What Metals Can You Print?

The products we use every day contain thousands of different metals, each selected for their balance of cost, performance, and manufacturability. All conventional metal manufacturing processes, from casting to stamping, are limited in the alloys they can manufacture (material compatibility). Additive manufacturing (AM) processes are no different. Each metal AM process has distinct sets of materials with which it is compatible, and this determines the applications that are viable for each process.

Materials are usually chosen early in the product development process. The compatibility of these materials with manufacturing methods is essential to selecting a viable material and process combination. Successfully implementing metal AM requires an understanding of which processes and metals are compatible, today and in the future. This post highlights material compatibility for the most popular metal AM technologies and discusses the most important factors influencing compatibility.

The below table summarizes which metals are commercially available (green) and in development / proven in a lab environment (yellow) for popular AM processes:

full, high-resolution table available: www.digitalalloys.com/blog/what-metals-can-you-print/

A metal’s compatibility with an AM process is dependent on three factors:

1. Feedstock Manufacturing: the metal must be able to be processed into a feedstock of suitable type and quality for the AM process
2. Printability: the feedstock material must behave acceptably during the manufacturing process
3. As-Printed State: the final quality and state (hardness, grain size, etc.) of the printed material must meet requirements

Feedstock Manufacturing

Powder and wire are the primary feedstocks for most metal AM processes. Wire is produced by drawing a metal rod through a die; a process that is compatible with most metals. Wire is commercially available in a very wide range of metals.

Powder-based metal AM processes often have stringent requirements on the powder’s size and shape (morphology). Due to these requirements, metal AM powders are typically produced through slow, expensive atomization processes (diagrams below), limiting the supply and range of metals produced.

Printability and the As-Printed State

Metal AM technologies use a wide range of different processing conditions. These include the physics governing the movement of material and energy, and the dynamics of melting and solidification. Please see our post on The Physics of Metal AM.

The most common metal AM process challenge is the rapid melting and cooling cycles that produce large temperature gradients. When materials heat and cool they expand and shrink, creating residual stresses that can cause metals to warp and/or crack. This same challenge exists in welding. As such, the weld-ability of a material often predicts whether a metal is compatible with AM processes. The exceptions to this test are binder jetting, where the whole part is sintered in one post-processing step, and cold spray, which is a low-temperature, steady-state process. Below we provide more detail on metal printability and the as-printed state for popular metal AM processes.

Powder Bed Fusion (PBF)

PBF is characterized by high-energy welding of a powder bed. In a typical part, this small line-width process produces the equivalent of miles of welded toolpath. This results in the printed material seeing many cycles of melting and solidification. The rapid solidification rates and large thermal gradients make PBF prone to anisotropic and small-grain microstructures as well as micro-cracking. These effects are especially pronounced in low ductility materials that cannot accommodate high stresses. One expert notes that PBF’s “microscopic defects often constrain the types of alloys that can be printed effectively, and limit the mechanical performance of those that can” (source: Metal AM Magazine).

Another PBF process compatibility challenge is the absorption/reflectivity of the heat source. Highly reflective materials such as copper require machines with special wavelength lasers to increase energy absorption into the powder being melted

Binder Jetting

Unlike most other metal AM technologies, Binder Jetting applies heat separately from the printing process, in a secondary sintering step. During sintering, parts experience more gradual, uniform heating than they do in direct melt AM processes. For this reason, Binder Jetting has unique material compatibility factors that allow the printing of very low ductility materials like tungsten carbide. Similar to conventional powder metallurgy processes, the secondary sintering process also provides a large-grain microstructure.

...

To continue reading, please access the full, original blog post here: https://www.digitalalloys.com/blog/what-metals-can-you-print/