Surface Roughness in Metal AM

The surface roughness of a part is critical to its function and long-term performance. Metal Additive Manufacturing (AM) processes alone cannot usually meet surface roughness requirements. This necessitates slow, expensive post-processing such as machining or polishing. To choose the optimal manufacturing workflow, one must understand the surface roughness capabilities of metal AM, as well as post-processing techniques and their associated time and cost.

Surface roughness is a measure of the variance in a part’s surface topology. The engineering requirements for most parts include surface roughness specifications. Roughness affects part aesthetics (e.g. shiny or matte) and mechanical behavior like crack initiation, wear resistance, fatigue life, mating, sealing, bearing, and fluid dynamics. Because metal AM processes produce relatively rough surfaces, secondary post-processing is required.  This requirement has a large impact on total production time and cost.

This post provides an overview of surface roughness including: the ranges achievable by various metal AM and secondary processes; key process parameters that influence surface roughness; and the effects of processes and parameters on total production time and cost.

Measuring Surface Roughness

Surface roughness is usually specified in Ra, which measures the surface’s average absolute deviation from its mean height. Ra can be measured physically (below, left image) or optically. Either method can provide a precise profile of surface height which is then used to calculate Ra (below, right image).

Causes of Surface Roughness

In 3D printing metal, there are three major contributors to surface roughness:

• Surface artifacts due to low process resolution and layering effects,
• Granular micro surface textures from melting and binding powder feedstocks, and
• Support structures, and the remnants and surface marks left by their removal.

The following 5 categories of factors are most important in determining the surface roughness that can be achieved by a metal AM process:

1. Core process resolution and precision

The resolution and precision of a 3D printing process is the most important determinant of surface roughness. Because 3D printing builds parts by layers, the process resolution can be broken down by XY and Z axis resolution.

In the XY axes, resolution is dependent on the specific mechanism of the process. In Powder Bed Fusion (PBF) processes, resolution is determined by the diameter of the laser or electron-beam.  In Binder Jetting, the resolution of the jetting process is measured in dots per inch (DPI). The resolution of wire-based process (like DED and Joule Printing?) is defined determined by the width of the deposited bead.  In the Z axis, resolution for most processes is defined by the layer thickness. 

2. Material feedstock – type, size, and quality

For powder-based processes (PBF and Binder Jetting), the morphology (size and shape) and quality of the material feedstock affects the surface roughness.  In these processes, the size and shape of powder grains stuck to the outside of the part impact the surface roughness. Powder feedstocks can vary from highly spherical particles as small as 5 μm, up to irregularly shaped 120 μm particles. The quality of the powder is also an important factor because low quality powders can clump, preventing proper flow and distribution in the process and further exacerbating surface issues.

3. Surface orientation with respect to process

The orientation of the surface with respect to the printing process also plays an important role in surface roughness. Below is an example of PBF surface roughness as a function of surface orientation (measured in degrees to horizontal for both upward-facing (“upskin”) and downward-facing (“downskin”) surfaces).

4. Support Interface

Some metal AM processes require support structures in order to build overhangs and to anchor the part or certain features to the build plate. Where support structures connect to the part, the standard physical removal process (manually with pliers) leaves remnants that create roughness.

5. Other key processing parameters

There are many other processing parameters that can influence the surface roughness of metal AM parts. These include the power input, print speed, location in build, and cooling rates. Many parameters must be controlled and refined in order to optimize and ensure surface quality.

Post-Processing to Improve Surface Roughness

Near-net-shape manufacturing processes create parts that are close to the final design but require secondary material removal to reach the desired final dimensions and smoothness. Most engineered metal parts have surface roughness requirements that exceed the capabilities of metal AM processes (and of conventional processes like casting and forging). As a result, for most applications metal AM technologies – regardless of the particular technology chosen – are near-net-shape processes. (Accuracy requirements also drive the need for secondary operations.  We will cover accuracy in a future post).

The below chart compares the range of typical surface roughness achieved in by different additive and conventional manufacturing processes. Metal AM processes produce higher surface roughness than almost all conventional processes. For this reason, post-processing techniques are generally included in metal AM workflows. The post-processing methods most widely used include CNC milling and turning, grinding, and polishing. Many conventional processes can achieve less than 1 μm Ra surface roughness whereas the smoothest surface possible from any metal AM process (Binder Jetting) is in the range of 3-13 μm Ra.

Thanks for making it this far! If you would like to keep reading, you can find the full article here: https://www.digitalalloys.com/blog/surface-roughness/