Friction stir welding, Whisker growth, and ductile fracture

December 31, 2010

Back-of-the-envelope calculations in friction stir welding – Velocities, peak temperature, torque, and hardness

A Arora et al

Given the complexity and resource requirements of numerical models of friction stir welding (FSW), well-tested analytical models of materials flow, peak temperatures, torque, and weld properties are needed. Here an approximate analytical technique for the calculation of three-dimensional material flow during FSW is proposed considering the motion of an incompressible fluid induced by a solid rotating disk. The accuracy of the calculations is examined for the welding of three alloys. For the estimation of peak temperatures, the accuracy of an existing dimensionless correlation is improved using a large volume of recently published data. The improved correlation is tested against experimental data for three aluminum alloys. It is shown that the torque can be calculated analytically from the yield stress using estimated peak temperatures. An approximate relation between the hardness of the thermomechanically affected zone and the chemical composition of the aluminum alloys is proposed.

Interface flow mechanism for tin whisker growth

H P Howard et al

Tin coatings, widely used in electronics, are susceptible to the spontaneous eruption of fine metal filaments or “whiskers”. Tin whiskers are a serious reliability issue in microelectronics, as they can cause short circuits and device failure. While it is generally accepted that whiskers grow to relieve compressive stresses, the specific mechanism for whisker formation is yet unknown. Data are presented to support an interface-transport mechanism for whisker nucleation and growth. This mechanism, involving the formation of a viscous layer at the interface between substrate and coating, could explain the extremely rapid growth of whiskers that has been observed experimentally.

In situ observation of ductile fracture using X-ray tomography technique

H Toda et al

Fast microtomography combined with local crack driving force analysis has been employed to analyze crack-tip stress/strain singularities in an aluminum alloy. The application of fast microtomography has made it possible to observe real crack initiation and propagation behaviors without intermediate unloading. The details of a crack and its local propagation behaviors are readily observed with this technique along with evidence of microstructure/crack interactions. After a preliminary investigation of the achieved spatial resolution, we show that conventional stationary and growing crack singularities can be quantitatively validated by deriving the local crack opening displacement. This is to our knowledge the first three-dimensional validation of conventional fracture mechanics during a real time continuous experiment that has been mainly developed via surface observations so far. We also reveal that there is a spatial transition from a stationary crack singularity to a growing crack singularity in addition to the well-known temporal transition that occurs with the onset of crack propagation. Local crack propagation behaviors are also discussed on the basis of this validation. To separate the effects of complex crack geometry from those of microstructure, we also perform an image-based numerical simulation.

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