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Abrasive cuts

Waterjet cutting addresses some of the issues encountered with titanium

Titanium is the preferred material for components in a variety of demanding applications. Due to its light weight, strength and hardness over traditional materials like steel as well as its beneficial properties such as corrosion resistance, titanium is the material of choice for many industries including aerospace, defense and medical. However, machining titanium is known for being a very slow and often costly process. There is a solution that makes titanium cutting faster, easier and more cost-effective: abrasive waterjet.

A 1-in.-thick part with a 40-in. cut path is shown in Omax’s IntelliMax Make controller software. Cutting the part in aluminum on a waterjet with a 50-hp pump takes about 5 min., and in steel, the same part requires 15 min. The part in titanium only takes 10 min. and is a stronger option than the other two.

“Titanium is notorious for wearing out ‘hard’ tools,” says Vlad Bucur, applications lab supervisor for Omax Corp. “Waterjet erodes the material and there is no tool wear in the classic sense. The only wear is on the mixing tube, which remains constant due to the constant abrasive usage. The preferred approach to incorporating waterjet in titanium manufacturing is to near net shape cut a part on the waterjet machine and then complete the part using a mill. This way, the shop doesn’t waste as much time or the cutting tools needed to mill down the titanium.

“Near net shape cutting titanium with a plasma or laser cutter heats up and hardens the edges,” he continues, “adding costly procedures to correct.”

For decades, aluminum has been the go-to material for aerospace. The shiny aluminum alloy passenger and military aircraft of the 1950s became iconic to America’s adventurous spirit. The 1960s ushered in the production of second-generation jet fighter military aircraft, such as the SR-71 Blackbird and the XB-70 Bomber, as a result of technological advances in aerodynamics, propulsion and aluminum alloys. Aluminum also became known as “space metal” because it was credited in large part for the success of the 1969 Apollo Lunar Eagle landing on the moon.

Today, the metal is lightweight, abundant and relatively cheap. Yet, as air travel has advanced, aluminum hasn’t been able to maintain its industry foothold. The pressure and beating aircraft take at high speeds requires much more aluminum to be used to maintain rigidity and form. With more material comes more weight, and because flight is a fight against gravity, adding more weight isn’t practical.

That’s where titanium comes in. According to Bucur, titanium is heavier than aluminum by volume. It’s also about twice the strength of aluminum (depending on grade) so less can be used and still achieve the same result. “It is a light, durable material perfectly suited for airplanes and satellites.”

The medical industry is looking into future uses for waterjet-machined titanium, including the production of heart stents.

Near net cutting

Manufacturing titanium parts for the aerospace industry does have its challenges, and it’s not cheap. Given the benefits as well as the limitations of waterjet technology, near net shape cutting on a waterjet machine helps reduce the cost associated with machining titanium.

“The waterjet process avoids imposing a heat-affected zone upon the material when processing titanium near net shapes,” says Dave Smith, director at ThyssenKrupp Aerospace Waterjet. “Unlike other near net processes, it does not affect the metal properties. Heat-affected zones mean more metal use, which results in increased cost. The ability to nest parts saves in the total metal used per part, both in scrap and metal per unit. Titanium is very expensive and the metal savings drives our investment into waterjet.”

To perform the near net shape cutting, aerospace shops cut a component’s general shape from the raw material using a waterjet machine equipped with the nesting software option and then perform finishing operations on a conventional CNC machine tool. Processing aerospace parts in this fashion eliminates extensive material removal operations, which, in turn, reduces overall part cycle times, tool wear or breakage, and material waste. With the release of several auxiliary devices such as the Omax Rotary Axis and A-Jet, waterjet technology has expanded from 2-D to 3-D machining.

Another benefit of cutting titanium with waterjet is the excess material is solid and not in chip form. The solid titanium can be sold back or recycled for much more than chips. On top of that, when cutting titanium with waterjet, it makes some pretty cool sparks.

Medical cutting

Titanium has been used in the medical industry since the 1950s – and maybe even earlier than that in some experimental dentistry. It’s considered the most biocompatible metal because of its corrosion-resistant, bioinert and high-wear properties. Titanium is found in bone conduction hearing aids, spinal fusion cages and hip and knee replacements. The metal’s ability to withstand harsh biological environments is a result of the protective oxide film that forms naturally in the presence of oxygen.

The preferred approach to incorporating waterjet in titanium manufacturing is to near net a part on the waterjet machine and then complete the part using a mill.

Though titanium has been used for nearly 70 years for medical applications, the manufacturing of prosthetics and other devices has long been relatively cumbersome. As discussed before, traditional mills are not the fastest or most effective at machining titanium.

“A majority of prosthetic and orthopedic components are made of titanium because of its physical qualities of high strength, durability, low density, corrosion resistance and, most importantly, biological compatibility,” says Dr. Peter Liu, senior scientist, Omax. “While there are considerable challenges to micro machining titanium with contact tools, abrasive waterjet cuts titanium fast and effortlessly. The advancement of micro cutting in abrasive waterjets has greatly sped up the process, allowing for on-demand fabrication.”

And on-demand production is one of the big advantages to using waterjet to fabricate medical prosthetics. If a patient needs a replacement knee or other bone, the speed in which waterjet can produce the part allows for an exact replacement in a short period of time.

The medical industry is looking into future uses for waterjet-machined titanium, including fabricating heart stents, which are small mesh tubes used for treating narrow or weak arteries that improve blood flow and help prevent arteries from bursting.

Whether processing starts with plate or sheet, titanium is faster and easier to cut and is stronger than aluminum or steel.

“Medical manufacturing shops are most drawn to abrasive waterjet technology because it’s material independent,” Dr. Liu says. “Shops making orthopedic implants from titanium can safely and effortlessly cut these materials without fear of inducing thermal damage or somehow adversely modifying the integrity of a medical component’s parent material. Plus, abrasive waterjet systems cut tough-to-machine, exotic and high-strength material much faster than other conventional machining methods.”

Omax Corp.