Steel powders can be sub-divided into either tool steels or stainless steels, and there has been a vast body of work carried out over the last 15 years or so to study the performance of these alloys both in the AM process and in actual use.
Renishaw reports that there is a general rule of thumb that carbon steels are more difficult to process, and that most, if not all successful work has been carried out on standard metal laser powder bed fusion platforms using low, or even no carbon, equivalent grades of steel.
The most popular alloy in use today is the maraging steel, often referred to as M300 or 1.2709, and classified within the AM sector as a tool steel. This alloy was first developed for and used by the aerospace sector for missile and rocket motor cases, recoil springs, flexures, actuators, landing gear components, high performance shafting, gears, and fasteners. The alloy then later was used in extrusion tooling, and in the die casting industry for long-run dies and also as core pins.
Maraging steels can be heat treated in several ways to obtain different properties. Unlike titanium, it is possible to use the maraging steel in the as-built condition without further heat treatment. As such the tensile strength can range from around 1000 MPa in the as-built condition up to, and in excess of, 2000 MPa in the age hardened condition.
For stainless steels, popular choices are the precipitation hardenable martensitic stainless steels, either 17-4PH or 15-5PH. The latter supposedly offers better higher temperature oxidation resistance. Also popular are ASM 300 series austenitic stainless steels, 316L and 304L. All these steels are commonly used in the aerospace sector, for medical devices, and in heavy industries, such as the oil and gas sector.
The precipitation hardenable (PH) stainless steels typically offer high tensile strength, corrosion resistance and moderately good high temperature oxidation resistance, along with having good toughness. Both alloys are typically used in the age-hardened condition and this is achieved through heat treatment cycles after the parts have been built and removed from the AM machine. Even without any initial solution treatment these PH stainless steels can have tensile strengths above 1400 MPa.
Of the austenitic stainless steels, 316L is the most popular used in metal AM. It resists corrosion, especially to some acid environments, and its superior strength is comparable to 304 stainless steel.
These single-phase stainless steels are non-heat treatable, meaning that their strength cannot be increased significantly through heat treatment. However, one of the quirks of the AM process route is that in this case it typically results in higher strength alloys compared to conventionally wrought products. Some reported figures for tensile strength suggest that greater than 700 MPa is achievable, but most AM users have reported values greater than 600 MPa, which is still higher than the typical wrought material, unless it has been cold worked. However, unlike the PH stainless steels there have been generally acceptable levels of corrosion resistance found in AM produced parts with the 316L alloy.
This information is excerpted from a Renishaw paper, The status quo of metal alloys for additive manufacturing.
