength, r2=0.85.

Phase-Field Modeling of θ′ Precipitation Kinetics in W319 Alloys

Understanding and predicting the morphology, kinetics and hardening effects of precipitates are critical in improving the mechanical properties of Al-Cu-based alloys through controlling the temperature and duration of the heat treatment process. In this work, we present a comprehensive phase-field framework for simulating the kinetics of θ′ precipitates in W319 alloys, integrating the thermodynamic and diffusion mobility databases of the system, the key precipitate anisotropic energy contributions from literature and first-principles calculations, as well as a nucleation model based on the classical nucleation theory. By systematically performing phase-field simulations, assuming the precipitate peak number densities determined from experiments, we optimize the model parameters to obtain the best possible match to the average diameters, thicknesses and volume fractions of precipitates from experimental measurements at 190, 230 and 260 °C. With these parameters available, the phase-field simulations can be performed at other aging temperatures. The possible extensions of the current phase-field model for more accurate prediction of the precipitate behaviors in W319 alloys will also be discussed.

Ultrasonic maps of porosity in aluminum castings

The use of cast aluminum in the automotive industry has grown dramatically in recent years, leading to increased need for quantitative characterization of microporosity. As previously reported in the literature, the attenuation of ultrasound can be used to measure the porosity volume fraction and the mean pore size. An immersion ultrasound system has been built utilizing this technique to scan castings with high spatial resolution. Maps of attenuation are shown to locate areas of varying porosity readily and reliably.

Fatigue Behavior of Friction Stir Linear Welded Dissimilar Aluminum-to-Magnesium Alloys

In this paper, we present the results of fatigue testing and analysis of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration. The overlap linear welds were created by joining AA6022 aluminum alloy to AM60 magnesium alloy. In general, the test data exhibited significant scatter in the fatigue life results and the corresponding failure modes. In fact, observations from fractography analysis revealed two distinct modes of failure. In the first mode of failure observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kinked crack formation. Interestingly, frettinglike debris was observed at the initiation sites for this failure mode. In the second mode of failure observed, fracture occurred by interfacial weld separation. In this mode, fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the intermetallic phase.

Friction Stir Welding of Dissimilar Lightweight Metals with Addition of Adhesive

Dissimilar welding of aluminum to magnesium alloy sheets is desired for automotive body structures, but challenging due to the formation of brittle intermetallics at the interface. In a previous study, friction stir welding was utilized to evaluate the joining potential of a cast magnesium alloy to aluminum alloy sheets, and specifically to produce robust and reliable friction stir welds between sheets of wrought AA6022-T4 aluminum alloy and cast AM60B magnesium alloy. The welds are generally stronger when welding is conducted from the aluminum-sheet side, given the thickness of aluminum is half that of magnesium. In the present study, friction stir welding was further developed to produce structural welds between these sheets from the aluminum side, with and without adhesive at the faying surface. The effect of adhesive on the interface was examined for resultant material mixing and microstructure. Mechanical performance of welds was also evaluated.

Future Directions in Sensors

There is a growing interest in applications of embedded and integrated sensors to ensure the reliability of structural components. The results of a panel discussion to elucidate future directions and key issues are presented. Views of three organizations, with different missions and objectives, on needs and opportunities are first presented, followed by a summary of general audience discussion. Emphasis is placed on identifying the commonalities and differences in those needs and opportunities.

Role of Pore Size Distribution in Ultrasonic Characterization of Microporosity in Aluminum Castings

Microshrinkage porosity in cast aluminum plates was characterized utilizing the frequency dependence of ultrasonic attenuation caused by pore scattering. The back‐surface echo spectral shape was fitted assuming various size distributions of spherical pores. Best fits were obtained for a lognormal distribution. Pore volume fraction inferred from these fits overestimates the actual volume fraction by a factor related to the complicated, nonspherical pore shapes. These results permit accurate, nondestructive laboratory measurements of porosity with 2‐mm spatial resolution.