EFFECT OF MICROSTRUCTURES ON DEFORMATION BEHAVIOUR OF ALUMINIUM MATRIX COMPOSITES IN LASER BENDING

It is well known that the mechanical and deformation properties of metal matrix composites (MMCs) are related to their microstructures. However, less work has been carried out in laser bending to examine the dependency of microstructures of MMCs on deformation behaviour. In this paper, two aluminium-based metal matrix composites, A12009/ 20vol% SiCw and A12009/20vol% SiCp were investigated. AYAG laser was used to scan the composites both parallel and perpendicular to their rolling directions. It was reported that under the same processing conditions, a larger bending angle was obtained for the AI2009/SiCp composite. No significant change in microstructures was observed for both composites after bending. Experimental findings also revealed that for the particulate reinforced composite, a larger laser bending angle was obtained when the laser scanning direction was perpendicular to the rolling direction, whereas no significant difference was observed for the A12009/SiCw composite. These phenomena were shown to relate to the shape anddistribution of reinforcements.


INTRODUCTION
In the last decade, there have.beena considerable amount of experi- mental and theoretical investigations on laser forming (Vollertsen,  1994a; Vollertsen et al., 1995; Yau et al., 1998).Relatively less work has been reported on laser forming of hard and brittle materials, though it is considered to be a suitable process for these materials.Metal matrix composites (MMCs) are attractive for many structural applications because of their high specific strength, modulus of elasticity, thermal conductivity and structural stability (Hull and Clyne, 1996).However, they are relatively brittle and not easy to form at room temperature.There are commonly two types ofcommercially available discontinuous reinforced MMCs, i.e., whisker and particulate reinforced.Recently, the author and his workers have carried out preliminary investigations on the deformation behaviour oftwo aluminium-based MMC materials in laser bending.It has been reported that a large bending angle can be achieved for an A12009/SiC composite and the bending angle is strongly related to the laser power, the processing velocity, and the number of irradiations.They have also examined the effect of sheet thickness, orientation and width on bending angle of an A16013/SiCp composite (Yau et al., 1996; Chan and Liang, 1999).The experimental results show that smaller bending angles are obtained when the laser scanning direction is parallel to the rolling direction.A linear rela- tionship between bending angle and threshold line energy is obtained.It is, however, very worthwhile to further examine the effect of micro- structures of MMCs in laser bending.In this paper, two aluminium- based MMCs with different types of reinforcements are used for investigations.

EXPERIMENTAL MATERIAL AND PROCEDURES
Two commercially available aluminium-based MMCs, A12009 alloy reinforced with 20vo1% SiC particles and the same matrix alloy with 20vo1% SiC whiskers were used in this study.The materials supplied by Advanced Composite Corporation were manufactured by powder metallurgy.The nominal composition of the aluminium alloy is listed in Table I.Samples of thickness of 0.5mm and dimensions of 10mm x 20mm were prepared by wire electro-discharge machining.
An 80 W YAG laser unit operating in continuous wave (CW) mode was used in the experiment to scan the specimens as shown in Fig. 1.In order to examine the effect of sheet orientation on the bending angle, the composites were scanned both parallel and perpendicular to their rolling directions.No graphite coating was applied and a defocused laser beam with a diameter of 1.32mm was used to avoid surface melting.A machine vision system including a high-resolution CCD camera, a specially designed fixture, a graphics interface board and a personal computer were equipped with the laser unit for angle mea- surements.The accuracy of the system is +0.5In order to examine the effect of microstructures on bending angle, the two composites were deformed under the same processing conditions.
A thermal mechanical analyzer was used to measure coefficient of thermal expansion (CTE) of the composites along the rolling and transverse directions.CTE measurements were performed between temperatures of 25C and 350C at a rate of 10C min-.The crystallographic textures of the matrix alloy were measured by a Philips X-ray diffraction (XRD) system.Three incomplete pole figures: (111), ( 200) and ( 220) are obtained by the back reflection method at 5 incre- ments using Cu Ka radiation.From the pole figures, the orientation distribution function (ODF) and the quantitative texture component were calculated by the software developed by Cai and Lee (1995) using the series expansion method.Tensile tests were carried out at room temperature to determine the mechanical properties of the composites.
A scanning electron microscope (SEM) was used to examine the microstructures of the specimens at different locations, before and after laser bending.The relationship between laser bending angle and number of irradia- tions in two different scanning directions is shown in Fig. 2. A relatively linear relationship is observed, which is in agreement with that reported by Namba (1985), Scully (1987), Yau et al. (1996), Chan and Liang  (1999).It is however inconsistent with the findings of Sprenger et al.No. of passes (times)

