Centerline Cracks Research Articles

Vapor pressure and void size effects on failure of a constrained ductile film

To achieve certain properties, semiconductor adhesives and molding compounds are made by blending filler particles with polymer matrix. Moisture collects at filler particle/polymer matrix interfaces and within voids of the composite. At reflow temperatures, the moisture vaporizes. The rapidly expanding vapor creates high internal pressure on pre-existing voids and particle/matrix interfaces. The simultaneous action of thermal stresses and internal vapor pressure drives both pre-existing and newly nucleated voids to grow and coalesce causing material failure. Particularly susceptible are polymeric films and adhesives joining elastic substrates, e.g. Ag filled epoxy. Several competing failure mechanisms are studied including: near-tip void growth and coalescence with the crack; extensive void growth and formation of an extended damaged zone emanating from the crack; and rapid void growth at highly stressed sites at large distances ahead of the crack, leading to multiple damaged zones. This competition is driven by the interplay between stress elevation induced by constrained plastic flow and stress relaxation due to vapor pressure assisted void growth. A model problem of a ductile film bonded between two elastic substrates, with a centerline crack, is studied. The computational study employs a Gurson porous material model incorporating vapor pressure effects. The formation of multiple damaged zones is favored when the film contains small voids or dilute second-phase particle distribution. The presence of large voids or high vapor pressure favor the growth of a self-similar damage zone emanating from the crack. High vapor pressure accelerates film cracking that can cause device failures.

Considerations for the weldability of types 304L and 316L stainless steel

The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur. High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels (less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle control during fusion welding. A calculated Creq to Nieq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type 304L while the Creq/Nieq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.

Characterization of CO2 and diode laser welding of high carbon steels

Deep penetration welding with a high power CO2 and diode laser may offer an attractive means to join metal for certain applications, for instance: welding car drive shafts and gear plant. The rapid cooling rate of laser welding results in high hardness discontinuities across the welded joint; however, this leads to brittle weld and fatigue failure. To avoid this critical problem, it is useful to optimize the laser operating parameters in order to improve the mechanical properties of the weld. In this study, an experimental analysis was used to predict the significant effect of the weld quality using CO2 and high power diode laser (HPDL) laser welding, operating at 10.6 μm and 810 nm, respectively. Investigations into the weld quality were done to quantify the effect of different welding velocities by examining the hardness profiles, tensile strength, aspect, weld volume formation rate, and microstructure formation. In all cases, the results showed that for the HPDL weld configurations, cracking was observed in the fusion zone, whereas, for a CO2 laser weld, a greater weld strength and wider weld width were observed. For HPDL welding, center-line cracking was found along the fusion zone at higher welding velocities.

連鋳鋳片の中心割れの発生機構

連鋳鋳片の中心割れの発生機構

Microstructural features of cracking in autogenous beryllium weldments

An investigation was conducted to determine the causes of centerline cracking in autogenous Be weldments. In agreement with earlier studies, features on the fracture surfaces consistent with hot shortness cracking were observed. However, the present study reveals a more complicated solidification path than previously described. Analytical transmission electron microscopy (TEM) studies of the weld centerline revealed low-melting constituents consisting of a MBe12 grain boundary film and nodules containing elemental Al, elemental Si, and AlFeBe4 phases. Possible solidifcation pathways to produce this structure are discussed in terms of available binary and ternary phase diagrams.

Defect formation mechanisms in laser welding techniques for semiconductor laser packaging

In this work, the authors experimentally investigate defect formation mechanisms in spot-welding techniques for semiconductor laser packaging. Results obtained from the stainless-to-stainless steel joints indicate that the dimension of hole formation depends on the laser power density, and the hole disappears as the power density is below 3/spl times/10/sup 5/ W/cm/sup 2/. In the stainless-to-Kovar/sup TM/ joints, surface cracks are eliminated by reducing the gold thickness from the Kovar/sup TM/ before welding, while the centerline cracks in the fusion zone are eliminated by the air gap tightness between the Kovar/sup TM/ and stainless steel. The excess laser energy is the possible cause for hole formation. The low solubility of gold in the Kovar/sup TM/ and the large air gap between the joints are the possible causes for surface cracks and enhancing centerline crack propagation, respectively. A technique for reduction of post weld shift (PWS) in semiconductor laser packaging is also presented. Preliminary reliability data demonstrated that these laser packages, which do not have hole and crack defects in the welded joints, are reliable.

Computational analysis and prediction of weld-solidification cracking

A numerical analysis model is developed for weld-solidification cracking. The metal in the solid-liquid coexisting zone during weld solidification is modeled as a visco-elastoplastic substance. Its constitutive equations are established on basis of rheological mechanics. The transient stress and strain across the weld axis are analyzed for bead-on-plate welds of 2024 aluminium alloy. The analysis results have shown the characteristics of transverse stress and strain development during solidification and the influences of welding conditions and additional synchronous heating of the base metal on weld-solidification cracking. The numerical results agree well with the observed experimental phenomena. A method for prediction of weld centerline cracking is presented on the basis of the computed transient transverse strain at the weld center.

