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Material Evolution and Revolution

 

New products and processes

 

By Robert B. Aronson
Senior Editor


A lot of materials available now can be called "new." Most are evolutionary changes in well-known materials such as Inconel and ceramics. Others are fresh out of the box, chiefly composites that combine unique matrix and reinforcing materials.

All of these materials present new challenges to manufacturers in that they bring new properties that have to be understood before they can be efficiently processed. The problems come at a couple of levels. Some require new machining techniques because they are harder, gummier, or softer than materials previously experienced. Others present new challenges because government restrictions have forced changes in either the material's use (i.e., the limits on cadmium) or the manufacturing process (i.e., machining without oil).

The first manufacturing group to react to new materials is often the cutting-tool makers. Their initial concern is to match existing products to the new materials, or material application. If not, the next decision is to determine if the potential developing market is large enough to support the R&D needed to create a new product.

At Sandvik Coromant (Fairlawn, NJ) a business development unit works with key industry personnel in early stages of new projects and new concepts of component and material combinations. This activity triggers the company's R&D efforts.

"Within tool materials, metallurgical components can be manipulated to give different properties," explains Roger Grey, grade development specialist. "Typically in tungsten carbide materials the cobalt level has an influence on the final toughness capability of the tool material and can be adjusted to the requirement of the final product development.

"The choice of coating such as MTCVD/PVD, etc., all have specific applications relative to final properties of the tool material. Thicker MTCVD coatings, for instance, as used in GC4005 grade for steels, give superior properties in high-heat machining especially dry machining, whereas PVD coatings have less benefit in this area.

"When materials become more 'gummy' for example, the other factors come into play such as edge treatment and types of coatings. MTCVD coatings are usually used with more edge treatment than, say, PVD where sharper edges can be achieved," says Grey.

"This factor now becomes important particularly in heat-resistant materials and gummy materials where we must use sharper edges to penetrate the workpiece material more efficiently. Sandvik Coromant works with the whole profile of tool materials from tungsten carbide through to PCD, ceramics, and PCBN, and has the R&D resource capability to support continual development of products," concludes Grey.

A big event in the materials market is the dramatic rise in costs. "One of the reasons behind this change is the tremendous 20% growth rate of the Chinese economy," explains Sandvik's John Ridgeway, product manager, stainless bar and hollow bar. "China is now the world's largest consumer of stainless and nickel-based metals. Most of the European producers are sending their materials there. At the same time, the US has lost production capacity and there is little sign there will be major investments in new production facilities. The end result is a shortage that is driving prices up. Conventional carbon steels have doubled in price and it's worse for stainless. It isn't lack of capacity that has driven stainless prices up, but the surcharges on raw materials caused by the large Chinese consumption."

Iscar (Arlington, TX) has the philosophy of starting off all new cutting tool designs with finite-element modeling. "We do a lot of theoretical work before investing in hardware," explains Mike Gadzinski, national training manager. "In each case we look at hardness, tensile, and yield strengths of what is to be cut as well as tool geometry. After that we evaluate thermal conductivity, that has a big influence on the grade of carbide and coating to be used. With low-thermal-conductivity workpieces you can't have a high cutting speed. If that material can't carry away heat in the chip, that heat goes to the cutting tool or workpiece.

"We also do chemical analysis, that influences the tool geometry as well as reactivity with the workpiece. From this information, we can calculate the new tool's performance and recommend optimum cutting speed, depth of cut, and feed rate.

"We don't do these analyses for all requests for cutting tools intended for new materials. For example, researchers come up with a lot of new alloys for aerospace and medical works. But the quantity of material that's actually machined is rather small and does not warrant a major R&D program. More likely, we will be able to meet the new need with an existing or modified product. It's not always necessary to change the tool physically. You might only need to change the operating specs to make it work acceptably.

"Our company is doing a lot of work in carbide coating development needed for hard machining. These coatings are now more common with mold and die work. Hard machining is more of a change in process than in materials because these tools have to take a lot of pressure. There are hardness changes in the mold and die during the process. The material may be initially at RC 32, but it might workharden to RC 52. You have to consider the changes in material.

