Hot Properties

Recent trends in manufacturing have turned up the heat at the cutting edge. Manufacturers are pushing their tools to cut faster. They are also trying to limit or eliminate the use of coolant at the cutting interface, and working with materials that are more challenging to machine. The high temperatures generated in high-speed machining, dry machining, and applications involving difficult-to-machine materials can accelerate tool wear. In turn, the poor tool life can result in more frequent tool changes, increased tool inventory, longer machine downtime, and lower throughput.

Some coatings can extend tool life by providing a protective layer that insulates the cutting edge from extreme temperatures. But not all coatings can provide this protection. Tools coated with titanium nitride (TiN) or titanium carbonitride (TiCN) typically don't last longer than uncoated tools in high-temperature cutting operations. Both of these physical-vapor-deposition (PVD) coatings have relatively low oxidation temperatures. When exposed to high heat, they transform into brittle oxides that do not resist the wear caused by cutting forces.

Titanium aluminum nitride (TiAlN) has proven more effective than TiN or TiCN in protecting tools from heat, because it has a higher oxidation temperature (1450° F). TiAlN can withstand higher temperatures without breaking down and degrading the tool substrate. Furthermore, when TiAlN's oxidation threshold is exceeded, its outer surface transforms into aluminum oxide, which has excellent hot hardness, low thermal conductivity, and high chemical stability. As a result, most of the heat generated by the cutting operation flows into the chips rather than the cutting tool. Furthermore, TiAlN provides the increased lubricity typical of PVD coatings.

Harder Is Better
Every coating manufacturer currently offers an aluminum-base TiN coating. Multi-Arc Inc., Rockaway, NJ, introduced the first TiAlN coating in North America in 1991. It was a graded, titanium-rich, mono-layer coating with a hardness of 2600 HV (micro-Vickers). Since then, the company has developed a TiAlN coating with increased aluminum content and a hardness of 4500 HV. Although the original coating and the harder coating have similar oxidation and surface-roughness properties, the harder coating has higher wear resistance and better thermal and chemical stability as the aluminum oxidizes. The harder TiAlN coating also has a lower friction coefficient that facilitates chip evacuation and maintains low, stable cutting forces. Technological advancements have allowed both TiAlN coatings to be deposited with adhesion levels comparable to those for TiN and TiCN exceeding 70 Newtons in normal-force scratch testing.

Without understanding the advantages of TiAlN over TiN and TiCN in high-temperature applications, a shop may be hesitant to pay the higher price for the premium coating. Table 1 demonstrates the merits of TiAlN by comparing its properties with those of TiN and TiCN. TiN's hardness is typically around 3000 HV, with an oxidation temperature near 950° F. TiCN has a higher hardness, approximately 4000 HV, but it suffers from a low oxidation temperature of 750° F. This makes the use of TiCN difficult without other means to manage the heat produced during high-speed machining, dry machining, and machining difficult materials.

TiN and TiCN may perform just as well as TiAlN in cutting processes that do not generate excessive heat. Therefore, a shop may not be justified in paying extra for the TiAlN coating for these applications. However, the properties of TiAlN make it the only choice for high-temperature operations. TiAlN's thermal and chemical stability allows tools to be run at higher speeds and feeds than TiN- or TiCN-coated tools, and it enables certain processes to be run dry. TiAlN's properties also make it suitable for machining a broad range of materials, including heat-resistant alloys, nickel- and cobalt-base alloys, titanium, stainless steels, tool and die steels, cast iron (gray, nodular, and austempered), heat-treated steels, and die molds.

A Variety of Uses
Many OEM tool manufacturers and tool users have achieved excellent results with TiAlN-coated tools. The coating has reportedly enabled tools to last two to five times longer than uncoated, TiN-coated, and TiCN-coated tools. Here are a few examples of TiAlN's performance in a variety of high-temperature cutting operations.

In the machining of titanium alloys, many users have reaped the benefits of TiAlN coatings by increasing production levels in endmilling and turning applications. This has been achieved mainly by increasing the cutting speed to between 400 and 500 sfm, which is significantly higher than the conventional range of 100 to 250 sfm for uncoated, TiN-coated, or TiCN-coated tools. This higher speed, combined with longer tool life, has helped to reduce overall production costs.

