Metal Fabrication: A Guide to Manufacturing Metal Parts
Exploring material selection and manufacturing techniques for metal prototypes and production parts
Next time you take a hike, look down. It is not just dirt and dust. Almost every material comes from the ground, locked up in ores and minerals.
I expect you remember the Periodic Table well. But it is worth remembering that these simple elements make up all of the complexity around us. Just 94 are naturally occurring, and of those only 9 are 'abundant' in the earth's crust making up over 99% of its mass.
First up oxygen (46%) unless we’re short of breath after a brisk walk, most of us take it for granted. It's favourite partner is Hydrogen, and H₂O must be the most memorable chemical formula (Water!). Surprisingly its far from an equal partnership Hydrogen makes up less than 0.14% of the earth's crust by mass. Having just one proton makes the element the lightest, which explains it's low % by mass; it is both under-counted and will escape gravity if free to do so.
Oxygen makes up 21% of our atmosphere. So where is the rest of it? Mostly in those ores, metals make up 6 of the abundant 9 but they only occur naturally in an oxidised form (aluminium 8.3%, iron 5.6%, sodium 2.5%, magnesium 2.4%, potassium 2.0%, and titanium 0.61%).
They are the building blocks of modern society. Without the raw materials trapped in the earth’s crust, and the technology to extract and process those minerals into various alloys, humans would still be using stone, wood and bone.
Protolabs offer you a wide range of metals for manufacturing, characterised by 'hard' and 'soft' alloys. Hard alloys have a harder and tougher structure, which resists wear and impact damage. They are also 'Hard' to machine, because they are tough, even harder materials are needed to cut them (normally tungsten carbide). Soft alloys are much more malleable and so easier and quicker to cut.
Soft Metals: Alloys containing Aluminium, Magnesium, tin, and Copper. Such as aluminium alloys and brass
Pure aluminium is soft and malleable, making it a poor candidate for mechanical purposes. Instead, aluminium is usually alloyed with a mixture of other elements, including silicon, copper, magnesium, and zinc, then heat-treated to make the strong, lightweight materials used today in airframes, automobiles, and various consumer products.
Protolabs’ CNC machining service holds stock of five types of aluminium: 2024-T351, 5083-H111, 6082-T651, 7075-T651 and 7075-T7351.
The 4-digit number is the International Alloy Designation System, 2000 use copper as their primary alloying element, 5000 magnesium, 6000 both magnesium and silicon and 7000 zinc.
The suffix denotes how the material has been heat treated. T denotes solution heat treated (or tempered), H strain hardened (or work hardened). T6 is Solution heat treated then artificially aged to minimise residual stress, thus making it more consistent and stable when machined.
6082 aluminium is alloyed with magnesium and manganese, and offers a yield strength of 250 MPa. It is very corrosion resistant and weldable with the proper equipment, making it an ideal choice for low-fatigue applications such as structural components in machinery, hydraulic valve bodies, marine, and automotive parts, and almost any application requiring robust, lightweight material.
A stronger option 7075 aluminium. Harder and stronger than 6082, it offers a yield strength of over 500 MPa, twice that of 6082, but at a higher cost. Its primary alloying elements are zinc, magnesium, and copper. Most modern aircraft use 7075 in areas of compressive load, for example wing spars. It’s high toughness, strength. In fact, the only place where 6082 wins out is in corrosion resistance, and in parts that need a little more “give” than those made of 7075. Both materials offer easy machining, although 7075 is a bit abrasive.
Copper and its alloys like brass, are very versatile. With the exception of environments high in ammonia and some acids, they are extremely weather and corrosion resistant. If you’ve ever replaced a car radiator, soldered a kitchen faucet, or played the French horn, you’ve handled parts made of brass.
Protolabs Europe offers brass, both Cz121 | C360 (free cutting brass) and Cz112 | C464 (naval brass).
