Why Compacted Graphite Iron?
Cutting Developments Make Application Now Practical
In 1949, a now well-known material called ductile iron was patented. At the same time, a lesser-known material called Compacted Graphite Iron (CGI) was also patented, though it was just considered a curiosity at the time.
While ductile iron became a manufacturing staple, CGI never was seriously utilized despite possessing some very interesting properties. While not quite as strong as ductile iron, CGI is 75 percent stronger and up to 75 percent stiffer than gray iron.
The thermal and damping characteristics of CGI are midway between ductile and gray iron. It is five times more fatigue resistant than aluminum at elevated temperatures, and twice as resistant to metal fatigue as gray iron.
The nodularity and tensile strength of the material also increases as wall-section decreases.
These properties have most recently been found to make CGI ideally suited for engine manufacturing, where lighter and stronger materials are needed which can absorb more power.
An assembled automotive engine can be made nine percent lighter with CGI. The engine block weight alone can be reduced by 22 percent. This corresponds to a 15 percent reduction in length and a five percent reduction in height and width.
The first commercial CGI applications were the high-speed rail trains (175+ mph) in Europe. Initially, they had cast iron disc brakes that were simply not up to the task. They suffered severe heat-checking and cracks, which were potentially catastrophic. CGI cured all that, and has since been found to be the perfect solution for other real-world applications.
Diesel truck engines are about six times heavier than equal displacement gasoline engines, so the potential for total weight reduction is even greater. A CGI 12L engine block weighs about 660 pounds and the head weighs about 275 pounds, making for a total of 935 pounds; this is a nearly 160 pound reduction.
It has been widely reported that several engine builders and automotive manufacturers are beginning to rely on CGI to meet efficiency and strength requirements.
DAF Trucks of Eindhoven in the Netherlands changed a 12.6L, 480 hp cast iron engine into a 530 hp engine merely by changing the material to CGI. That equated to a 10 percent increase in power and a significant decrease in weight. Now all of its production is being switched to CGI, creating a tremendous market advantage.
Many Caterpillar engine components are being switched to CGI, such as modular heads.
Rolls Royce Power Engineering uses CGI for the engine frames and cylinder heads it casts at VDF in Italy for its industrial engines, with individual engine frames weighing from nine to 17 tons.
Europeans are the leading proponents and users of diesel engines in the world, and consequently have great interest in CGI utilization. It is called Grau Guss Vermikular (GGV) or Gray Iron Vermicular in German-vermicular refers to the worm-like or coral-like micro-structure appearance of CGI.
Alleviates Engine Design Weakness
All V-engines share a common design weakness, as there is a lot of flexing in the V-area between the cylinders when it is under power. CGI strengthens this physical area considerably.
Audi is an early user of CGI material in its power production. All Audi 2.7L, 3.0 V6 and 4.0 V8 diesel engine blocks are now being made of CGI. The V8 main bearing caps are also cast in place, laser etched and then fractured for an absolutely perfect fit after bearing sizing.
The BMW Series 7 V8 engine was also cast in CGI. Hyundai, currently the number seven automobile company in the world, and rising, has V6 CGI blocks scheduled for full series production during 2006. Its World Rally Championship car (1997-99) also had a CGI engine block.
The new 2005 Jaguar 2.7L Ford/PSA V6 diesel is made of CGI. At 445 pounds fully assembled, it is lighter than a comparable aluminum diesel. Even the new James Bond will soon be driving a diesel powered Jaguar R-D6.
Other CGI Users
Nearly all NASCAR teams are running CGI engine blocks, or blocks with CGI liners. These liners are usually plated with a hard-surface coating. It has been reported that some NASCAR teams are able to run a whole season without having to re-bore the blocks.
The Toyota Racing Development (TRD) campaign has the #12 Craftsman pick-up truck. This 5.8L V8 CGI engine block weighs 89 kg (195 pounds), has a 3 mm (0.118 inch) cylinder wall thickness and produces 650 hp (83.5 kW/liter). For comparison purposes, standard CGI diesel engines will soon be producing 66kW/liter.
General Motors’ Opel subsidiary has used CGI for the engine block of its 2.5-liter V6 DTM racing engine. Theoretically, a CGI engine block can be fabricated lighter than an aluminum block for equal power densities.
A recent 500cc Suzuki Grand Prix motorcycle engine had a crankcase fabricated from CGI. Nothing is put on these racing machines that would pose any kind of a weight penalty, and this is an extreme example showing the real potential of CGI applications.
