Yoshiharu Mutoh

Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy A5052-O

Abstract Joining technology of lightweight dissimilar metals between magnesium and aluminum alloys is essential for realizing hybrid structure cars and other engineering applications. In the present study, the normal center-line welding of lap joint was carried out by laser welding. It was found that the intermetallic layer formed near interface between two metals significantly degraded the joining strength. FEM heat transfer analysis was carried out to find out an available method to control penetration depth and width of molten metal, which contributes to control thickness of intermetallic compound layer. Based on the results of FEM analysis, the edge-line welding of lap joint was carried out, which could easily control the thickness of intermetallic layer and successfully obtained high joining strength.

Monotonic tension, fatigue and creep behavior of SiC-fiber-reinforced SiC-matrix composites: a review

The monotonic tension, fatigue and creep behaviour of SiC-fiber-reinforced SiC-matrix composites (SiC/SiC) has been reviewed. Although the short-term properties of SiC/SiC at high temperatures are very desirable, fatigue and creep resistance at high temperatures in argon was much lower than at room temperature. Enhanced SiC/SiC exhibits excellent fatigue and creep properties in air, but the mechanisms are not well understood. The present Hi-Nicalon/SiC has similar properties to enhanced SiC/SiC, but at higher cost. Improvement of Hi-Nicalon/SiC therefore seems necessary for the development of a high-performance SiC/SiC material.

Low cycle fatigue test for solders using non-contact digital image measurement system

In a strain controlled low cycle fatigue test it is customary to use a contacting extensometer. However the extensometer can cause premature fracture at the contact points by inducing localized plastic deformation, particularly in a soft metal such as a lead– tin solder. In order to avoid this problem, a non-contacting, digital-image measurement system has been developed to measure the displacement of the specimen gage length during a cyclic loading. The capability of this system for strain-controlled fatigue testing was studied in preliminary experiments, and the effect of variables in the digital image measurement system on the accuracy and the reliability were established. The results confirmed that this system was capable of measuring the displacements in straincontrolled fatigue tests. Low cycle fatigue tests on three solder materials at 20°C were then carried out using this system. The solders were: Ag–Sn eutectic solder (3.5Ag/96.5Sn), Sn–Pb eutectic solder (63Sn/37Pb) and Sn–Pb solid solution solder (5Sn/956Pb). It was found that at a given strain range the Sn–Pb eutectic solder (63Sn/37Pb) had the lowest low cycle fatigue resistance. The lead free eutectic alloy (3.5Ag/96.5Sn) has the highest low cycle fatigue resistance in low strain range regime (p 1%). At higher strain range regime (p 1%), the 5Sn/95Pb alloy has better low cycle fatigue resistance. For all three types of solder, the initial failure mode at the surface of specimens was intergranular. For the 63Sn/37Pb alloy the subsequent fracture path was also intergranular, but for the 3.5Ag/96.5Sn and the 5Sn/95Pb alloys, the subsequent crack paths were generally transgranular.  2001 Elsevier Science Ltd. All rights reserved.

Low cycle fatigue behavior and mechanisms of a eutectic Sn-Pb solder 63Sn/37Pb

Abstract Low cycle fatigue tests of as-casted Sn–Pb eutectic solder (63Sn/37Pb) were carried out using the non-contact strain controlled system at 20°C. The fatigue behavior followed the Coffin–Manson equation with the fatigue ductility exponent of 0.63. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, the crack initiation and propagation were studied by SEM examination of specimen surface with replication technique, longitudinal cross section and fracture surface. Colony boundary sliding and microcracks along colony boundaries were detected in the early stage of fatigue life. Cavities on colony boundaries, especially on the interphases between Sn–Pb phases, and wedge cracks at the triple-point colony coners were the initiation sites of microcracks. Stage II crack propagated in intergranular manner along the eutectic colony boundaries. This propagation of stage II cracks could be expressed by the relation of dac/dN=1.6×10−10 [ΔJ]1.2, where ac is average crack length and ΔJ is J-integral range. After fatigue tests, small grains were observed in Sn-rich phases near fracture surface.

Effect of rhenium addition on fracture toughness of tungsten at elevated temperatures

Fracture toughness tests of tungsten and tungsten-rhenium alloy specimens were carried out at elevated temperatures. Temperature dependence of fracture toughness and effect of rhenium content on fracture toughness were investigated. Although fracture toughnesses of three kinds of specimens with rhenium contents of 0, 5 and 10 wt% were almost identical at room temperature, fracture toughness at elevated temperatures increased with increasing rhenium content. The brittle-ductile transition, similar to steels, and subsequent transition of the fracture mode from ductile dimple to intergranular were observed for all three kinds of specimens. With increasing rhenium content, the transition temperatures increased. A significant grain growth was found, not for tungsten-rhenium alloy specimens, but for a tungsten specimen without rhenium in a temperature range higher than the recrystallizing temperature, which resulted in transition of the fracture mode from dimple to intergranular.