Cz121 | C360 has 36% zinc and 3% lead and is the most general purpose of all the brass alloys. A but like pewter the lead remains insoluble in the microstructure of the brass, but it helps lubricate, hence the name "free cutting"
Cz112 | C464 is well suited to marine applications, it is also one third zinc, but an important addition of just 1% tin gives improved corrosion resistance, and a harder and stronger duplex structure.
Copper alloys in the form of bronze and brass have been about a long time, and there are dozens of grades each with subtle differences and distinct uses. Softer brass suitable for rivets and screws, Muntz metal (also known as yellow metal), invented for lining the bottom of boats now used to create striking architecture.
To a machinist, brass is as easy tool life exceptional, and feed rates quite high. However, brass is sturdy stuff, offering tensile strength rivalling that of mild steel.
Even though it’s the primary element in brass, copper is a different story. Pure copper’s machinability is roughly five times worse. Chips don't break, due to copper’s malleable and stringy nature. The material heats up very quickly during cutting, due to its high thermal conductivity.
Copper is only second to silver in electrical conductivity, a factor that makes it one of the most important metals in use today. Copper (and some aluminium) wiring basically make electricity possible. Without it, lights would remain unlit, cars wouldn’t run and it would be impossible to read this article on-line.
Copper is easy to braze but difficult to weld. Its extreme ductility makes it both strong and flexible, a rare occurrence among metals. Yet copper does far more than conducting the power needed to heat our grills. It’s used in semiconductor manufacturing as an element of high-temperature superconducting, in glass-to-metal seals such as those needed for vacuum tubes and has even been approved by the for use in hospitals and public places as an antimicrobial surface.
Because elemental copper exists in nature, people first started pounding it into coins and cutlery millennia ago. Today, it’s an ingredient in more than 570 different metallic alloys. Copper can also be used for electrodes in electrical discharge machining (EDM), a technology often seen in injection moulding and metal stamping.
Hard Metals: Alloys containng steel, Chromium, Nickel, and Titanium. Such as tool steels, stainless steels, Inconel and 17-4 PH.
The world needs hard metals as well. Steel is used in almost everything, cars, ships, bridges. Regardless of alloy type, steel is mostly composed of iron. Our UK manufacturing facility is located near Ironbridge, built in 1779, It was the first structure in the world made from cast iron. Iron smelting has been around a long time, but it wasn’t until the Bessemer steel process, invented in the mid-1800s, that mass production of high-quality steel was made possible.
As with the soft metals, a small quantity of alloying elements can have a dramatic effect on steel’s properties, it is just a fraction of a percent of carbon that changes iron into steel.
Protolabs machine Mild Steel S275JR, which also contains 1.6% manganese and a little silicon.
Carbon Steel EN8 has a little more carbon 0.4% making it much harder.
There is a solution, in 1913 stainless steel was invented. The 304 and 316 stainless steels offered by Protolabs carry at least 20 percent chromium along with a fair amount of nickel, making them more difficult to machine. Still, these popular materials are commonly used for medical instruments, vacuum and pressure vessels, and for food and beverage equipment. 300-series stainless is quite tough but cannot be hardened like carbon steel. If hardness is a requirement for your application, consider 17-4 PH, with 17% chromium and 4% nickel the alloy can be Precipitation Hardened (hence the PH).
This versatile but very tough material is part of the stainless-steel family, but its machinability in the annealed state approaches superalloy status. When heat treated, it easily achieves hardness of 25 Rockwell C (HRC) and in combination with nitride hardening 67 HRC. An ultimate tensile strength of 1,200 MPa or higher, three times higher than most steels. It is most used in the medical, aerospace, and nuclear industries, or anywhere a combination of high strength and good corrosion resistance is needed.
Stainless steel is widely used in the chemical industry, textile processing, and for marine applications. Many stainless steels are temperature resistant as well and are able to withstand temperatures over of 900 °C (intermittently). Hot enough to turn aluminium, brass and copper into molten puddles. 316 stainless, for example, is excellent for heat exchangers, and sees regular use in steam turbines and exhaust manifolds.
