Hwa Teng Lee

Influence of interfacial intermetallic compound on fracture behavior of solder joints

Abstract This paper studies the solder joints of an Sn–Ag lead-free solder system with pure copper wires. The study focuses upon the interrelationships, which exist between the adhesive strength of the joint, its shear strength, the formation of interfacial intermetallic compounds (IMC) and the fractographic morphology. Additionally, the paper determines how these characteristics, and the relationships between them, are influenced by the storage duration and the storage temperature. Experimental results show that both the adhesive strength and the shear strength of the solder joints decrease significantly following short-term thermal storage. As the storage time is increased, it is noted that both the thickness and the roughness of the interfacial IMC layers increase. Regarding the fracture of the solder joints, fractographic observation reveals that fracture morphology under adhesive loading are similar to those observed under shear loading conditions. In the as-soldered condition, the fracture surface appears to be flat, and some broken Cu6Sn5 and residual solder pieces are evident. When the total thickness of the IMC layer lies within the range 1–10 μm, it is observed that the fracture morphology gradually becomes a dimple-like structure. This phenomenon may be attributed to the residual stresses caused by phase transformation, and by the increasing roughness of the IMC layers which causes an increase in the stress concentration within the Cu6Sn5 layer, and which ultimately results in fracturing of this layer. When the total thickness of the interfacial IMC layers exceeds 10 μm, the roughness of the IMC layers and the residual stress between them and the solder both continue to increase. Eventually this results in a fracture being initiated and propagated within the Cu6Sn5 layer. Fractographic observation shows the fracture to have a cleavage-like morphology.

Influence of intermetallic compounds on the adhesive strength of solder joints

Abstract This study investigates the influence of intermetallic compound (IMC) growth on the adhesive strength of solder joints. 100Sn and 60Sn40Pb solders were used to solder electrolytic copper wires end to end. Soldered samples were then subjected to high temperature storage testing. The resulting interfacial IMCs in both solders were composed of Cu 6 Sn 5 and Cu 3 Sn. IMC in the 100Sn solder joint grew faster than in the 60Sn40Pb. The growth behavior of Cu 6 Sn 5 and Cu 3 Sn were subject to lead concentration. Results of tensile testing revealed an adhesive strength for 60Sn40Pb of 77.3 MPa, higher than the 50.0 MPa for 100Sn. 60Sn40Pb was found more sensitive to high temperature storage than 100Sn.

Effects of filler metal composition on joining properties of alloy 690 weldments

Abstract This work investigates the influence of filler metal composition on the corrosion resistance and mechanical properties of alloy 690 weldments. Alloy 690 (61 wt.% Ni, 30 wt.% Cr) was used as the base metal. Inconel I-52 (61 wt.% Ni, 29 wt.% Cr) and I-82 (73 wt.% Ni, 20 wt.% Cr) rods were used as filler metals. Manual gas tungsten arc welding was performed using four weld passes in three layers for a single-V groove butt weld. The subgrain structure near the centerline of the fusion zone middle layer was cellular and columnar dendritic in the I-52 weld, but was dominantly equiaxed dendritic in the I-82 weld. Both had white particles dispersed in the fusion zone. The I-82 weld had more white particles and a denser subgrain structure. Compositional analysis showed the I-52 welds interdendritic region had higher Al, Si, Ti, N content than the dendritic core. The I-82 welds interdendritic region had higher Al, Si, and Nb content than the dendritic core. The Ni and Cr content of interdendritic white particles of the I-52 and I-82 welds decreased to 38, 23 wt.% and 11, 9 wt.%, respectively, much lower than the base and filler metals. This situation causes high corrosion at the white particle sites during Modified Huey testing. Thus, the I-52 weld had better corrosive resistance than the I-82 weld. However, I-82s superior weld joint strength is attributed to its finer fusion zones subgrain structure and giving it higher tensile strength and elongation.

Characteristics of dissimilar welding of alloy 690 to 304L stainless steel

Abstract The object of the present work is to research the dissimilar welding of nickel based 690 alloy and SUS 304L stainless steel using two alternative Inconel filler metals, namely, 82 (I–82) and 52 (I–52). Gas tungsten arc welding with identical parameters and procedures was used to carry out single V groove butt welding with six passes in four layers on nickel based alloy 690 and 304L stainless steel. Mechanical and corrosion resistance tests were performed. Metallographical, fractographical, and compositional analysis were used to study filler metal dissimilarities. Mechanical tests show that the I–82 weldment, owing to its denser dendrites and formation of abundant niobium enriched precipitates, has a higher strength and hardness than the I–52 weldment. Rupture occurred in the alloy 690 base metal. In comparison, the I–52 weldment, with coarse dendrites and no niobium enriched precipitates, exhibits superior corrosion resistance to the I–82 weldment but has a lower tensile strength and ruptured in the fusion zone. Microstructural investigation reveals that I–52 is a mixture of cellular dendrites and columnar dendrites whereas I–82 is mainly columnar dendrites with niobium enriched precipitates.

