voestalpine is an international steel company headquartered in Austria. In recent years it has opened dedicated research centres to 3D printing, and has also opened a number of production plants around the world focused on the development of metal additive manufacturing materials. TCT’s Head of Content, Daniel O’Connor posed a few questions.
Q:When does a material become worthwhile to develop for additive manufacturing?
A: The core competence of the voestalpine mills is material expertise, in combination with material application and existing customer requirements. We have therefore tapped into those resources to establish the Additive Manufacturing (AM) powder ranges. AM powders developed with properties to provide specific solutions that meet the requirements and expectations of our customers. This makes them more competitive in the market.
The route to developing AM powders is two-pronged
- Optimise existing alloys in the market for improved processability. For example, the Uddeholm AM Corrax was developed to aid in plastic injection moulding markets as it has improved corrosion resistance, wear resistance and polishability.
- Developing innovative and novel alloys to corner the market especially for tool steels as this is the most under-developed market for AM.
Q: What are the cost implications of developing an AM specific material?
A: Alloy development is a time and cost consuming activity but the Uddeholm mill has 350 years’ experience in developing new grades, so it’s nothing new for them. The mills have been able to embrace new technologies in the past and harness their knowledge into developing new and/or improved alloys, with AM being the latest step in technical development. Powder development for AM needs a lot of effort but this is necessary, the cost implications are not much higher that of developing new alloys for other applications/technologies. Some of the costs in developing AM specific powders are driven by:
- Carbon content of the metallic alloys (as tool steels tend to have higher carbon content than the AM procedure can usually process)
- Metallic alloys are required to have specific properties (e.g. low melting/solidification range) that would make them suitable for the process.
- Considerable investments to ensure alloy development and handling would meet both Quality and Health & Safety requirements are undertaken. This includes the building of new Gas atomisers, test melt plants and various powder handling/storing equipment.
Q: How does the R&D process work?
A: Inert gas atomisers are utilised in melting the bars (which have been obtained from the mill directly hence ensuring full traceability). VIM furnaces are installed directly above the atomiser to increase capacity and powders obtained are usually spherical with particle sizes suitable for the AM process. The particle size are influenced by varying the gas flow rate dependent on the alloy been atomised. Mass spectrometry is conducted to verify the chemical composition during the atomisation process and monitor the oxygen content. Once melted, powders are sieved into two sizes – 0.015 mm – 0.045 mm and 0.045 mm – 0.150 mm.
Q: What does the traceability of the material look like (in terms of contamination risks)?
A: Traceability is integral to production in the mills and AM powders are no different. Bars for atomising are obtained, with already established melting routes, heat number and lot traceability number. Each batch of powder after atomisation is further assigned its own heat number and material certificate that incorporates the chemistry, particle size distribution and aspect ratio.
Q: How is a new material tested?
A: As metal particles are susceptible to environmental degradation (moisture and oxygen), testing is carried out under controlled atmospheres during the melting stage. During gas atomisation, a test sample of the current powder is sent to the test lab for analysis. The analysis includes the morphology/rheology (Shape and surface) of the powder particles, porosity, flow rate, density (relative and tap) and particle size measurements.
Once established that the powder is viable for the PBF/LMD process, we take the testing further in determining its viability for melting and full densification as a part. This involves testing on several different machines available in the group AM Centres. Dusseldorf and Toronto (for Powder beds) and Singapore (for LMD). This is done with a view to establish a suitable process window, also process robustness tests through its physical and thermochemical properties. When a working process window (through the modification of the laser spot size, layer thickness and scanning speed of the equipment) is obtained, we continue to investigate the heat treatment and other post-processes to verify the properties essential for the targeted application. This is an important step as our alloys should not just be printable but also offer benefits to our customers.