If you’re looking for some truly robust alloys, look no further than cobalt chrome and Inconel. Protolabs doesn’t machine these materials, but its 3D printing service is happy to fuse them into solid objects for you using the Laser Powder Bed Fusion process (LPBF or DMLS). Each material has unique, high-performance properties.
Inconel contains 50 percent or more of nickel, giving it excellent strength at a range of temperatures. It’s used for extreme demands such as gas turbine blades, jet engine compressor discs, and even nuclear reactors and jet engine combustion chambers. The high nickel content makes Inconel one of the most difficult materials to a machine, requiring wear resistant coated carbide and a rigid machine tool. Sitting right next to nickel on the periodic table is cobalt, the main ingredient in cobalt chrome alloy. This material is known for superb wear resistance and human biocompatibility, making it ideal for dental implants, hip and knee replacements, and arterial stents.
Finally, there’s titanium. This lightweight element is alloyed with aluminium and vanadium, providing a strong, corrosion-resistant material (the most common is Grade 5 Ti6Al4V). Like cobalt chrome, titanium is biocompatible and is used extensively for bone screws, pins, and plates. Its tensile strength is more than twice that of mild steel but weighs just half as much. This makes titanium appealing to the aerospace industry and high-performance vehicle manufacturers.
CNC Machining: The Foundation of Metal Manufacturing
Metallurgy is the foundation of metal manufacturing, a dozen or so raw elements provide for hundreds of important, life enhancing materials.
But none of these metals would be particularly useful without the means to accurately shape them. The key process is machining, which evolved with steel processing. Over the past 150 years, machine tools have gone from crude steam driven devices to the high-tech, ultra-precise, computer numerical control (CNC) equipment of today.
Protolabs provide you access to several hundred machine tools, ready to machine custom parts from most of the materials just discussed.
Chief among these are machining centres, which work by rotating a cutting tool such as an end mill or drill to remove material. The workpiece is gripped in a vise or similar clamping device and moved in one or more axes against the cutter, thus creating complex geometries. Five-axis machining centres may use all axes simultaneously to generate the free-form shapes common in artificial knees and propellers or indexed to machine multiple sides of the workpiece in one clamping.
CNC lathes use a chuck or collet to grip the workpiece and rotate it against a fixed cutting tool. Need to cut a set of candlestick holders or a fitting for a garden hose? Lathes make short work of these parts and more. Mill-turn machines, like Protolabs uses, take lathes one step further with the addition of rotating tools and secondary spindles, eliminating what were once secondary machining operations.
Metal 3D Printing for Complex Geometries
For parts impossible to manufacture through any of the methods previously discussed, there’s the additive manufacturing process of direct metal laser sintering. DMLS boldly goes where other manufacturing processes can’t. Like its plastic counterpart selective laser sintering (SLS), which uses a laser to fuse nylon-based powder into almost any shape imaginable, DMLS achieves similar results in metals such as aluminium, cobalt chrome, Inconel, stainless steel, and titanium.
Like most additive processes, DMLS builds the parts from the bottom up, like Like most additive processes, DMLS builds the parts from the bottom up, like layers of a cake. It begins with a 3D CAD model, which is sliced into layers roughly 0.02 to 0.06 mm thick, depending on resolution. A laser then “draws” each tissue-thin slice of the CAD model on a bed of metal powder, one with the consistency of flour. As the laser passes, the metal particles are melted and fused to their neighbours, creating metal with the same mechanical properties as that emerging from the business end of a steel mill. As each layer is completed, a rubber blade pulls additional material across the in-process part and the laser goes back to work, successively fusing each level to its predecessor. Several hours later, a finished part emerges.
Tolerances of +/- 0.1 mm to +/- 0.2 mm + 0.005mm /mm are expected to be achieved with DMLS, along with part features smaller than the period at the end of this sentence. Because very high temperatures are involved, small struts are often required to support the workpiece during the build process and to prevent warping—post processing is necessary to remove these supports. Also, as you can expect surface roughness of 0.004 to 0.010mm Ra, depending on material and resolution, some sort of secondary polishing or machining operation may be necessary.
