The difficulty of machining and consequently costs are highly dependent on material properties. And let’s look at a few “special” metals that are challenging to machine and understand the properties of the materials that make it hard to machine, increasing the machining costs. All the materials are compared relative to Aluminium and Brass because they are among the easiest materials to machine, which are common too.
Inconel is actually a trademarked word of Special Metals Corporation and it refers to a branch of high nickel superalloys that are resilient to corrosion and oxidation at elevated temperatures and has a high-temperature coefficient of strength. It is predominantly used in the aerospace industry. Machining Inconel is extremely hard because it does not get any softer when the temperature rises. Cold working increases its hardness, making it difficult to machine, whereas metals like stainless steel get softer when the temperature rises, making it easier to cut. When we machine Inconel, its hardness increases non-linearly and wearing the tool disproportionately. So, the cutting tool must be inspected often for excessive wear to ensure quality. The best way to machine Inconel is to start with a “solutionized” metal. Solutionizing is a heat treatment process that involves heating the material beyond the solvus temperature and soaked until a homogenized solid-solution is formed. It reduces the toughness and the amount of work hardening happening during the process, increasing the tool life. It is also advisable to use Ceramic tools with an aggressive depth of cut but slowly when machining Inconel because it reduces the amount of heat produced in the process. Similarly, peck drilling (repeatedly drilling and retracting the drilling tool) must be avoided for the same reason as above. For cutting plates, water jet cutting does a good job because it doesn’t generate heat.
Kovar is a special nickel-cobalt ferrous alloy that is designed to have the same thermal coefficient of expansion as glass. It is referred to as a controlled expansion alloy. It is initially developed for sealing electronic components like vacuum tubes, CRTs, and light bulbs. Normal metals don’t work for this purpose because they expand and contract quite readily with changes in temperature and breaking the seal. Whereas Kovar, being a controlled expansion ally, expands at the same rate as glass, creating a tight seal. Today, it has found profound usage in medical and electronic equipment that require tight metal to glass seals.
Like most Nickel alloys, Kovar also exhibits work hardening when machining, slowing the machining process, and even warping the work. So, it has to be mounted firmly. Sulphurised mineral oils must be used as a coolant when machining it. Kovar also tends to be gummy when machining and tools will just plow through rather than chipping the metal away. So, the feed rate of the tool must be slower. Since a sulfur-based coolant is used, it must be washed properly after the machining is done since it can affect the performance of the electronics where Kovar is generally used.
Invar is an extremely low thermal expansion nickel-ferrous alloy. It differs from Kovar in that it is a controlled expansion metal (designed to have the same coefficient of thermal expansion as borosilicate glass), whereas Invar is an extremely low coefficient of thermal expansion metal. It is used to make extreme precision equipment like scientific laboratory equipment, gauges, clocks, stencils, etc.
Invar does not get hardened with temperature, but only with cold working. So, the hardness increase during machining is relatively lower. But it does not make the material much easier to machine. It is softer than Kovar and thus it does not chip off. The tool plunges through. So, care must be exercised when machining. The chips coming off will also get stuck in the tool. So, they must be inspected often, as it can generate heat quite readily. There is also a Free-cut variation of Invar which is much easier to cut, and can improve productivity by up to 250 % compared to normal Invar. So, it should be used whenever possible.
Magnesium is among the easiest material to machine – the least cutting force of any structural metal, so it can reduce machining time and energy consumption -, but the complexity with magnesium comes up with its combustible nature. Despite that, magnesium is the lightest structural metal there is. So, if proper machining is done, machining magnesium can be highly beneficial. The chips and dust generated from machining are extremely combustible and they must be handled with care. As a precaution, Class D fire extinguisher should always be present close to the machine; water and nitrogen won’t extinguish magnesium fires. So consequently, water-based coolants cannot be used too. Compressed air or mineral oil coolants must be used. Blunt tools can rub and cause friction, which can increase the fire risk. So sharp tools must be maintained. Tight clearance angles with the work also must be avoided, as they can cause chips to get stuck and might result in combustion.
Titanium is increasingly replacing aluminium as the most popular for aerospace applications, and it is not unwarranted. Titanium’s high strength to weight ratio is one of the reasons for this. But it is one of those properties that make Titanium difficult to machine.
Titanium is not as rigid as steel, so it needs to be secured in an extremely rigid way to reduce the vibrations. Titanium’s low Young’s modulus causes spring back and chatter during machining. So, Titanium is usually machined with low feeds. Titanium is also sticky, making it stick to the cutting tool generating excessive heat. So, machining titanium requires carbide coated tools that will resist sticking and break it into chips. We also need high-pressure cooling to keep the work temperatures low. A technique called climb milling is proven to be effective when machining Titanium as it increases the tool life.
Copper is renowned for its thermal and electricity conducting properties, and it has made copper and its alloys indispensable for electric applications. Copper is relatively easier to machine compared to all the materials above. But its high ductility makes it challenging. Because of that, the small chips that break away won’t form, instead, it forms long tubular chips form. It increases the temperature of the work. So efficient cooling is required. There is also a risk of a build-up edge in the tool that can deteriorate the surface finish.
Hardened Steel Machining
When machining HRC 50 or 60 hardened steel, we have to take a lot of care because it is quite easy to get it wrong. Tools selection, machine set-up and feeds are the most crucial factors when machining hardened steel. One cannot use normal grade carbide or HSS for machining hardened steel. You have to go for superior grade ultra-fine carbide tools. The coating of that tool is an also important consideration, because normal coatings like TiCN won’t be effective above 800 degrees. The feed rate must also be slow and constant to improve the tool life. Proper cooling must also be provided as it may work harden the already hardened steel, thereby further deteriorating the tool life. We, at Custiv, have hundreds of vendors who specialize in machining special exotic materials and we can do it in the fastest lead time in the market with competitive costs. If you are in a dilemma in choosing the material because of its machining costs, you can request a quote to understand how materials can impact the machining costs.