Cavity microstructure and kinetics during gas tungsten arc welding of helium-containing stainless steel

Helium was implanted in type 316 stainless steel, through tritium decay, to levels of 0. 18, 2. 5, 27, 105, and 256 atomic parts per million (appm). Bead-on-sheet welds were then made using the gas tungsten arc (GTA) process. Intergranular cracking occurred in the heat-affected zones (HAZs) of specimens with helium concentrations equal to or greater than 2.5 appm. No such cracking was observed in helium-free control specimens or in specimens containing the lowest helium concentration. In addition to the HAZ cracking, brittle, centerline cracking occurred in the fusion zone of specimens containing 105 and 256 appm helium. Transmission and scanning electron microscopy results indicated that both the HAZ cracking and centerline cracking in the fusion zone resulted from the stress-induced growth and coalescence of cavities initiated at helium bubbles on interfaces. For the HAZ case, the cavity growth rate is modeled and shown to predict the experimentally measured 1-second time lag between peak weld temperature and the onset of cracking.

A New Laser Hermetic Sealing Technique for Aluminum Package

A new laser hermetic sealing technique which combines nickel plating with laser welding has been developed for aluminum packages. Laser welding has proved to be the most promising hermetic sealing technique for the aluminum packages. However, laser welding of the package has been difficult because of aluminum's tendency to suffer centerline cracks in the weld zone. Nickel was added in the weld zone by laser welding nickel-plated aluminum to strengthen the weld metal and prevent centerline cracks. With this aim, the optimum plating method and plating thickness were investigated. Consequently, electroless Ni-1wt%B, 5 µm in thickness was found to be best suited to the new technique, because the laser weld zone of the package exhibits a high joint strength almost equal to the tensile strength of the parent metal, and a package leakage of less than 10-9atm · cm3/s. The reliability of the hermetically sealed package was tested in accordance with MIL-STD-202F. It was confirmed that the new technique is suitable for the aluminum package for use in space, in the stratosphere and in salt-laden air.

Determination of stress-intensity factors of fillet-welded T-joints by computer-assisted photoelasticity

Fillet-welded T-joints with crack-like discontinuities are analyzed using the photoelasticity method. Stress-intensity factors of opening modes are determined for undercut, overlap and center-line crack of fillet joints with respect to the various loading conditions and the weld dimensions. The photoelastic analysis uses a 20-point overdeterministic approach. The accuracy of fringe measurement is obtained by iteration of the measured results on a pattern-recognition software.

Observations of solidification cracking in Ti alloy weldments

Solidification in weldments normally results from an inability of the partially solidified fusion zone to withstand contractual stresses produced during cooling [1]. The susceptibility of a particular weldment to such hot cracking or hot tearing has been shown to be dependent on the interrelated metallurgical and thermomechanical characteristics of the weldzone [2, 3]. Observations of solidification and theories explaining the mechanisms involved have been documented for nearly every major alloy system. Studies performed by Savage and Lundin [4] have shown that titanium alloys are highly resistant to solidification cracking, particularly when compared to aluminum and nickel-base alloys. Nonetheless, studies at the Air Force Materials Laboratory have found that, under certain welding conditions, solidification in titantium alloys can occur. This paper briefly describes the morphological characteristics of solidification cracks in weldments on the metastable-/3 titanium alloy Ti-3AI-13V-l lCr and the ot-fl titanium alloy Ti-6AI-6V2Sn. The application of scanning electron microscopy techniques provided information concerning the microscopic characteristics of these weld defects not obtainable with conventional optical light microscopy. Figure l(a) illustrates a longitudinal centerline crack in an autogenous pulsed-current gas tungsten-arc weldment on Ti-3A1-13V11Cr. Cracking in each weld crater of the full-penetration weldment appeared to nucleate at or near the trailing edge of the weld pool from an existing crack in the previously solidified region. Fracture of this weldment when stressed transversely in three-point bending occurred at the longitudinal solidification crack after minimal deformation. Observation of the exposed surfaces showed little evidence of mechanical room-temperature fracture, indicating that solidification essentially separated the

The casting of steel

The solidification of metals will be considered as it relates to the casting process. Current theories and mechanisms will be examined with reference to their applicability to situations encountered in industry. The solidification process in static cast steel ingots and continuously cast steel will be considered in some detail, based on experimental observations. These observations were made from in-plant tests in which radioactive gold was added to the liquid pool during solidification. The results will be related to a mathematical model of the system based on a heat flow analysis. Using the model, predictions of middle and center line cracking of continuously cast billets have been made which conform with observed cracking.