The thermomechanical behavior for aluminum alloy under uniaxial tensile loading

Abstract This paper studies temperature variation phenomenon induced by the thermomechanical effect in aluminum alloys 2024-T3 and 7075-T6 under uniaxial tensile loading. Experimental results show that cooling occurs under elastic deformation. When load yielding is attained the temperature approaches its minimum. Beyond the temperature minimum the slope of the temperature curve inverts and becomes positive as heating occurs under plastic deformation. The point of temperature curve inversion coincides with the point at which the sample’s elastic behavior yields to stress and becomes plastic. At strain rates of 2×10 −2 s −1 –2×10 −4 s −1 , the amount of temperature drop was found to be reasonably close to the theoretical values, as determined by the standard thermoelastic model. Likewise, within this strain range, the relative difference between the stress value at lowest temperature and the stress value at the yielding point was minimum. Experiments demonstrate that the temperature method of defining yield strength is a valid technique and, moreover, for the aluminum alloys tested, superior to the tensile method, at least within the strain rate range of 2×10 −2 s −1 –2×10 −4 s −1 . The curve of experimental estimated thermoelastic factor, K m , showed two different regions when plotted in relation to strain rate. The two regions had inverse slopes and dramatically different curve characteristics. The regions met at a strain rate of about 10 −3 s −1 , which was also the point of closest approach to the theoretical values.

Analysis of microstructure and mechanical properties in alloy 690 weldments using filler metals I-82 and I-52

AbstractThis work investigated the weldability and mechanical properties of weldments made with Inconel filler metals I-52 and I-82 in the welding of Inconel alloy 690. Gas tungsten arc welding was used with different multipass sequences. The microstructures of the fusion and heat affected zones were examined and weldment properties were compared by tensile, hardness, and impact tests. Fracture surfaces were examined by scanning electron microscopy. Experimental results indicate that the subgrain structure near the fusion zone centreline was dominated by equiaxed dendrites in I-82 weldments but by columnar dendrites in I-52 weldments. In addition, the I-82 weldments had a finer subgrain structure near the fusion zone centreline and smaller cellular spacing near the fusion line than I-52 weldments. Mechanical test results demonstrate that the I-82 weldments had higher tensile strength (622–630 MPa) with rupture occurring in the base metal. In comparison, the I-52 weldments had lower tensile strength (568–5...

A general solution on stress singularities in the junction of two anisotropic materials

The general solution on stress singularities of a junction composed of two dissimilar anisotropic materials is presented in this paper. Based on the Lekhnitskiis approach, the characteristic equation of the generalized plane deformation problem is developed. The influencing parameters on the stress singularity are material constants, fiber orientations and the bonding wedge angle. The results of the stress singularity order are contour plotted in a circular region. With these figures, the conditions for a minimum, or even vanishing, singularity order can be determined. The accuracy of this approach is guaranteed as the results are compared with several degenerated cases.

Evaluation of effects of niobium and manganese addition on nickel base weldments

Abstract The present work investigates the influence of different concentrations of Nb (from 0.1 to 3.35 wt-%) or Mn (from 0.78 to 3.32 wt-%) on the microstructure and mechanical properties of an Inconel 690 weldment. Welding electrodes are produced by coating Inconel filler metal 52 with a flux containing various percentages of Nb or Mn. Weldments with a bevel edge are butt welded via a manual shielded metal arc welding process, using identical parameters and procedures. The microstructure and mechanical properties of the resulting weldments are then analysed. The experimental results indicate that the subgrain structures of the welds are primarily dendritic. Under tensile testing, it is found that each specimen ruptures in the fusion zone and that the fracture surfaces exhibit entirely ductile features. It is noted that as the content of Nb increases, the welds tend to show a finer subgrain structure, i.e. having smaller dendritic spacing. Consequently, the tensile strength and microhardness of the fusion zone increase slightly and the tensile rupture mode changes from slant to flat fracture. It is determined that the interdendritic precipitates are mainly Nb rich eutectic type and Nb rich type constituents. The presence of these precipitates increases with higher concentrations of Nb in the flux and results in a significant decrease in the ductility of the weldment. Regarding the relative influence of the Mn additions, there appears to be no significant change in the subgrain structure as the percentage of Mn increases. However, the ductility tends to increase and it is also found that the tensile strength and microhardness of the fusion zone also increase slightly. Accordingly, the tensile rupture mode exhibits a slight tendency to change from flat to slant fracture. Although the interdendritic precipitates identified in the Mn series are similar to those in the Nb series, it should be noted that the precipitates appear in lower numbers and are smaller than their Nb counterparts.

Application of EDM Hole-Drilling Method to the Measurement of Residual Stress in Tool and Carbon Steels

This study adopts the application of the electrodischarge machining (EDM) hole-drilling method to the measurement of residual stress in AISI D2 cold work tool steel, AISI H13 hot work tool steel, and AISI 1045 medium carbon steel. A calibration procedure based on the thermal conductivity of the material is conducted to compensate for the additional compressive stress induced in the workpiece by the EDM hole-drilling operation. Since the formation of this white layer influences the magnitude of the induced stress, the scanning electron microscopy, transmission electron microscopy, and nanoindentation techniques are used to examine the microstructure and hardness of the white layer resolidified on the EDMed surface. The experimental results reveal that combination of the hole-drilling strain-gage method (ASTM standard E837) with an EDM drilling process provides the effective means of determining the residual stress in materials with high hardness and good wear resistance.

Magnetic structure of an Fe–Pd–Rh alloy

Martensitic phase transition in an aged Fe–Pd–Rh alloy was studied using transmission electron microscopy (TEM) and x-ray diffraction. Both TEM and x-ray diffraction reveal the appearance of an intermediate structure between the fcc→L10 martensitic transformation. The intermediate phase, denoted L1m, has a monoclinic structure with lattice parameters of a=3.193A, b=3.684A, c=3.141A, and β=92.042°, and the normal L10 martensitic phase has an ordered structure with lattice parameters of a=3.876A, c=3.684A, and c∕a=0.950, as confirmed by TEM and x-ray diffraction.