CONCLUSION

Dimensional analysis, FEA and experimental studies have been performed with reference to various variables such as welding condition, rigidity and restraint to identify the principal factors controlling welding distortion and to establish a predictive equation of welding distortion for hulls. The main results obtained are summarized as follow.

1. Angular distortion at the fillet and butt weldment was defined as a function of heat intensity, bending rigidity, and internal and external restraint condition.

2. Angular distortion decreased by effects of internal restraint. However, reduction rate of angular distortion at the fillet weldment depends on restraint intensity alone, while that of angular distortion is determined by internal restraint and ratio of heat intensity and bending rigidity.

3. The control effect of external restraint (by jig) on the angular distortion increased as the function of ratio of heat intensity and bending rigidity and bending restraint intensity increased.

REFERENCES

1. N. R. N. Rao and L. Tall, 1961, "Residual stress in welded plates", Welding Journal, Vol.40, p.468s-480s.

2. Rodgers and Fetcher, 1938, "Determination of internal stress from the temperature history to a butt-welded plate", Welding Journal, Vol. 17, p.4-7s

3. Griffith and H. R. George, 1941, "Residual stress in butt-welded steel plates", Welding Journal, Vol.20, 1941, p.410s-414s

4. Satoh K., Matsui S., 1967, "Reaction stress and weld cracking under hindered condition", Technological Reports of the Osaka Univ., 1967

5. Satoh K., 1972, “An analytical approach to the problem of restraint intensity in slit weld", IIW, Document X-661-72,

6. Yukio Ueda and Taketo Yamakawa, 1971, "Analysis of thermal elastic-plastic stress and strain during welding by finite element analysis", Trans. of JWS, Vol. 2, No. 2, p.90s-98s

7. E. Friedman, 1975, " Thermo-mechanical analysis of the welding process using the finite element method", Trans. of ASME, p.209s-213s

8. Satoh K., Terasaki T., 1976, "Effect of welding conditions on welding deformations in welded structural materials", Trans. of JWS, Vol. 45, No. 4, p.54s-60s

NUMERICAL SIMULATION OF SHEAR STRESS DISTRIBUTION
IN THE FILLET OF AN Al-Al2O3 SOLDERING ASSEMBLY

X. Ma*, Y.Y. Qian*

ABSTRACT

By means of finite element numerical simulation, the shear stress distribution in the Al-Al2O3 soldering assembly and the effect of the coefficient of thermal expansion of interlayer alloy and the fillet geometry had been analyzed. The calculating results showed that, maximum level of shear stress occurred at the soldering fillet and the interface between Al2O3ceramic and coated Cu. Meanwhile, in order to get better shear stress distribution, the coefficient of thermal expansion of interlayer alloy should match with the Al base metal and the optimized geometry of soldering fillet is the concave shape with extruding length a little more than the gap height.

KEYWORDS

Finite element numerical simulation, Al-Al2O3 soldering, shear stress, coefficient of thermal expansion, fillet geometry

INTRODUCTION

Metal-ceramic soldering assembly has been widely used in industrial applications [Ref. 1-2]. As an assembly with different materials joining, cooling stage after soldering will lead to thermal stress in the assembly due to the mismatch of coefficients of thermal expansion (CTE) of different base materials. Such thermal stress will further cause crack at the joining interface in some cases and failure of the assembly [Ref. 3-4]. With the consideration of the special geometry of soldering seam, the mechanical response which is most affected is the shear stress distribution in the soldering fillet [Ref. 5]. Undoubtedly, the CTE of base materials and the geometry of soldering fillet will play important roles on the shear stress distribution. Similar problem has been investigated in reliability evaluation of surface mount solder joints used in microelectronic packaging [Ref. 6-7]. But few works had been done for metal-ceramic soldering assembly. In this work, shear stress distribution characteristics in the soldering fillet of Al-Al2O3 assembly caused by cooling stage has been studied by finite element numerical simulation, the effects of the CTE of base materials and the geometry of the soldering fillet had been illustrated.

National Key Laboratory of Reliability Physics of Electronic Product, CEPREI, Guangzhou 510610, P.R.China * National Key Laboratory of Welding, Harbin Institute of Technology, Harbin 150001, P.R.China

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FINITE ELEMENT MODEL

Figure 1 is the schematic of Al-Al2O3 soldering assembly. Cu with 0.2mm thickness was coated on the surface of Al2O3 ceramic in order to improve the solderability. The interlayer filler is Sn-based, low melting point (180-190°C) alloy with 0.1mm thickness. The corresponding two-dimensional finite element model with 8 nodes isoparametric elements is shown in Figure 2. Since it is axial symmetric, only half part of the assembly was modelled in this simulation.

Figure 2: Two-dimensional finite element model of Al-Al2O3 soldering assembly

Commercial finite element analysis software ANSYS5.6 was used in this work. All the materials were assumed as elastic and the materials properties used in this calculation is listed in Table 1. Cooling stage is the only outer thermal loading, cooling temperature range is from 180 °C to 20 °C since the melting point of interlayer alloy is rather low.

Table 1 Material parameters used in FEM calculation

EFFECT OF SOLDERING FILLET SHAPE

From the point of view of geometry, the shape of soldering fillet can be divided into three types: concave, 45° line style and convex. Their corresponding finite element models are shown in figure 3.

The shear stress distribution in the above three types soldering fillets are shown in figures 4, 5 and 6, respectively. By comparing the shear stress level, it can be seen that convex fillet is the worse shape because the maximum shear stress in the fillet is 50MPa more than concave shape and 45° line style. As to the latter two fillet shapes, the shear stress level is similar, but the site of maximum shear stress is a little different. The maximum shear stress occurs at the fillet centre for concave shape while at the upper side of fillet for 45° line style. Furthermore, it can be seen from the shear stress contours that stress concentration is more serious in 45° line style than concave shape. Therefore, the mechanical response in the concave fillet shape can be considered as the best.

Figure 3: Finite element models of different soldering fillets geometry

(c) Convex fillet

Figure 4: Shear stress distribution corresponding to concave soldering fillet (Unit: MPa)