Structured graphite gray iron (CGI) is increasingly being used in the manufacture of diesel and racing engine parts. The choice of tool can determine the effectiveness of the shop in the processing of this challenging material. The development of new tough workpiece materials has prompted tool manufacturers to continually develop suitable new tool geometries, various grades of carbide carbide and tool surface spray technology. For example, in a processing plant that serves the aerospace industry, an effective method must be found to process 5553 titanium alloys and their composite materials. The same problems are encountered in the processing shops that serve the medical industry, such as requiring them to process PEEK polymer materials, stainless steel and some other special materials. On the way forward in the automotive industry, there is also a material that is not easy to process. This is the dense graphite gray iron (CGI). This material is primarily used in the production of engine blocks, cylinder heads and bearing cap castings, which are typically used in large diesel trucks. The aim is to better improve the fuel efficiency of long-haul vehicles because CGI materials weigh only half as much as ordinary gray cast iron. In addition, its strength and rigidity are equivalent to twice that of gray cast iron, which helps designers to minimize the wall thickness of the engine block. Therefore, in general, engines made and assembled from CGI materials are 9% lighter than those made from gray cast iron. CGI materials have been used in Europe for some time and are accepted by most people in the United States. In diesel engines, this material can withstand peak combustion pressures, a capability not found in aluminum engines with cast iron cylinder liners. Some high-performance, V-type racing engines are also made of CGI materials, mainly because they not only reduce the weight of the car, but also increase the rigidity, especially in the valley between the cylinders. Robert McAnally, an industrial expert in the automotive milling department at Sandvik Coromant (Fair Lawn, NJ), explains that one of the more challenging CGI materials is due to its tensile strength equivalent to that of grey cast iron. 3 times. In milling, the higher the tensile strength, the greater the cutting force required – the processing power required to process CGI materials is 15% to 25% higher than that of gray cast iron. Therefore, the shop equipment for gray cast iron processing may not have the ability to process CGI materials. Mr. McAnally pointed out that they also face several other challenges: the thermal conductivity of CGI materials is relatively low, so the heat generated in the process is easily transferred to the workpiece, which will inevitably affect the wear of the tool. In contrast, gray cast iron has a high thermal conductivity, so the heat generated during the cutting process is easily carried away by the chips. The cast iron surface on the CGI component is a trivalent iron structure that is easily bonded to the cutting edge of the tool during processing. This does not happen when machining grey cast iron because the grey cast iron exhibits a pearlite structure. Unlike gray cast iron, CGI materials do not contain sulfur. The sulfur content of the grey cast iron deposits on the cutting edge of the tool, which acts as a lubricating tool and extends the life of the tool. In the casting process of CGI materials, titanium can be used as an alloying element to make the surface of the cast iron more tough. But this also creates a free carbide with friction in the entire cast iron. The amount of alloying elements in CGI materials has a great influence on the processing properties of the materials and the service life of the tools. Due to these factors, the tool life of cutting CGI materials is generally only half of that of cutting gray cast iron tools. The milling surface finish (Rz) of milling and boring CGI materials is about 50% stronger than that of gray cast iron, which means that the number of passes during machining can be reduced, and no separate finishing tool can be used. But it can achieve the required finish. During the machining process, the cutting of the tool does not cause the edge of the CGI component to collapse. Gray cast iron is prone to chipping and chipping, which can cause the workpiece to be scrapped. In this respect, CGI materials are more like steel and are prone to burrs, but they do not break. If a common process is used, since the CGI material needs to be reduced in cutting speed, the time required to process it is almost three times that of the gray cast iron. Sandvik has conducted a variety of tests to determine more efficient processing methods for processing CGI materials. For milling, the company has determined that the best material for tooling is carbide alloy, which is coated with a thick layer of titanium nitride (TiCn) and a layer of alumina (Al203). ). Mr. McAnally believes that the thickness of the thicker spray coating should be 7-10 μm, and the thinner coating is typically 2~3 μm. For turning and honing, the company recommends a carbide matrix material and uses chemical vapor deposition (CVD) to add a thicker layer on the surface that has higher friction and wear resistance and wear resistance. Characteristic coating. It has been found that the CBN inserts are used for boring CGI materials, and the tool life is only 1/10 of that of boring gray cast iron. It is appropriate to use a tool with a slightly positive-angle geometry (between 5 and 10°) and it is recommended that no coolant be used for the machining of CGI materials.
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