Not Just For Engine Blocks
The Aston Martin Vanquish V12 engine block utilizes flywheel and clutch components made from CGI. Mr. Richard Cooke, engineering director at AP Driveline, the tier-level supplier of these components, said, “Compacted Graphite Iron was the only material that could satisfy the performance, durability and manufacturing requirements of this demanding application.”
The bedplate of the 4.7L V8 Jeep Grand Cherokee is made of Enhanced Compacted Graphite (EGC) with even greater nodularity and improved tensile strength. This utilization eliminated a costly 100 percent magnetic-particle inspection procedure, as well as that it produced the “power sound” the market desired. Daros in Sweden also supplies industrial piston rings in CGI for large stationary and marine diesel engines.
Webb Wheel Products offers CGI brake drums for Class 8 highway and heavy-service trucks that are virtually indestructible. During tests, conventional cast iron drums had 0.027 inches of wear, while steel drums had 0.030 inches of wear. The CGI had only 0.003 inches of wear, which is 10 times better while weighing up to 20 percent less.
High titanium content CGI is also commonly used in exhaust manifold and power steering pump parts. Although this alloy is difficult to machine, it has very good thermal conductivity and high-temperature strength.
The Swedish company SinterCast® licenses a foundry technology to accurately produce CGI. This material is being actively scrutinized by all engine OEM’s in the world. Many have already secured their SinterCast®-CGI licenses.
Initial CGI uses in the United States will be in diesel engine blocks. However, gasoline engine block usage will not be too far behind.
All of these uses for engine blocks and components do have some potentially negative effects on the machinability and the production results. In order to achieve production results and per-piece tool costs similar to gray iron, every aspect of the production machinery process must be optimized. Tooling, fixtures, processes, programming and machine platforms must be analyzed for utilization potential.
As the chart below shows, even the same tool used in a different manner will have different power requirements.
Previous Machining Solutions
Prior to 2006, there were no real high-speed machining solutions for CGI. It was best considered the titanium of the ferrous world. It is very speed sensitive, and if that speed limit is exceeded, the tool life will decrease quickly and drastically.
Typical CGI Vc or surface speeds in m/min (SFM), with through-coolant spindle machining technology:
|130 (430) Dry
That means that you will need to use higher insert densities and higher feedrates to achieve the desired gray iron MRR’s (Material Removal Rates). This would normally translate into heavier construction for machines, fixtures and associated components that translates to increased costs for these components.
Tool life for milling and drilling is less affected by CGI material attributes than cylinder boring, or any continuous operations. If milling cutters are selected that are of positive geometry and the right diameter to take advantage of the spindle power curves, then the increase in forces is less of a problem. For example, this includes smaller diameter face mills with higher insert density and greater depth of cut.
Drills are being developed with cutting geometries and special coatings that are designed specifically for CGI to reduce tool wear and enhance cutting forces. Two such companies are Titex (a subsidiary of Sandvik) and Guhring. Balzar has also developed some aluminum/chromium (AlCr) based coatings that may prove to work well in CGI.
Engine block cylinder boring and finishing efforts are challenges regarding tool life versus cycle times. The high insert density and feedrates also raise the thrust requirements. As with milling processes, if all the aspects are optimized then the cutting force penalties are not as severe.
Tools and processes thus had to be designed to deal with the disposal of extra graphite and to gain speed. CGI has been known to hone and grind very well. However, due to its microstructure, CGI has considerably more graphite content in the cutting swarf. Because of the reduced Vc cutting, CGI can take nearly three times as long as similar cast iron with conventional processes.
Makino recently began working with Sandvik Coromant to develop better solutions for cutting CGI.
After extensive tests, the two companies have developed a revolutionary process and tooling that allows CGI cylinders to be finished at near traditional cast iron process times.
The key to this new process is three-fold. The first is an investment in technology in order to change the entire support operation and processing methods. Makino developed such new processes using advanced machinery like the Makino a81M used in these tests.
Makino machines are ideally suited to CGI because these platforms have been engineered to higher limits for heavy cutting operations like CGI. The a81M horizontal machining center spindle’s performance and rigidity is specifically designed for high-power, high-thrust machining and is ideally suited for CGI production machining.