Post-Processing Methods for Metal
Secondary operations are common with many manufacturing processes, especially with metal parts.
Heat-treating improves strength and removes internal stresses created during raw material processing and with heavy machining.
Carbon steels such as EN8 can be through or case hardened through nitriding or carburisation, EN8 can be hardened to 250 Hardness Brinell (HB) equivalent to 25 Rockwell C (HRC), or up to 67 HRC using nitride coating.
Soft metals such as aluminium and magnesium are not hardened in the same way, although they may be stress-relieved or “aged” using heat treatment.
Surface treatment is another common post-machining process. Aluminium is often anodised, giving it a scratch resistant surface in almost any colour. For electrically conductive protection, chromate is a good option. For a much harder wearing surface hardcoat, or hard anodising produces a thicker layer of aluminium oxide, and extremely hardwearing 'ceramic' coating.
Copper and brass discolour when exposed to oxygen, so electroless nickel or chrome plating may be applied if protection is needed. Stainless steel and superalloys don't normally require protection, but passivation can be used to further improve stainless steel. Carbon, mild and tools steels are commonly given black oxide surface treatment, or plated with nickel, or zinc. Painting is also a popular choice, but bead blasting or some other forms of abrasive preparation is recommended to provide a clean, rust-free surface prior to paint application.
Protolabs’ machining service offers bead blasting to remove burrs and provide a uniform appearance to machined surfaces, as does its 3D printing service for DMLS parts. Bead blasting, as the name implies, uses a high-pressure stream of particles such as ceramic or glass beads to smooth sharp edges and remove burrs. Tumbling uses small ceramic or plastic media in a tumbler bowl to achieve the same effect. It is possible to get parts bead blasted within standard lead times at Protolabs. In most cases, customers can have their parts plated, painted, or anodised outside of Protolabs immediately afterwards, or use our digital network who are able to offer a greater range of finishes.
Balancing Materials and Processes at Protolabs
Protolabs offers you a variety of metals. Couple those metals with several different machining processes and pretty much anything you can dream up and draw is possible! And for the impossible, metal additive manufacturing turns almost any CAD model into a physical reality.
They are some pretty tall statements, here’s the fine print. Parts made in Protolabs’ 3D printing, and machining processes have minimum and maximum size restrictions. Since part envelopes are continually expanding at Protolabs, please look at the design guidelines listed for each process online at protolabs.com/en-gb.
Regarding machined parts, walls thinner than 0.5 mm are also not advisable, and tool depths should be no greater than 50 mm from any given side. Part tolerances of ±0.1 mm are typical. At this time, cobalt chrome and Inconel are not machining candidates at Protolabs.
Direct metal laser sintering produces parts from many of the materials mentioned earlier, and adds cobalt chrome and Inconel to the mix. Protolabs’ DMLS capabilities include three levels of precision: normal, high and fine resolutions. Each can achieve very thin layer thicknesses and minimum feature sizes, regularly improving capabilities that can again be found online at protolabs.co.uk. Parts made with DMLS typically require additional processes to improve surface finish, and grinding or machining to remove the part supports created during the build process.
Of course, some parts may need additional machining after leaving Protolabs. Quick turnaround and competitive pricing means Protolabs is not a one-size-fits-all solution. Square internal corners may require EDM, close tolerances holes might need boring, external diameters and surfaces can be ground or lapped. Despite these restrictions, Protolabs produces one-offs and low-volume metal parts faster than anyone around, in a range of materials that suit the majority of our customers’ requirements.
Dozens of plastic materials can also be machined at Protolabs, along with hundreds of mouldable thermoplastics and liquid silicone rubber materials. Regardless of your project, we offer materials, a suite of manufacturing services—3D printing, CNC machining, and injection moulding—and pricing designed to meet your part-making needs.