The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m² were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m². Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds. Formerly Graduate Student.     

Peak load and energy absorption of DP600 advanced steel resistance spot welds

The paper aims at investigating the microstructure, failure mode transition, peak load and energy absorption of DP600 dual phase steel during the tensile-shear test. It was found that the welding current has profound effect on the load–displacement characteristics. In the low welding current, welds failed in interfacial failure mode. Increasing welding current resulted in sufficient weld nugget growth to promote double-sided pullout failure mode with improved mechanical properties. Further increase in the welding current caused expulsion and failure mode was changed to single-sided pullout with reduced energy absorption capability. It was found that the fusion zone size is the key parameter controlling the mechanical properties of DP600 resistance spot welds in terms of peak load, maximum displacement and failure energy.     

Failure of resistance spot welds: tensile shear versus coach peel loading conditions

Abstract

This paper aims at investigating the failure behaviour of resistance spot welds under tensile shear (TS) and coach peel (CP) loading conditions. A failure mechanism was proposed to describe both interfacial and pullout failure modes in each loading condition. The mechanisms were confirmed by SEM investigations, examining the cross-sections of the fractured welds to detail the fracture path. The experimental results showed that in the pullout failure mode during TS testing, necking is initiated at the nugget circumference in the base metal, and then failure propagates along the nugget circumference in the sheet, leading to the final fracture, while pullout failure during the CP test occurred by crack initiation and propagation near the weld nugget/heat affected zone boundary. The interfacial failure to pullout failure mode transition in the TS and CP tests was also studied. The critical weld nugget size required to ensure the pullout failure mode was obtained for each loading condition. The critical fusion zone size to ensure pullout failure mode during the TS test was larger than that of the CP test. It was found that the load bearing capacity of the spot welds under CP is significantly lower than that of the TS test.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part II. AA 1100 aluminum and comparison to AISI 409 stainless steel

In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties. The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were produced in aluminum at power densities ranging from 3.2 to 10 GW/m². The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction- to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m², compared to about 4 GW/m² for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding. Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown to completely characterize weld pool development in conduction-mode welds made in both materials.     

The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation

A digital image correlation (DIC) method has been used to characterize the constitutive tensile stress-strain response in 304L austenitic stainless steel weldments produced by both continuous-wave (CW) and pulsed-wave (PW) laser welding. The method provides quantitative two-dimensional (2-D) strain maps of the deformation field across the transverse weld samples throughout the tensile test. Local stress-strain response was extracted from regions within the fusion zone and compared to base metal response. The weldments were found to have a higher yield strength than the base metal. The metallurgical origin for the fusion zone strengthening was largely attributed to Hall-Petch and ferrite content effects. While failures localized in the fusion zone with little appreciable necking, the material within the fusion zone retained considerable local ductility: more than 45 pct strain at failure. Significant weld root porosity found in the PW condition and absent in the CW condition appeared to have no deleterious effect on the mechanical performance under the present test conditions in this very ductile, flaw-tolerant alloy.     

Simulation of time-dependent pool shape during laser spot welding: Transient effects

The shape and depth of the area molten during a welding process is of immense technical importance. This study investigates how the melt pool shape during laser welding is influenced by Marangoni convection and tries to establish general qualitative rules of melt pool dynamics. A parameter study shows how different welding powers lead to extremely different pool shapes. Special attention is paid to transient effects that occur during the melting process as well as after switching off the laser source. It is shown that the final pool shape can depend strongly on the welding duration. The authors use an axisymmetric two-dimensional (2-D) control-volume-method (CVM) code based on the volume-averaged two-phase model of alloy solidification by Ni and Beckermann^[1] and the SIMPLER algorithm by Patankar.^[2] They calculate the transient distribution of temperatures, phase fractions, flow velocities, pressures, and concentrations of alloying elements in the melt and two solid phases (peritectic solidification) for a stationary laser welding process. Marangoni flow is described using a semiempirical model for the temperature-dependent surface tension gradient. The software was parallelized using the shared memory standard OpenMP.     