The a81M comes standard with a CAT 50 (HSK 100) taper tool holder, an 8,000 RPM spindle, and a 60 tool automatic tool changer (ATC). The twin pallet, 500mm capacity of the machine is ideal for large automotive production, such as a V8 engine block.
Tooling and Process Steps
Along with utilizing an advanced machine tool is the second step, which is an investment in simple, special carbide tooling from Sandvik Coromant. CBN, ceramic and other special tooling that could enhance CGI cutting speeds cannot be used on the material due to its microstructure and lack of sulfur. This led to the development of new tooling and processes to recover this loss of cutting speed.
The revolutionary Long Edge Tool, which is an engineered solution utilizing serration technology, is ideal for finished mill cylinder boring of CGI due to its ability to produce a complete length of bore in one pass.
The third factor is utilization of a new process jointly developed by Makino and Sandvik Coromant. This new, tested and proven process is based in part on Makino’s patented Flush-Fine machining process, in combination with the machine and tooling technology that allows for the elimination of the semi-finishing boring operation.
The elimination of this semi-finishing step is a substantial reason for the saving of time and enhanced competitiveness of this process in reducing CGI cutting time. Additionally, all special tooling, machine and support costs for this operation are eliminated. The combined quality of the machine and cutting tool provides for this improvement, and allows for operations to proceed smoothly from the rough cuts straight to the finish honing operation.
Project Scope Methodology
Phase 1 in the development of the Makino and Sandvik Coromant Process was to investigate the general function of the helical interpolation process and find a tool concept and cutting data that will give a desired result on the machined bore and cycle time. The concept that was tested was the CoroMill 790.
Two inserts with different corner radii were tested so that the effect of the cutting force direction could be studied. It was pre-determined that the direction of the cutting force would most likely affect the shape of the machined bore.
Phase 2 of the effort was based upon the test results and feedback from Phase 1. A decision was made at this point relative to whether or not to fully optimize both roughing and finishing tooling in line with “real-world” cylinder bore production today, up to the final honing stage.
This phase also considered the process economics, tool life development, overall cycle time calculations based on current production models with GCI material today. A third party (engine manufacturer) was invited into this project to supply actual CGI cylinder block and process capability experience.
Project planning included an evaluation of basic ideas, use of available standard tools and certified CGI test pieces for various designs of experiment (DoE). Work piece conditions prior to machining evaluation and test material specification represented industry standards. This was done to ensure consistent quality of material. Chip evacuation, cutting force, tool bending and vibration characteristics were also controlled and studied.
For the experiment analysis, with 19 test cuts equating to 288 test cuts, DoE variations were used to find the most significant influences and responses from several variables. These included: cutter diameter versus cylinder diameter; radial engagement; axial engagement; ramp angle (a function of axial engagement and cylinder diameter); cutting speed; Hex (chip thickness); table feed (a function of Vc and Fz.); small tool path radii to check the actual feeds; and conventional milling for some critical sets of variables.
Results and Conclusions
The model generated determined that the two most important factors having an affect on the bore diameter are the axial cutting depth and the feed per tooth. Three important factors having an effect on the roundness of the bore are the radial cutting depth, the cutting speed, and the axial cutting depth, where there is significant interaction.
All factors have an affect on the roundness of the bore, such as the radial cutting depth, the cutting speed, the feed per tooth and the axial cutting depth.
The answer to “Why CGI?” is quickly becoming “Why Not?” Continued research and test data constantly being proven out by Makino and Sandvik Coromant will have a tremendous impact on the CGI machining marketplace for years to come.
Since CGI rough machines very well, there is the potential with grinding utilization to eliminate some other semi-finishing operations. Makino is also developing proprietary grinding processes and tooling to take advantage of this aspect. There is some promise for ceramic use in this area, but no viable production solutions exist at this time.
Makino and Sandvik Coromant will continue to keep the production parts industry up-to-date on the latest developments and trends regarding future practical metal cutting developments and the growing potential of compacted graphite iron.
David C. Woodruff, CMfgE, is a Makino Process Development Engineer responsible for CGI research, development and utilization in Mason, Ohio. Wayne Mason is the Senior Specialist - Automotive Engine for Sandvik Coromant. All pictured components noted utilize SinterCast®-CGI. Information sources include: American Foundry Society, www.afsinc.org; DieselForum.org, www.dieselforum.com; Ductile Iron Society, www.ductile.org; Modern Casting, www.moderncasting.com; and Sintercast AB® www.sintercast.com.