Endmilling titanium. One company that has achieved this level of performance is Pratt & Whitney. The manufacturer's East Hartford, CT, plant makes hollow fan blades for aircraft engines used to power the Boeing 777-300. The titanium 6A1-4V blade has a dovetail form that is milled at the ends. After experimenting with several styles of endmills and different substrate materials, Pratt & Whitney selected a serrated-tooth, uncoated carbide endmill. The tools were run at a speed of 300 rpm and feed rates of 2.5 ipm for the roughing pass and 3.0 ipm for the finishing pass. At these cutting parameters, average endmill life was five to 10 parts.

Looking to improve tool life and reduce machining time, Pratt & Whitney tested two TiAlN coatings with different hardnesses. The coating with the lower hardness enabled the endmill to cut 22 parts while running at the same speed and feed as the uncoated tool. For the endmills with the harder TiAlN coating, Pratt & Whitney increased the speed 33% to 400 rpm and increased the feed rates to 3.0 ipm for the roughing cut and 3.5 ipm for the finishing cut. Even though these higher cutting parameters produced more heat, the tool with the harder coating cut 28 parts, 30% more than the previous tool and almost three times more than the uncoated tool. Pratt & Whitney hopes to achieve even better performance with its TiAlN-coated tools. Using endmills coated with the harder TiAlN, the company is looking to increase the speed of the finishing cut to 600 rpm and the feed rates to between 4.5 and 5.0 ipm.

Facemilling titanium. In another application involving an aerospace titanium alloy, a pump body was being facemilled with standard 15°-rake, TiCN-coated inserts. TiAlN-coated inserts were used on a 2"-dia. milling cutter with a 45° lead angle and five teeth. Cutting conditions were 200 sfm, 3.0 ipm, and 0.100" depth of cut, with coolant. Full-diameter cuts were taken up to a 45° shoulder. The TiAlN-coated inserts lasted 3 1/2 times longer than the TiCN-coated inserts being supplied by a tool vendor.

Shops have used TiAlN-coated tools to cut other materials in addition to titanium and in applications other than milling. The switch to TiAlN resulted in higher productivity for these manufacturers as well.

Broaching Inconel. One aerospace manufacturer tested the high-hardness TiAlN coating in the broaching of Inconel 718. Typically, this material was cut with uncoated tools, with an average broach life of 10 to 12 parts. After TiAlN was applied to the surface of the tool, broach life was doubled, with an average of 23 parts per grind. In addition to minimizing the flank wear, the coating made the wear pattern more predictable. The results were less downtime, reduced tooling investment, and lower overall part cost. And the manufacturer could continue to regrind its TiAlN-coated tools, recoating them after sharpening. The predictable wear pattern of the TiAlN-coated tools meant that the manufacturer could schedule a tool's regrinding before the tool suffered significant damage in most instances.

Drilling aluminum. Although coatings are not typically used in the machining of aluminum, TiAlN was used in the drilling of 10% silicon aluminum to provide added abrasion resistance. In this application, TiAlN-coated solid-carbide drills produced more than three times the number of holes that uncoated drills did. Because the operation was on a transfer line, the user could not increase the cutting parameters on one machine without overloading machines down the line, but the improvement in tool life justified the cost of the TiAlN coating. In another application on the same transfer line, TiAlN-coated indexable drills running at more than 1700 sfm lasted more than twice as long as uncoated drills, reducing transfer-line downtime.

Dry hobbing steel. With coolant disposal becoming more complex and costly, an increasing number of shops are pursuing dry machining. An automotive-transmission manufacturer has been investigating the dry hobbing of gears made of medium-carbon steel. The TiAlN-coated carbide hobs allowed the company to machine parts dry at increased cutting parameters and still hob nearly as many parts per regrind throughout the regrindable life of the hob as it did with conventional methods using coolant.
With continued success in a variety of operations, TiAlN is expanding the realm in which cutting tools are applied in today's manufacturing environment. Higher productivity, reduced costs, and environmental liability are growing concerns in manufacturing. While addressing these issues, TiAlN has demonstrated the ability to run faster, longer, and cooler in repeated applications. Through a combination of properties, including thermal stability and a high hot hardness, TiAlN coatings open new opportunities for manufacturing.

About the Authors
John Earnhardt is general manager, northeast regional coating center, Multi-Arc Inc., Rockaway, NJ. Raymond May is general manager, Multi-Arc de Mexico, Queretaro, Mexico.