Influence of welding parameters on peak load and energy absorption of dissimilar resistance spot welds of DP600 and AISI 1008 steels

Abstract

This paper addresses the mechanical performance of dissimilar resistance spot welds between DP600 and AISI 1008 low carbon steels. The weldability lobe was established and proper welding conditions to produce welds with sufficient size and without expulsion were determined. Correlations among the process parameters (welding current and welding time), physical spot weld attributes (fusion zone size and electrode indentation depth) and mechanical performance (peak load and energy absorption) were analysed. It was shown that the increases in welding current and welding time result in increases in fusion zone size and electrode indentation depth. In the low heat input welding condition, welds failed in interfacial failure mode. Increasing welding heat input results in sufficient weld size and promotes pullout failure mode with improved mechanical properties in terms of peak load and failure energy. However, a further increase in heat input caused metal expulsion and the failure mode was changed to partial pullout–partial thickness mode with reduced energy absorption capability.

Cet article discute du rendement mécanique de soudures dissimilaires par points entre les alliages à faible carbone DP600 et AISI 1008. On a établi le lobe de soudabilité et l’on a déterminé les conditions appropriées de soudage afin de produire des soudures d’une taille suffisante et sans expulsion. On analyse la corrélation entre les paramètres du processus (courant de soudage et temps de soudage), les attributs physiques de la soudure par points (taille de la zone de fusion et profondeur d’indentation de l’électrode) et le rendement mécanique (charge maximale et absorption d’énergie). On a montré qu’une augmentation du courant de soudage et du temps de soudage avait pour résultat une augmentation de la taille de la zone de fusion et de la profondeur d’indentation de l’électrode. Sous condition de soudage à faible apport de chaleur, les soudures défaillaient en mode de défaillance à l’interface. L’augmentation de l’apport de chaleur de soudage avait pour résultat une taille de soudure suffisante et favorisait le mode de défaillance par arrachement avec des propriétés mécaniques améliorées quant à la charge maximale et à l’énergie de défaillance. Cependant, une augmentation additionnelle de l’apport de chaleur résultait en l’expulsion du métal et le mode de défaillance se changeait en un mode d’arrachement partiel-épaisseur partielle avec capacité réduite d’absorption d’énergie.     

Photorefractive keratectomy for hyperopia and aphakia with a scanning spot excimer laser

OBJECTIVE: To study the safety, efficacy, predictability, and stability of photorefractive keratectomy (PRK) for hyperopia and aphakia. METHODS: Fifteen eyes of 15 patients (mean age, 33 +/- 5.95 yrs) were enrolled in the study and divided into three groups. The first group was comprised of six eyes that had hyperopia ranging from +1.75 to +4.75 D; the second group had seven hyperopic eyes ranging from +5.00 to +9.75 D; the third group included two eyes of two aphakic patients. All eyes had PRK with a 193 nm argon fluoride excimer laser (Chiron-Technolas, Keracor 116) with a 10 Hz repetition rate and a fluence of 120 mJ/cm2. The total follow-up time in all eyes was 12 months. RESULTS: In the lower hyperopia group, 0% eyes were within +/- 0.50 D and 66% (N = 4) of eyes were within +/- 1.00 D of emmetropia with the other two eyes between +1.00 and +2.00 D at 1 year after PRK. In the higher hyperopia group, all eyes had at least +3.00 D of hyperopia at 1 year. In the aphakic group, both eyes achieved less than 50% of the target correction of +10.00 D at 1 year. Final uncorrected visual acuity ranged from 20/20 to 20/30 in the lower hyperopia group, 20/30 to 20/50 in the higher hyperopia group, and count fingers in the aphakic group. CONCLUSIONS: PRK is a relatively safe, stable, and effective procedure with reasonably good predictability for eyes with less than +5.00 D of baseline hyperopia, and poor predictability for eyes with more than +5.00 D of baseline hyperopia. PRK is ineffective in the correction of aphakia.     