"Europe and Asia are doing more dry machining and it's becoming more common in the US. So, carbide substrates and coatings have to be able to withstand higher temperatures. Because of the higher pressure, you also need a very sharp cutting tool. Much of this work has to do with metal matrix composites.

"Composites are also more common in the aerospace industry. They are not tough to machine, but are very abrasive. Often this can be handled by a geometry change. We find that simple changes in helix angles have more influence on cutting ability than a change in carbide grade or coating. It's Iscar's philosophy through the years that geometry is a prime consideration, rather than coatings and insert materials," Gadzinski concludes.

Kennametal analysts see two materials growing in popularity for the automotive industry, CGI, and, to some extent, austempered ductile iron (ADI)."The properties of CGI are between gray and ductile iron," explains Mark S. Greenfield, director, materials technology. "Generally, the same cutting tools used on ductile iron will work on CGI, but you have to look at each particular operation. The challenge is not just the tools, but the entire manufacturing system.

 

"ADI has mechanical properties that are better than those of conventional ductile iron cast, forged aluminum, and many cast steels," according to Greenfield. "This material has generally poor machinability, but good strength-to-weight ratio, good ductility, and good wear resistance. However, a modified version (MADI) has the advantages of ADI plus good machinability. It's easier to machine than CGI, has higher tensile strength and, surprisingly, low cost. It has everything going for it.

"As to specific alloys, we are trying to improve the cutting efficiency of machining Inconel 718. Interest in this alloy comes from a demand for more engine power, which means using higher temperature, more difficult-to-cut materials.

"In aerospace, we see more of a shift to composites and titanium. In addition, there is greater use of powder metal parts to take advantage of their near net shape. But they are more abrasive and require engineered cutting tools with tougher substrates and coatings with high abrasion resistance.

"Different applications require tailored edge preparation (degree of cutting-edge sharpness) and this influences the coating choice for that application. For example, PVD coatings work best over sharp edge inserts while CVD coatings are used when honed edges are appropriate," Greenfield concludes.

Metal producers report a number of evolutionary trends in the market. Mostly these are developments that improve existing alloys and fill niche markets.

At Carpenter Technologies (Reading, PA), they have found that the aerospace market is a big driver in materials change as the industry moves from high-strength steel to stainless for both airframes and engines.

Other issues involve environmental regulations. These laws are limiting what can and cannot be used. This is particularly true in Europe where restrictions are often more harsh than in the US. One is designed to stop the use of cadmium plating. This is commonly used on many aircraft parts, particularly landing gear.

The plating is needed for corrosion resistance, but the newer material does not need plating and lasts longer. So despite the higher initial cost, there is a long-term benefit to the user because of reduced maintenance. With new materials, machining takes a little more effort and care, but does not represent any major problems.

Another alloy, a carburizable stainless, is good for bearing applications, particularly for parts exposed to the environment.

In the automotive area, the company is looking primarily at materials for the fuel injection systems. These materials must not only have the necessary strength and machinability but optimum magnetic properties, because the metal is part of the control circuit. These are primarily soft magnetic ferritic stainless.

To meet the need for fuel injection system materials for marine applications there is a new family of free-machining alloys. They are tailored to provide both corrosion resistance and good magnetic performance, and are particularly good in a chlorine environment.

Compressed Graphite Iron (CGI) is one of the few revolutionary materials to come along in a while. This material has been growing in popularity in Europe for some time and is slowly being accepted in the US. One of the most dramatic developments in materials is the acceptance of CGI. Because of its great tensile (at least 75% higher than conventional gray cast iron), and fatigue strength (about double that of conventional gray cast iron), it is apparently an ideal material for diesel engine cylinder blocks and heads. The auto industry has tried for some time to develop an economic, reliable diesel engine for passenger vehicles. CGI seems to be the answer to the ever-increasing demand for performance, durability, and noise reduction.

The advent of this material is coming on the scene when emission regulations are becoming more rigid, and car manufacturers and regulatory bodies alike see the diesel as a possible answer. So far most of the development has been in Europe, but all of the Big Three are linked to European affiliates and can draw on this technology.

