In recent months, there have been reports on the discovery of a super heavy new element tentatively named element 117. While verifications are still ongoing, people are already speculating the potential impact it will bring to science and the industry. As we wait to see if element 117 makes it to the periodic table and join the likes of exotic-sounding elements such as Francium and Rubidium, the metalworking industry continues to deal with its existing list of exotic materials.

There are many definitions of exotic materials in metalworking and truth be told, an ‘exotic’ material in job shop A could well be a ‘normal’ material in job shop B. In addition, thanks to technical advancements, what was exotic a few years back could be nothing more than a material that requires special care now during machining. 

What’s So Difficult?

“Exotic materials are used in harsh environments, in high pressure, high temperature and in corrosive conditions. Inconel 718 and titanium alloys are some of the examples,” said Dr Liu Kui, a scientist specialising in machining technology at SIMTech. “In Singapore, majority of these materials are used for the aerospace and oil and gas industry applications.”

From the laser cutting point of view however, the definition of exotic materials can be very different indeed. 

“Exotic materials may also include those that are very difficult to machine with conventional methods such as surface hardened glasses and ceramics,” said Dr Lim Gnian Cher, principal scientist in machining technology, also from SIMTech. 

Working on collaborative industrial projects, the duo has amassed a wealth of experience on working with difficult materials. 

Dr Liu has worked with a group of companies from the oil and gas industry and the project involved deep hole drilling of Inconel 718. “This material has high yield strength of above 150 ksi. When carrying out deep hole drilling, tool wear is very rapid. There are also some instances where we suffer catastrophic failure of the cutting edge. Another challenge is that the temperature in the cutting region is quite high,” he said. 

According to him, the tool failure problem can be attributed to the work hardening nature found in Inconel 718.

Dr Liu (left) and Dr Lim

“When you cut the material, it hardens. Coupling that with its low thermal conductivity, heat generated in the cutting region will be more on the tool edge, resulting in higher cutting edge temperature. This will degrade the cutting tool hardness and cause those failures.” He believes as a direct result of the material’s nature, it takes about 20 times longer to drill Inconel 718 with high aspect ratio than common harden steel.

Like machining, heat is also a delicate issue in laser cutting. According to Dr Lim, cutting hard and brittle materials like Sapphire and Silicon Carbide can be tough, be it for drilling, cutting and slicing.

“In the cutting operation, some materials will redeposit, causing contamination. In certain applications, especially in the microelectronic industry, you really do not want that. On the other hand, surface quality and thermal issues are concerns,” he said.

The introduction of heat in particular is a risky situation that cannot be eradicated. As such, operators have to walk a fine line. 

“All materials are sensitive to high temperature damage but some are not as sensitive. Sometimes the issue is not the material itself, it is in the devices. As a result, we have to consider the whole system or device. One example is the semiconductor device where integrated circuitries are fabricated on Silicon and more recently on Sapphire and Silicon Carbide. As there are very sensitive microelectronic circuitries present, we have to see how to reduce or minimise heat retention on the substrate,” he said.   

Getting Around It

Thermal degradation seems to be the main mechanism for shorter than expected tool life and in order to nip the problem in its bud, most solutions or strategies used to tackle difficult materials are designed to reduce heat at the cutting zone. 

“For common Inconel 718 machining, we apply high-pressure coolant. In the market, they are 70 bar high-pressure coolant. This can help prolong tool life by maybe two times. The second potential solution is by using cryogenic (liquids) to cool the area, reducing heat in order to increase productivity,” said Dr Liu.

He revealed studies are still being carried out at the institute to optimise the coolant delivery system and process parameters. He added that there is a technology that combines ECM and EDM and it involves applying high-pressure dielectric on the finishing area for material removal. Another potential solution is using harder tool materials like PCBN. 

If there are reasons to prevent the adoption of high-pressure coolant or other technologies, Dr Liu said that by using lower cutting speed, operators can prolong tool life when cutting Inconel. 

Thermal degradation may well be the biggest obstacle to overcome when cutting ceramic glasses with laser.

In laser cutting, things happen quickly and are often invisible to the naked eye. As such, it is important to know the mechanisms behind the cutting process. 

“Laser cutting of exotic materials relies on heat to a great extent. The laser beam energy is concentrated on a small spot, raising the temperature to the point of vapourisation. To avoid the problems caused by heat, the solution is to make sure as little heat is retained by the material (as possible). The way to do it is to use a short pulse laser beam. The shorter it is, the more efficient the materials will absorb the heat energy in order for the temperature to rise to the melting and decomposition range very quickly, allowing very little or any heat (at all) to retain on the material (and for it) to be conducted away,” said Dr Lim. 

Extremely short pulse lasers are now available to the industry users. For a femtosecond laser that produces pulses less than 200 fs each, the laser light travels a distance of less than the diameter of a strand of hair. When each pulse hits the material, the energy absorbed stays within the atomic range of the material because it takes longer than 200 fs for heat to pass from one atom to another.  

At the end of the day, using short laser pulse rate would help mitigate the thermal issue and the quality of the part would be better too, according to him. 

Plan Ahead

Before going on a vacation to an exotic location, many will spend some time planning for their holiday before they hop on a plane. The same mentality will serve operators well when they find themselves with an exotic material to cut because knowledge is often the sharpest tool in the shed.

“Whether you are cutting glass ceramic or any other material, it is important to understand the properties of the materials that you are cutting. Laser cutting depends on a large number of factors. There are some questions to answer: How does the material absorb the laser beam? Glass is transparent to most laser beams but yet you can use transparent laser beam to cut glass. Why? You also have to understand the property of the laser itself,” said Dr Lim.

Armed with the theoretical knowledge and technical know-how, metalworkers should be able to deal with exotic materials in a normal way and await fresh challenges posed by a new generation of exotic materials that will no doubt find their way into a machine tool before we know it.