Texture characterization of autogenous Nd:YAG laser welds in AA5182-O and AA6111-T4 aluminum alloys

This article reports a study of texture characterization in Nd:YAG laser welds of AA5182-O and AA6111-T4 alloys. Electron backscattering diffraction (EBSD) in the scanning electron microscope was used to determine the texture. The determination was made as a function of thickness through the sample. The results show that the welds can develop significant texture. In particular, the columnar grains that grow from the base metal into the weld have a strong 001 texture along the direction of growth. The work at Brown University was supported by GM through the GM Collaborative Research Lab at Brown University.     

Computer modeling of heat flow in welds

This paper summarizes progress in the development of methods, models, and software for analyzing or simulating the flow of heat in welds as realistically and accurately as possible. First the fundamental equations for heat transfer are presented and then a formulation for a nonlinear transient finite element analysis (FEA) to solve them is described. Next the magnetohydrodynamics of the arc and the fluid mechanics of the weld pool are approximated by a flux or power density distribution selected to predict the temperature field as accurately as possible. To assess the accuracy of a model, the computed and experimentally determined fusion zone boundaries are compared. For arc welds, accurate results are obtained with a power density distribution in which surfaces of constant power density are ellipsoids and on radial lines the power density obeys a Gaussian distribution. Three dimensional, in-plane and cross-sectional kinematic models for heat flow are defined. Guidelines for spatial and time discretization are discussed. The FEA computed and experimentally measured temperature field,T(x, y, z, t), for several welding situations is used to demonstrate the effect of temperature dependent thermal properties, radiation, convection, and the distribution of energy in the arc.     

Ductility of stabilized ferritic stainless steel welds

An investigation was made into the mechanism of ductility loss in low interstitial 18 Cr-2Mo ferritic stainless steel welds stabilized with Ti and Nb. It was found that stabilizing TiN or Nb(C,N) precipitates are dissolved during the welding process, resulting in a finer distribution of precipitates in the weld metal than in the base metal. Furthermore, the FATT was found to increase by more than 200°C, leading to decreased room temperature ductility. Such an increase in FATT may not be explained solely in terms of grain growth. Internal friction measurements indicate that no free nitrogen is present in the weld metal, yet wet chemical analysis reveals that the nitrogen is present in a soluble form. Kinetic arguments suggest that the stabilized nitrogen dissolved during welding tends to reprecipitate during solidification in the form of a chromium rich nitride phase.     

Geometry of shadows

Shadows provide a strong source of information about the shapes of surfaces. We analyze the local geometric structure of shadow contours on piecewise smooth surfaces. Particular attention is paid to intrinsic shadows on a surface: that is, shadows created on a surface by the surface's own shape and placement relative to a light source. Intrinsic shadow contours provide useful information about the direction of the light source and the qualitative shape of the underlying surface. We analyze the invariants relating surface shape and light-source direction to the shapes and singularities of intrinsic shadow contours. The results suggest that intrinsic shadows can be used to directly infer illuminant tilt, qualitative global surface structure, and, at intersections with surface creases, the concavity/convexity of a surface. We show that the results obtained for point sources of light generalize in a straightforward way to extended light sources, under the assumption that light sources are convex.     