Because the material is stronger than conventional cast iron, it is more difficult to machine. But working with CGI is no longer an issue, according to Steve Dawson, president and CEO of SinterCast Inc. (Stockholm, Sweden), the world's leading provider of CGI production technology. "Every major supplier has, or is developing, machining solutions for CGI. There has been a lot of development on the foundry side. Last autumn, Ford and PSA Peugot-Citroen launched production of the first high-volume CGI engine, a 2.7-L, V - 6 diesel that will be used in up to 10 different vehicle models. Many OEMs have announced plans to bring diesels to the US market, and some will rely on CGI cylinder blocks."

According to Dawson, "With its higher strength and stiffness, CGI is the answer to the fatigue and noise issues that have plagued some diesel designs. SinterCast is also developing other nonautomotive CGI applications.

Machine tools are another key element in the successful use of newer, harder materials. One company specializing in this work is Corecron Corp. (Portland, CT). Most of the work is with prototypes and work process development. "Much of this precision is achieved through the use of high-quality machine tools, such as those provided by Yasda Precision America [Elk Grove Village, IL]," says company president Steve Hanson. The toolholder is important too. Accuracy (runout), stiffness, and access geometry are vital. The right combination of equipment provides the needed rigidity, spindle performance, and thermal control we need for working with hard metals and ceramics.

The materials the company works with have an effective hardness and abrasiveness beyond that of any tool steel. "They would have a Rockwell hardness of 70 or 80, if the scale went that far," says Hansen.

Measuring hardness is difficult in polyphase material. Most tests assume relatively homogenous materials. With polyphase or composite material, the reading depends on whether the penetrator hits a hard or a soft area.

"Our cutting tools are chiefly PCB made in-house or by specialty houses. We have typically cut to 0.0001" (0.003mm) accuracy," says Dawson.

"Because of the demands for stronger, tougher, longer-lived materials, I believe that the usefulness of many materials is coming to an end in some areas. The newer materials, particularly the composites, are the key to large-volume production in the future," he concludes.

Materials research is one of the major programs at the Oak Ridge National Laboratory (Oak Ridge, TN). Most of their efforts are in support of Department of Energy programs, but they also have a number of user program projects from US universities and private industry. A typical project would be to provide material characterization such as friction, wear, and machinability information for a proposed MMC brake rotor material. Measurements of basic material properties and studies of molecular structure can also be conducted. If the information developed at the lab becomes published research, there is no fee to the user. If it's proprietary, there is a charge.

As for trends, according to Peter Blau, Surface Processing and Mechanics Group, Metals and Ceramics Div., "Currently, materials related to lighter weight and more fuel-efficient vehicles are the main focus. We look at issues such as machining, fabrication, and finishing. Tribology of a material is also important for applications involving friction, wear, and lubrication.

"I believe there has been a major shift in research emphasis from ceramics to composites--both metal and polymer-matrix types. For ceramic machining R&D, oil-based coolants are often best, but that coolant is falling from favor, chiefly because of the cost of handling and disposal."

Much of the work is not related to conventional machining. "Processes are changing. Because of the interest in near-net shape forming to avoid machining, there is more effort going into finishing," Blau says.

"One of our labs is working on a new process that uses high-intensity infrared heating to melt, form, and heat-treat metal. The heating is so rapid there is little influence on the properties of the underlying material, despite the intense heat. It can reduce a sheet of titanium to liquid in a few seconds.

"Another area of interest for us is selective coating and surface treating. There are two ideas. One is to coat or surface-treat low-grade material. You get the surface you want without making the part from costly materials. The second idea is to coat only those limited areas that need the wear resistance or other protection. For example, on a brake rotor you don't need wear resistance on the whole part, just the friction surface.

"Another novel idea we are looking at to improve the joining of sheet material is friction drilling. In this process you use a spinning tool to heat and puncture the metal, usually a thin section. This action extrudes the metal, creating a sleeve or boss. That means there is more area to tap and hold a threaded fastener.

"One potential trend is more computer modeling of machining processes such as chip formation. The work is just getting underway at ORNL, but could save a lot of time and trial-and-error work if reliable models can be developed and validated," Blau concludes.

 

This article was first published in the July 2004 edition of Manufacturing Engineering magazine. 


Published Date : 7/1/2004

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