ELSEVIER Composites Science and Technology 58 (1998) 285-291 0 1998 Elm&r Science Limited. All rights reserved Printed in Northern Ireland PII: SO266-3538(97)00126-7 0266-3538/98 $19.00 CHIP FORMATION IN THE MACHINING OF SiC-PARTICLE- REINFORCED ALUMINIUM-MATRIX COMPOSITES J. T. Lin,” D. Bhattacharyya” & W. G. Fergusod “Composites Research Group, Department of Mechanical Engineering, School of Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand bDepartment of Chemical and Materials Engineering, School of Engineering, University of Auckland, Auckland, New Zealand (Received 11 April 1996; revised 2 Decl ember 1996; accepted 7 July 1997) zyxwvutsrqponmlkjihgfedcb A bstract As a consequence of the widening range of applications of metal-matrix composites (MMCs), the machining of these materials has become a very important subject for research. Aluminium-matrix composites are widely used .for their favourable speciJic strengthlstlflness and corro- sion resistance properties. This paper describes a study of chip formation during the machining of a DiJRALCAfl aluminium-matrix composite (A359/SiC/ZOp). For good machinability, it is desirable to have continuous chips in short segments without the use of a chip breaker. The chip-formation mechanism in machining this silicon-car- bide-particle-reinforced aluminium MA4C at three d@er- ent cutting speeds has therefore been investigated by using an explosive charged quick-stop device. An improved quick-stop device has enabled research to be carried out more easily on the mechanism of chip formation by achieving better chip control during machining. During the chip-breaking process, the primary chip-forming mechanism involves the initiation of cracks from the outer free surface of the chip due to the high shear stress. Meanwhile, some small voids are formed by the separa- tion of particles and the matrix material within the chip because of the stress concentration at the edges of the particles. The crack propagation is enhanced through the coalescence of these voids along the shear plane. The ,fracture and sliding of material then follow to form semi- continuous ‘saw-toothed’ chips. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: A. metal-matrix composites, chip formation, quick-stop device, saw-toothed, shear angle 1 INTRODUCTION With the increasing use of metal-matrix composites in various applications such as the aerospace industry, the automotive industry and in sports equipment, the machining of such materials has become a very impor- tant subject for study. Owing to the addition of rein- forcing materials which are normally harder and stiffer than the matrix, machining becomes significantly more difficult than is the case for conventional materials, as reported in earlier publications.‘-5 It has been shown5 that while machining the DURALCAN@ aluminium- matrix composite A359/SiC/20p, short chips formed without a chip breaker are desirable for a continuous machining operation. This not only improves the machinability of this composite, but also enhances its applicability in various industries. On the other hand, the importance of chip formation has been well recog- nised and studied by other researchers.6 Problems with surface finish, workpiece accuracy and tool life can be caused even by minor changes in the chip-formation process. Hence, it is necessary to understand the chip- forming mechanism for this material through further investigation. This will render the material more sui- table for advanced applications and more efficient chip control in machining can also be achieved. Basically, chip formation is a shear process involving plastic deformation within the shear zone. While study- ing the nature of the shear zone in metal cutting, con- flicting evidence has led to two basic schools of thought with regard to analysis of the deformation zone, namely the thin-plane (or thin-zone) model as shown in Fig. l(a) and the thick-zone model, Fig. l(b). The available experimental evidence indicates that at higher speeds a thin shear plane is approached.7 Thus it seems that the thin-zone model is likely to be the most useful for prac- tical cutting conditions since the shear angle can be more easily identified. However, the types of chip formed are not only rela- ted to the nature of the shear zone but are also influ- enced by material properties such as ductility, thermal conductivity and microstructure. Furthermore, physical phenomena such as instability in the cutting process can change the chip-formation mechanism.8-” The research into chip-formation mechanisms has been facilitated significantly by the employment of the quick-stop device,12,13 which provides a suitable method of ‘freezing’ the chips on the workpiece for an in situ 285