Some say we are on the cusp of another industrial revolution, namely the decentralization of manufacturing heralded by the growth of additive-manufacturing (AM) technology. The Doctor agrees. So, what is additive manufacturing, how does it differ from other conventional manufacturing technologies and how will it affect the heat-treat community? Let’s learn more.

Powder metallurgy has always been an attractive alternative to traditional manufacturing of products from wrought materials, and sintering (the bonding of adjacent powder particles together to form a cohesive metal component) is the thermal-treatment method associated with this technology. The automotive industry in particular has embraced its use. When higher densities are required, other sintering methods such as metal injection molding (MIM) are used.

What is additive manufacturing?

Additive manufacturing is not new, having been first introduced in the 1980s and developed for three-dimensional plastic parts with a thermoset polymer hardened by ultraviolet light. The technology was initially too slow for mass production and primarily used for rapid prototyping. Today, metals have been added to the list of materials that can be used, and the speed of the process has accelerated to the point where it is viable for high-volume manufacturing.

AM refers to a process in which the raw material is added layer upon layer to create a component part. This is the opposite of machining, often now referred to as “subtractive manufacturing,” which creates a part by removing material from a raw-material form.

One of the most promising versions of metal AM today is binder-jetting technology. Laser sintering and electron-beam methods are alternatives.^[4] Binder jetting of metals is a process in which a liquid binding agent is selectively deposited onto a bed of powder-metal particles as the layers of the component part are being built up. The goal is to reduce the amount of liquid binder used since less binder allows easier access to the pores and more rapid binder removal.

A moving print head (Fig. 1) strategically blends binder into the powder while it is being deposited on the printing bed. After each pass, the bed height is lowered by the thickness of a print layer, 25-100µm, and another layer of powder and binder is added on top of the previous. As this is repeated, layers of bonded metal are successively deposited until the fully formed part is created. 

After printing, sintering in a vacuum furnace is required, the same as with MIM technology. Binder jetting is used for creating parts made of Inconel, stainless steel, tungsten carbide, titanium, copper, brass and aluminum, among others. Since the printed layers can be extremely thin, the resulting part can be produced to an extremely high level of detail with very precise physical features.

    Typical tolerances and specifications of metal binder-jetting technology include:^[2]

• Maximal build envelope of 4,000 mm x 2,000 mm x 1,000 mm
• Minimum feature size of 0.1 mm
• Typical tolerance of ±0.13 mm
• Minimum layer thickness of 0.09 mm
• Fast build speed (in comparison to other additive technologies)

Binder jetting is the fastest metal AM method available. Maximum build speed is currently approximately 2,500 cm³/hour, and one manufacturer is planning to introduce an 8,200 cm³/hour machine in 2019. Car bodies and other large composite plastic parts have been printed using AM technology, and it is just a matter of time before this expands to metal printing.

AM is considered by most in the industry as a “disruptive” technology; one that will revolutionize many industrial sectors as it becomes faster and less expensive. It will also fundamentally affect how, when and where heat treatment is performed since sintering will become part of an AM manufacturing cell. As AM becomes more sophisticated and as understanding and awareness grows among manufacturers, machine shops as we know them will be fundamentally changed. AM offers clear advantages in that:

1. Small runs of unique or complex parts can be produced quickly and at low cost. Unlike MIM, casting or forging, no expensive molds are required. This reduces time to market, a very valuable commodity today.
2. Shrinkage is significantly less than that of MIM-produced parts, increasing accuracy and repeatability. One description of AM is that it is a MIM process without the distortion.
3. AM has the ability to pursue new innovations without extending the design cycle. This allows for many generations of design changes in the time that it would normally take to make a single change using conventional technologies. This might be the most revolutionary aspect of the technology.
4. Honeycomb designs are possible, reducing part weight while maintaining or even increasing strength.
5. AM offers the ability to make on-the-fly changes. If there’s one thing design engineers can count on, it is customer revisions and design changes. With AM technology, the designer simply makes a change in the 3D digital model, and it is downloaded to the printer for manufacture.
6. Highly complex parts can be produced (Fig. 2) that would be literally impossible with any other technology. There are some shapes and intricate features that cannot be cast, molded or machined but can be printed. This opens up new possibilities for designers.
7. A high degree of customization is possible without added cost. AM technology allows the manufacture of one-of-a-kind designs like medical implants that are custom made to fit a specific individual.
8. AM generates no waste. Since it is an additive technology, only the material that is needed is actually used. When printing very expensive metals such as titanium, this makes a huge difference in the price of the finished product and the feasibility of the project.

AM has always been an attractive choice when production volumes are low, changes are frequent and complexity is high. As print speed increases and costs come down, AM applications will expand to include more mainstream component parts. Machine shops and in-house manufacturing departments will then be able to choose the most cost-effective technology, with sintering being performed as part of the AM manufacturing cell as opposed to a heat-treatment department or an outsource location. This will lead to new opportunities and challenges for heat treaters because more parts will require secondary debinding and sintering under vacuum.

For example, one of the primary challenges for vacuum furnaces (Fig. 3) used for sintering is dealing with the binder liberated from the material during the secondary debinding process. Dry pumps are preferred since the binder can contaminate the oil used in rotary oil-sealed pumps requiring frequent oil changes.

There must also be provisions made for removing the binder. One approach is to locate a binder trap prior to the pump, which collects the binder and requires periodic removal and cleaning. Manual or automated traps are available – the latter heats up to liquefy the binder residue, which then flows to the bottom of the trap. A valve is opened to allow residue to be collected. A third method involves the use of a condensing filter.

Summing Up

The additive-manufacturing revolution has begun! It will soon have an impact on all types of industries and their manufacturing strategies, representing a paradigm shift in design and engineering that will affect every process in the factory, including heat treatment.