Fatigue crack growth behavior of 96.5Sn–3.5Ag lead-free solder

Abstract The fatigue crack growth tests of 96.5Sn–3.5Ag solder have been carried out at frequencies from 0.1 to 10 Hz and at stress ratios from 0.1 to 0.7. Fatigue cracks propagated in a transgranular manner for the specimens tested under low stress ratios and high frequencies (R=0.1 and 0.3 at 10 Hz, R=0.1 at 1 Hz), where the fatigue crack growth behavior was dominantly cyclic dependent. Intergranular crack growth was observed for the specimens tested at high stress ratios (R≥0.5) and low frequencies (f=0.1 Hz), where the fatigue crack growth behavior was dominantly time-dependent. Irregular eutectic structure of Sn and plate-like Ag3Sn intermetallics was formed in the material. Plate-like Ag3Sn intermetallics behaved as a strengthening phase and improved the transgranular crack growth resistance in the cyclic dependent regime, while it had no significant influence on the intergranular crack growth resistance in the time-dependent regime. The formation of small grains in the Sn matrix was observed during the fatigue crack growth tests, which might result from polygonization in the Sn matrix.

Strain-rate effects on low cycle fatigue mechanism of eutectic Sn–Pb solder

Abstract Low cycle fatigue tests of as-casted Sn–Pb eutectic solder (63Sn/37Pb) were carried out using the non-contact strain-controlled system at 20°C in order to avoid local deformation and stress concentration at contact points between the extensometer and the specimen surface. The fatigue mechanisms were studied by SEM examination of polished surface of specimens and fracture surfaces. Wedge cracking due to grain boundary sliding was the dominant mechanism in the low strain rate regime, while extensive cavitation was observed on the colony boundaries at high strain rate regime. An estimation of the transition strain rate for grain boundary cracking due to sliding ( e ∗ w ) yielded a value lowers than that of the experimental result by 2–3 orders of magnitude. Relationship between time to failure and strain rate for the present results and the reported results could be expressed by a double-linear curve with the transition strain rate of approximately 10 −3 s −1 .

Fracture mechanics approach to fretting fatigue and problems to be solved

A large number of research works have been devoted to fretting fatigue from both mechanical and metallurgical viewpoints. In the present paper, fracture mechanical approaches for evaluating fretting fatigue life and strength have been briefly reviewed. Furthermore, a new approach based on a singular stress field near the contact edge and on fracture mechanics has been proposed. The directions of crack initiation and propagation as well as fretting fatigue life, which have coincided with the experimental results, could be estimated according to the new approach, in which singular stress near the contact edge and mixed mode crack growth have been taken into consideration. In the application of the new method to predict the fretting fatigue behavior, there are still several problems to be clarified, which have also been discussed in detail.

Fatigue crack growth behavior of Sn-Pb and Sn-based lead-free solders

Abstract Solder alloys of lead-rich composition have been commonly used as joining materials in electronic package. However, because of environmental concerns, lead-free solders will replace lead-rich solders more and more in the future. The fatigue characteristics of the solders used are most important in assessing the reliability of joints in electronic packaging. In the present study, the fatigue crack growth (FCG) behavior of a wide variety of solders of both lead-rich and lead-free types has been investigated under a range of mean stresses and frequencies. Both time dependent and time independent (cyclic dependent) behaviors were observed. In the cyclic dependent crack growth regime, the FCG rates could be expressed as a function of either ΔKeff or ΔJ. Further, the lead-free solders were found to have a higher resistance to FCG than did the lead-rich solders. In the time dependent crack growth regime, the FCG rates were found to be a function of C ∗ . The point of transition between time dependent and time independent behavior was found to depend on the homologous temperature and strength of the alloys.

Utilising friction spot joining for dissimilar joint between aluminium alloy (A5052) and polyethylene terephthalate

Abstract The weld strength of thermoplastics with aluminium alloy, such as high density polyethylene and polypropylene sheets, is influenced by friction stir welding parameters. This paper focuses on the preliminary investigation of joining parameter at various levels as well as the mechanical properties of friction spot joining (FSJ) of aluminium alloy (A5052) to polyethylene terephthalate (PET). A number of FSJ experiments were carried out to obtain optimum mechanical properties by adjusting the plunge speed and plunge depth in the ranges of 5–40 mm min−1 and 0·4–0·7 mm respectively, while spindle speed remains constant at 3000 rev min−1. The results indicated that A5052 and PET successfully joined with the aid of frictional heat energy originated from the friction spot welding process. The effect of plunge speed on the joined area and the effect of formation of bubbles at the interface of joints on the shear strength of joint are discussed.