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The article presents a study on the use of Photon Doppler Velocimetry (PVD) in the analysis of electromagnetic metal forming. The study shows how PVD works with a dynamic ring expansion test experiment to the measure the tensile stress-strain relations at high strain rate as well as examine the compaction behavior of powdered materials. It suggests that PVD provide improved estimates of material constitutive behavior as well as experimentally measured data to evaluate numerical models.

Creep transients are commonly described either by a change in the isotropic strength of a material due to an increase in dislocation density or by a change in directional hardening, often described by a backstress. Here we look at transients as developed by load redistribution among regions of material with differing local strength in finite element simulations. No strain hardening or backstress exists in our material description. Two types of material models are used. The first contains grains of binary strength; the other uses grains with a continuous distribution of strengths. Both 2D and 3D analyses were performed. It was found that the load redistribution process is an important source of the transients in both creep and tensile testing. An equivalent stress quantity to track the strength of a sample during deformation is proposed. Using this quantity, a single load-shedding curve can be obtained that describes both the tensile and creep tests. This unifies the material constitutive behavior for the very different boundary conditions seen in creep and fixed-rate testing. Lastly, it is shown that from this general model we can also develop realistic anelasticity and Bauschinger transients. [ABSTRACT FROM AUTHOR]

Vaporizing foil actuator (VFA) spot welding, a type of spot impact welding, was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS). The interfacial microstructures and fracture surfaces were characterized using scanning electron microscopy (SEM). Lap shear tests that strained the samples to failure with digital imaging correlation (DIC) were conducted to study the mechanical performance of these welds. A mesh-insensitive structural stress method was used to understand the stress distribution and model the failure modes of VFA welds in ABAQUS. Despite experimental scatter in this developing joining method, most samples failed through the base metal, but multiple failure modes coexisted, including interface failure. These failure modes were used to classify the results. The failure process can be best understood through the lens of the spatial variation that is natural in this type of weld. The center is naturally unwelded, and there is an annulus of high strength material surrounding this unwelded zone that has a wavy morphology. The mesh-insensitive structural stress method could naturally provide a link between the joint structure and the mechanical properties of the spot impact welds. This could show that despite varied failure modes and nugget strength, strength itself is not usually affected adversely by the size of the central unbonded zone. [ABSTRACT FROM AUTHOR]

15-5 PH stainless steel (SS) possesses both high strength and good corrosion resistance and is often used in aerospace components. However, age-hardened 15-5 PH SS can lose up to 20% of its strength during fusion welding due to overaging brought on by heat effect. This work addresses the problem of strength reduction observed in 15-5 PH SS by employing the low-heat welding technique of impact welding. Both wrought (W) and additively manufactured (AM) versions of 15–5 PH SS were investigated through experimentation. All workpieces were age-hardened to condition H900 prior to welding. Best weld microstructures and peel strengths were obtained at impact angles of 12°, for both W and AM variants. Peak peel strength for W welds exceeded 178 N/mm, and for AM welds, 141 N/mm. Failure through base material, which was a most desirable failure mode, was obtained for both material types, though further work will be needed to obtain this failure mode repeatably. Thus, the capability of obtaining strong welds with age-hardened 15-5 PH SS was demonstrated. This capability can help eliminate the need for post-weld heat treatment, thus saving significant time and cost and opening up new design possibilities. [ABSTRACT FROM AUTHOR]

This work explores high-speed impact of fully age-hardened 15-5 PH stainless steel with itself at average impact speed of 682 m/s and impact angles ranging from 8° to 28°. It was assumed that higher angles corresponded with higher shear stresses at impact point. Microstructural examination indicated more pronounced shear-induced features at higher angles, such as nanoscale elongated grains and adiabatic shear bands. Interfacial waves, a feature associated with impact welding, were most prominent at 12° and 16°. White etching layers (WELs), a constituent formed by rapid heating and cooling at impact, were found along the impact interface and most prominently at the front and back vortices of the interfacial waves. WELs (along the interface) were softer than base materials due to overaging, whereas materials surrounding the interface were harder than base materials due to grain refinement. Numerical simulations based on smoothed particle hydrodynamics method were employed to predict the local temperature and strain distributions which supported microstructural findings. Thus, through high-speed impact experiments carried out at different angles along with advanced microscopy and numerical simulations, this work helped reveal the microstructural response of 15-5 PH stainless steel in a range of dynamic shear conditions. [ABSTRACT FROM AUTHOR]

