Publications

Abstract not provided.

The relationship between beam focus position and penetration depth in CW laser welding was studied numerically and experimentally for different welding conditions. Calculations were performed using a transient hydrodynamic model that incorporates the effect of evaporation recoil pressure and the associated melt expulsion. The simulation results are compared with measurements made on a series of test welds obtained using a 1650 W CO2 laser. The simulations predict, and the experiments confirm, that maximum penetration occurs with a specific location of the beam focus, with respect to the original sample surface, and that this relationship depends on the processing conditions. In particular, beam absorption in the plasma has a significant effect on the relationship between penetration and focus position. When the process parameters result in strong beam absorption in the keyhole plasma, the maximum penetration will occur when the laser focus is at or above the sample surface. In a case of weak absorption however, the penetration depth reaches its maximum value when the beam focus is located below the sample surface. In all cases, the numerical results are in good agreement with the experimental measurements.

A novel approach to simulating the dominant dynamic processes present during concentrated energy beam welding of metals is presented. A model for transient behavior of the front keyhole wall is developed. It is assumed that keyhole propagation is dominated by evaporation recoil-driven melt expulsion from the beam interaction zone. Results from the model show keyhole instabilities consistent with experimental observations of metal welding, metal cutting and ice welding.

Significant progress has occurred lately regarding the classification, characterization, and formation of white spots during vacuum arc remelting (VAR). White spots have been generally split into three categories: discrete white spots, which are believed to be associated with undissolved material which has fallen in from the shelf, crown, or torus regions; dendritic white spots, usually associated with dendrite clusters having fallen from the electrode; and solidification white spots, believed to be caused by local perturbations in the solidifications conditions. Characteristics and proposed formation mechanisms of white spots are reviewed and discussed in context of physical processes occurring during VAR, such as fluid flow and arc behavior. Where possible, their formation mechanisms will be considered with respect to specific operating parameters. In order to more fully understand the formation of solidification white spots, an experimental program has been begun to characterize the solidification stability of Alloy 718 and variants with respect to changes in growth rate and thermal environment. A description of the experimental program and preliminary results are included.

It is well known that two phase titanium alloy systems suffer from an abrupt drop in ductility at elevated temperatures in the range of 1,000 to 1,150 K. This loss of ductility is manifested by easy decohesion of polycrystalline aggregates along the grain boundaries of the high temperature beta phase. If the alloy is in a state of tensile stress at the aforementioned temperatures, cracks initiate at the grain boundaries and propagate readily through the alloy, leading to premature failure. This phenomenon is a cause of major concern in titanium alloy fabrication and welding. Several mechanisms have been proposed to explain high temperature crack nucleation and growth along the boundaries. A critical review of the phenomenon and possible mechanisms responsible for the observed behavior will be discussed.

Ten MC4073/4369 programmer base plates were analyzed. This component, a programmer base plate for the SRAM II (and later the SRAM A), is specified as a Grade C quality casting made of aluminum Alloy A356, heat treated to the T6 condition. A concern was expressed regarding the choice of an A356 casting for this application, given the complexity and severity of the loading environment. Preliminary tests and analyses suggested that the design was adequate, but noted the uncertainty involved in a number of their underlying assumptions. The uncertainty was compounded by the discovery that the casting used in the original series of mechanical tests failed. In this investigation, several production castings were examined and found to be of a quality superior to that required under current specifications. Their defect content and microstructure were studied and compared with published data to establish a mechanical property data base. The data base was supplemented with a series of X-direction static tests, which characterized the loading environment and measured the overall casting performance. It was found that the mechanical properties of the supplied castings were adequate for the anticipated X-direction loading environment, but the component is not over-designed. The established data base further indicates that a reduction in casting quality to the allowable level could result in failure of the component. Recommendations were made including (1) change the component specification to require higher casting quality in highly stressed areas, (2) supplement the inspection procedures to ensure adequate quality in critical regions, (3) alter the component design to reduce the stress levels in the mounting feet, (4) substitute a modified A356 alloy to improve the mechanical properties and their consistency, and (5) more thoroughly establish a data base for the mechanical property consequences of levels and configurations of casting defects.

Melt pool shape in VAR is controlled by fluid flow, which is governed by the balance between two opposing flow fields. At low melt currents, flow is dominated by thermal buoyancy. In these instances, metal is swept radially outward on the pool surface, resulting in relatively shallow melt pools but increased heat transfer to the crucible at the melt pool surface. At high melt currents, flow is primarily driven by magento-hydrodynamic forces. In these cases, the surface flow is radially inward and downward, resulting in a constricted arc, the pool depth and relative heat transfer to the crucible are intermediate, even though the melt rate is significantly lower than either diffuse arc condition. Constricted arc conditions also result in erratic heat transfer behavior and non-uniformities in pool shape.