RESULTS AND DISCUSSIONS
b-scanning direction parallel to the rolling direction (AI2009/SiCp) c-scanning direction perpendicular to the rolling direction (AI2009/SiCp) d-scanning direction parallel to the rolling direction (AI2009/SiCw) e-scanning direction perpendicular to the rolling direction (A12009/SiCw) FIGURE 2 Effect of sheet orientation on laser bending angle.
hardening is the cause for non-linearity of the curve.Since this effect is not significant when the sheet thickness and the number of irradiations are small, a relatively linear relationship between bending angle and number of irradiations is obtained in the present paper.Experimental findings also reveal that for the A12009/SiCp composite, a larger laser- bending angle is obtained when the scanning direction of laser beam is perpendicular to the rolling direction.But no significant difference is observed for the A12009/SiCw composite.Under the same processing conditions, the bending angle of the A12009/SiCp is shown to be larger than that of the A12009/SiCw.

Microstructural Examinations
Figure 3 reveals the initial microstructures of the A12009/SiC com- posite.A relatively random distribution of whiskers is observed.The dimensions of the whisker is 0.5 tm in diameter and 10-80 tm in length; whereas the distribution of particulates in A12009/SiCp is shown in Fig. 4, the particulates are not in ideal spherical shape and of an average size of 3 tm.After laser bending, a clear heat affect zone (HAZ) is observed as shown in Fig. 5. Figure 6 shows the microstructures of the whisker- reinforced composite which was bent to 7 after 30 irradiations, whereas, the microstructures of A12009/SiCp composite bent to 10 after 30 irradiations is shown in Fig. 7.No significant change in micro- structures is found in these composites.
ODF of the A12009/SiCp and A12009/SiCw composites are shown in Figs. 8 and 9 respectively.It is shown that the matrix of both com- posites consists of random texture and some weak components.The results are in agreement with findings of Chen et al. (1998).It is con- sidered that the matrix does not have any significant anisotropic properties and will not contribute to the overall anisotropic bending properties of A12009/SiCp.

Effect of Sheet Orientation and Reinforcement on Bending Angle
Vollertsen (1994b) have proposed the following analytical equation to predict the angle (as) in laser bending aB 3 athPlA pCpvl (1) where So is the sheet thickness, '/31 is the feed rate, A is the absorption coefficient, p is the heat input, ath is the coefficient of thermal expansion, p is the density, Cp is the heat capacity.This equation gives  an analytical expression for the bend angle as a function of the energy (laser power, absorption, path feed rate), geometric (sheet thickness) and material parameters (coefficient ofthermal expansion, density, heat capacity).Other researchers have further reported that the mechanical properties of materials are also an important material parameter affecting bending angle (Yau et al., 1997).It is found that materials with greater yield strength result in smaller bending angle.The mechanical properties of composites are known to relate to the types and dis- tribution ofreinforcements, and the microstructures ofthe matrix alloy.
For both A12009/SiCp and A12009/SiCw, a weak crystallographic tex- ture as shown in Figs. 8 and 9 is obtained in the matrix, which suggests that the matrix has relatively isotropic properties.Tensile tests were conducted at room temperature to reveal the overall mechanical properties of the composites.The results show that the yield strength of the whisker reinforced composite is 208.8MPa, which is significantly larger than that of the particulate reinforced one (121.2MPa).This is in

CONCLUSIONS
In this paper, the deformation behaviour of two MMCs in laser bending, A12009/20vol% SiCp and A12009/20vol% SiCw were exam- ined and compared.Under the same processing conditions, a larger bending angle was obtained for the A12009/SiCp composite.No sig- nificant change in microstructures was observed for both composites after bending.A larger laser bending angle was obtained when the laser scanning direction was perpendicular to the rolling direction for the A12009/SiCp, whereas relatively isotropic bending behaviour was observed for the A12009/SiCw composite.The misotropic properties of the A12009/SiCp composite are shown to strongly relate to the dis- tribution and shape of SiCp reinforcements, which result in a significant variation in CTE in different sheet orientations.
FIGURESchematic diagram of laser bending of MMC sheets.
(1994)  which show a degressive course of bending angle.Yau et al.  (1996) suggest that the influence of thickening in the edge and work

FIGURE 6
FIGURE 6 Microstructure of the laser HAZ of the A12009/SiC.

FIGURE 7
FIGURE 7 Microstructure of the laser HAZ of the A12009/SiCp.

FIGURE 11
FIGURE 11 Coefficient of thermal expansion of A12009/SiCp at two different sheet orientations.