Internal combustion engine designs are progressing to meet high-performance requirements with resepct to reduced fuel consumption and emissions. Compression ignition engines (diesel engines) involve high-bore pressures for combusting fuel. Advancements in diesel engine designs have placed higher performance demands on the materials used for manufacturing engine blocks, cylinder liners, and cylinder heads. Progress in diesel engine technology has created designs that require engine blocks with high-bore-pressure specifications. The next generation diesel engines involve peak bore pressures of the order of 135 BAR to 160 BAR during combustion. The increased bore pressures require the use of high-strength materials for engine blocks and cylinder heads. One such material under consideration by some automobile manufacturers is compacted graphite iron (CGI).

The unique graphite microstructure of the material provides high strength for the increased bore pressure requirements that are likely to be above 160 BAR in the future. However, challenges are faced in the area of machinability of the metal due to its high strength, stiffness, and hardness. Researchers S. Skvarenina and Yung Shin of the School of Mechanical Engineering, Purdue University, West Lafayette, in the United States, have developed a laser-assisted machining (LAM) technique to improve the machinability of CGI and therefore, reduce machining costs. The technique employs a laser to soften the material prior to machining so that machinability can be improved. Shin tells Technical Insights that the research on LAM for CGI was started based on a request from a company. He adds that currently, the machinability of CGI is poor. To use CGI as a next generation engine material, it is required to improve the machinability (by a factor of 2). This goal has been achieved through LAM, making the process cost-competitive. Tool life and material removal rates are improved by the technique. LAM was found to be successful for a material removal temperature of 400 degrees C with the microstructure remaining unaffected.

The lasers used for this study are a 1.5 kW carbon dioxide (CO2) laser and a neodymium-doped yttrium aluminium garnet (Nd: YAG) laser. The optical laser energy is directed on the work piece with the aid of focusing optics. A three component dynamometer with a suitable amplifier was used to measure cutting forces. The surface temperature of the work piece was measured by means of an infrared camera. The surface roughness of the work piece was measured using a profilometer after each trial. The tool wear was measured using an optical microscope. Due to the unique microstructure of CGI, excessive heat can alter the microstructure. By means of the study, the researchers have been able to control the temperature field precisely to achieve the desirable result. Shin says that the major difficulty was to maintain the original graphite microstructures during machining. However, this has been achieved within a specific operating range using the technique.

The research team has also pioneered the application of LAM to many difficult-to-machine materials, such as ceramics, high-temperature alloys, stainless steel, and metal matrix composites. Typical benefits of the technique include a 30% to 70% cost reduction, a 30% to 50% improvement in surface finish and improved subsurface integrity. CGI is being considered as a next generation material for engine blocks and heads at a few automotive companies in the United States. Shin says that the team has three patents pending. The team is collaborating with a number of companies in the United States, and with defense organizations.


Details

Yung C. Shin,

School of Mechanical Engineering

585 Purdue Mall, Purdue University

West Lafayette, IN 47907

Phone: 765-494-9775

Fax: 765-494-0539

E-mail: shin@ecn.purdue.edu

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