In this work, the mechanical properties of joints made with a ridged tack only, one-part heat-curing epoxy only, and a combination of the two were evaluated. All samples were constructed using 1.6 mm thick 5052 aluminum. Testing was performed in lap shear, cross tension, and at 45°. A set of tests was also performed to investigate the mechanical strength of joints in lap shear under various surface preparation methods. It was found that the peak failure load was governed solely by the adhesive in hybrid joints. The performance gain from the inclusion of the ridged tack was greater energy absorption. The absorbed energy for a hybrid joint in lap shear configuration is closely approximated by the sum of energy absorbed with adhesive only and ridged tack only. The effect was much less pronounced for samples in cross tension and at 45°. Additionally, the adhesive strength was found to be completely unaffected by the surface preparation of the samples. [ABSTRACT FROM AUTHOR]

Photonic Doppler velocimetry was applied to compare magnetic pulse welding and vaporizing foil actuator welding against each other in the form of lap joints made of 5000 series aluminum alloy sheets under identical experimental conditions which are: charging energies of the pulse generator, specimen geometry, initial distances between flyer and target plate. Impact velocities resulting from rapidly vaporizing aluminum foils were up to three times higher than those of purely electromagnetically accelerated flyer plates. No magnetic pulse welds were achieved, while every vaporizing foil experiment yielded a strong weld in that failure always occurred in the joining partners instead of in the weld seam during tensile tests. An analytical model to calculate the transient flyer velocity is presented and compared to the measurements. The average deviation between model and experiment is about 11% with regard to the impact velocity. Hence, the model may be used for the process design of collision welds generated by vaporizing foil actuators. [ABSTRACT FROM AUTHOR]

The objective of this study was to create impact welds between a Zr-based Bulk Metallic Glass (BMG) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications: impact welding of dissimilar materials being one of them. Its implementation for welding is termed as VFA Welding or VFAW. With 8 kJ input energy into an aluminum foil actuator, a 0.5 mm thick Cu110 alloy sheet was launched toward a BMG target resulting in an impact at a velocity of nearly 600 m/s. For this experiment, the welded interface was straight with a few BMG fragments embedded in the copper sheet in some regions. Hardness tests across the interface showed increase in strength on the copper side. Instrumented peel test resulted in failure in the parent copper sheet. A slower impact velocity during a separate experiment resulted in a weld, which had wavy regions along the interface and in peel failure again happened in the parent copper sheet. Some through-thickness cracks were observed in the BMG plate and there was some spall damage in the copper flyers. TEM electron diffraction on a sample, cut out from the wavy weld interface region using a focused ion beam, showed that devitrification of the BMG was completely avoided in this welding process. [ABSTRACT FROM AUTHOR]

Brittle intermetallics are easily formed during welding of dissimilar metals in many alloy system such as Al-Ti, Al-Fe, and Ti-Ni, usually resulting in poor mechanical performance. In this study, laser impact welding (LIW) was applied for joining of Ni-Ni, Al-Cu, Al-Ni and Al-Ti similar and dissimilar metal combinations. A series of experiments were conducted using a 3-Joule pulsed laser to study the influence of LIW parameters on the launch of the flyer component as well as the bonding between the flyer and target. Metallurgical bonds between Ni-Ni, Al-Cu, Al-Ni and Al-Ti combinations were successfully produced. A characteristic wavy interface was observed in all LIW samples. The impact angle plays a key role in the bonding between the flyer and target. The bonding area was significantly increased (up to 30 % of the impact region) when an appropriate impact angle was applied (nominally 7.5 degrees). In addition, Photon Doppler Velocimetry (PDV) was used to measure the impact velocity of the flyer during the LIW process. The full velocity of the flyer (over 400 m/sec) can be achieved within 1.1 μs. The impact velocity of the flyer was maintained at almost the same velocity even at relatively large standoff distance (32.5 mm). However, severe deformation of the flyer occurred when the displacement exceeded 3.0 mm. [ABSTRACT FROM AUTHOR]