Research Opportunities in Machining of Composite Material

Composite material is a macroscopic combination of two or more distinct materials, with a noticeable interface between them. Composite materials are extending the horizons of designers in all branches of engineering, and yet the degree to which this is happening can easily pass unperceived. Composite materials offer high strength, stiffness and toughness, good fatigue and impact resistance, good thermal and electrical conductivity, and good corrosion and wear resistance. These materials have wide applications in aerospace, transportation, construction and marine sectors, sporting goods and infrastructures. Advanced composites are optimized to achieve specific properties for special applications. These materials have balanced structural properties superior to either constituent material alone. Composites typically have a fiber or particle phase reinforced on relatively soft continuous matrix phase. Most of the reinforcements have good thermal conductivity along with electrical conductivity.

Composites are heterogeneous in nature because of very strong fibers interwoven into a softer matrix. Therefore traditional machining of composites is difficult due to anisotropy, low thermal conductivity, heat sensitivity, and high abrasiveness. Excessive cutting forces and cutting temperatures, high thermal conductivity of reinforcement results in heat affected zone, excessive tool wear and delamination of plies, low interlaminate strength allows damage in the composite structure, and chip-off of boundary films are usual difficulties in traditional machining of composites. Excellent machining properties are generally achieved by minimizing tool wear, delamination, cutting forces and heat affected zone. However these are very difficult in traditional machining including turning, drilling, milling, grinding, etc. of composites because tool wear increases with increase in cutting speed and feed rate.

In these circumstances, non-traditional machining processes become viable and economical way for machining of composites. These machining processes include water jet machining (WJM), abrasive water jet machining (AWJM), abrasive suspension jet machining, laser beam machining (LBM), laser assisted machining, ultrasonic machining, and electrical discharge machining (EDM). Out of all non-traditional machining processes, only AWJ, LBM and EDM of composites have received considerable attention in industries because these processes provide certain advantages over other machining processes. These include high production rates, greater flexibility, and capability to produce complex contours and shapes. In addition, these processes require no cutting tools and very minimum fixturing. But heat generated during LBM significantly affects the mechanical properties of polymer matrix composites. Different coefficients of thermal expansion of fibers and matrix result in residual stresses after curing. Thereafter deformation and part damage occurs due to residual stresses release while machining. Therefore it limits applications of LBM for composites. Only conductive materials can be machined by EDM process. However composite structure especially laminates in composite structure becomes obstacle in EDM due to change in phase of material through cross-section. AWJM is a cold machining process and it can cut any material. It is extensively used in various industrial applications such as cutting hard-to-cut materials, ceramics, alloys, composites and various materials in textile, food and leather industries. It has specific advantages such as no thermal damage to workpiece, no tool wear, low cutting forces and high productivity. Therefore this technology is proven as suitable process for machining of composites.

Dr. Ajit D. Dhanawade,

Associate Professor,

Department of Mechanical Engineering

TSSM's Bhivarabai Sawant College of Engineering & Research, Narhe, Pune