Metal powders for Additive Manufacturing will be a key part of the Laser Institute of America’s seventh Laser Additive Manufacturing Workshop (LAM 2015) in Orlando, USA, March 4-5. Speakers include representatives from GE’s Global Research Center, BMW and Siemens.
Dr Ingomar Kelbassa, General Chair of LAM 2015 and an Adjunct Professor at RMIT University in Melbourne, Australia, was part of the Fraunhofer ILT team that won an Aviation Week innovation award in 2012 for the company’s additively manufactured BLISK (blade-integrated disk). Kelbassa has overseen a programme of 24 presentations at LAM 2015 that focus on the Additive Manufacturing (AM) process chain, design and materials, wear and corrosion protection, maintenance, repair and overhaul and AM initiatives around the world.
Material and metal powder trends in metal Additive Manufacturing
Choosing high-quality powders is vital to metal Additive Manufacturing processes. Kelbassa stated, “The metal powder is the raw material for the entire AM process chain and its quality significantly determines final product quality. Powder quality can’t be manipulated and can’t be controlled by and during the AM process; it is the only process parameter that can’t be influenced. You can have the best AM process with the best process control, but if you use low-quality powder, then you are lost.”
Kelbassa added that to further optimise the various laser-based methods of Additive Manufacturing, specially tailored powders were needed. “We need AM-tailored powders. The current state of the art is that there isn’t any AM-tailored powder. All powders used for AM were developed and improved decades ago for thermal spray processes such as atmospheric plasma spraying, vacuum plasma spraying and high-velocity oxy-fuel.”
These powders, it was stated, may feature hollow particles and a higher content of atmospheric elements due to the atomising process used during fabrication. These issues aren’t crucial in thermal spraying, but they become crucial for a process such as laser metal deposition because they can lead to porosity and brittleness in the additively manufactured bulk part.
Another key concern that LAM 2015 will address is how often additive powders can be reused. “Usually, one can reuse/recycle the powder if you can completely avoid cross contamination with other materials and if you can guarantee a sieving in between, so that you can ensure the same quality when it comes to the used (constant) particle size distribution, for example 20 – 35 µm for selective laser melting and 35 – 100 µm for laser metal deposition,” Kelbassa explained. “A simple approach is to use the same machine, the same powder feeder and the same equipment for one material class only.”
The number of viable recycling steps depends, Kelbassa concluded, “on the chemical affinity of certain materials in terms of atmospheric elements. A highly affine material such as titanium can’t be recycled that often compared with conventional, not-that-affine metals such as iron and nickel-base steels. But if you can guarantee powder handling that always takes place in an inert-gas atmosphere throughout the entirety of processing, then repeated – theoretically infinite – recycling is possible with all these materials.”
The metal powders used in additive processes vary significantly by industry, stated Kelbassa. The aerospace and power generation sectors are primarily focused on titanium and nickel-base alloys such as Ti-6Al-4V, Ti-6246, Ti-6242, Ti-17 and polycrystalline Inconel 625, Inconel 718 and Nimonic PE 16; directionally solidified, for example Mar-M 247, and in a very few cases single crystals, such as CMSX-4 and PWA 1483/1484.
The automotive sector, however, has a strong interest in lightweight materials such as aluminium alloys, for example AlSi10Mg, whilst the tool, die and mould making industry has a focus primarily on iron-base steels.
Medical applications, usually in the form of implants, uses titanium and magnesium-base materials as well as CoCr alloys and ceramics such as ZrO2-Al2O3 compounds/hybrids or shape-memory alloys such as Nitinol (50% nickel, 50% titanium).