Microstructure of stainless steel single-crystal electron beam welds

Previously developed techniques by the authors for the microstructural analysis of welds, that included the effects of both the growth crystallography and the weld pool shape, are applied to several cases involving the single-crystal electron beam welding of an Fe-15Ni-15Cr alloy. This evaluation of weld microstructures and associated dendritic growth patterns is based on a three-dimensional (3-D) geometrical analysis. The present study includes examination of the effects observed in overlapping, multipass autogenous welds and butt welds of two single crystals with different orientations, as well as effects due to variations in the welding speed. Weld pool shapes were found to change significantly with increasing welding speed—becoming narrower in cross section but more elongated in the welding direction. Additionally, all electron beam welds showed evidence of a plateau region in the center of the weld pool. The pool shapes, however, were found to be independent of the crystallographic orientation. Therefore, it is possible to extend the pool shape results to crystals welded in any orientation and even to polycrystals. The over-lapping multipass welds showed remarkable reproducibility from pass to pass and duplicated the structural patterns found in single-pass welds. The similarity in dendritic patterns within each pass indicated that the weld pool shapes were identical in all of the passes. The micro-structure of butt welds of two single crystals with different relative orientations showed a remarkable relationship to that associated with each individual crystallographic orientation, and the micro structure was, in effect, simply a composite of two single-pass microstructures. Additional microstructural details were also examined. The tendency toward branching of dendrites was associated with the transition from one dendrite growth orientation to another. It was also found that the nonpenetrating welds exhibited a small protrusion at the bottom of the weld. It is suggested that the modeling of weld pool shapes can be directly evaluated by comparing the predicted dendritic growth patterns based on the modeled shapes with the actual experimentally observed dendritic growth patterns. Formerly Visiting Scientist, Solid State Division, Oak Ridge National Laboratory.     

Analysis of solidification microstructures in Fe-Ni-Cr single-crystal welds

A geometric analysis technique for the evaluation of the microstructures in autogenous single-crystal electron beam welds has been previously developed. In the present work, these analytical methods are further extended, and a general procedure for predicting the solidification microstructure of single-crystal welds with any arbitrary orientation is established. Examples of this general analysis are given for several welding orientations. It is shown that a nonsymmetric cell structure is expected in transverse micrographs for most welding geometries. The development of steady-state conditions in the weld pool is also examined in terms of the weld pool size, its shape (as revealed by the dendritic growth pattern), and the size of the dendritic cells. It is found that steady state is established within a few millimeters of the beginning of the weld. Furthermore, steady state is achieved faster in welds made at higher welding speeds. A general analysis of the three-dimensional (3-D) weld pool shape based on the dendritic structure as revealed in the two-dimensional (2-D) transverse micrographs is also developed. It is shown that in combination with information on the preferred growth direction as a function of the solidification front orientation, the entire dendritic growth pattern in single-crystal welds can be predicted. A comparison with the actual weld micrographs shows a reasonable agreement between the theory and experiment. Finally, the theoretical analysis of the dendrite tip radius is extended from binary systems to include the case of ternary systems. The theoretical dendrite trunk spacing in a ternary Fe-Ni-Cr alloy is calculated from the dendrite tip radius and is compared with the experimental values for several weld conditions. Good agreement between experiment and theory is found. Formerly Visiting Scientist, Solid State Division, Oak Ridge National Laboratory.     

Geometry of grain boundaries

The first part of this paper deals with the problem of describing and measuring microstructure in exact terms. The Euclidean parameters: volume, area, length, and angle can be measured and expressed only in terms of the total of each in unit volume. Average properties, such as average grain diameter, are accessible only through the topological parameters, specifically number in unit volume, which can be measured only by serial sectioning. The parameters which have been used to represent the concept of grain size are analyzed and shown in most cases to represent a function of grain boundary area. In a second section of this paper the geometric problem of plastic slip through a grain boundary is analyzed. A method is proposed by which all of the components of the deformation, as well as the crystallographic directions, can be manipulated simultaneously through the use of a stereographic projection. The third section of this paper is concerned with the geometry of grain growth. The polycrystalline state is described as a grain boundary network, which must respond to the requirements of surface tension. The several topological changes in a network which can contribute to grain growth are described.     

Karyotype evolution of marsupials: from higher to lower diploid numbers

A basic 2n = 14 ancestral marsupial karyotype giving rise to higher diploid numbers through chromosome fissions has been widely accepted for the last three decades. Our finding of interstitial telomeres in two South American species, one with the 2n = 14 "ancestral karyotype" and the other with 2n = 18, indicates that these complements evolved from a karyotype with a higher diploid number. A new scenario for the karyotype evolution in the group is put forward. In this scenario an ancestral karyotype with at least 22 chromosomes would have originated the basic karyotype with 2n = 14 before the radiation of marsupials.