Emissions of and exposure to hazardous chemical substances from selected additive manufacturing technologies

Abstract

Background: Additive Manufacturing (AM) is the process of joining feedstock materials one layer at a time in order to produce objects from a three dimensional (3D) computer aided design (CAD). Powder bed fusion (PBF) and direct energy deposition (DED) are two widely applied powder based metal AM processes used commercially in the South African AM industry. A number of studies have investigated small desktop polymer material extrusion fused deposition modelling (FDMTM) 3D printers and found that these printers are high emitters of particulate matter and volatile organic compounds. Chamber studies have been of value by contributing to a better understanding of emissions from FDMTM desktop printers. However, despite the numerous studies on particulate matter emissions from desktop FDMTM printers there is an unbridged gap to identify the hazardous chemical substance emissions from and the respiratory exposure of AM operators when utilising metal powders in AM. AM processes takes place in three distinct phases, namely the pre-processing, processing and post-processing phases, and occupational exposure may occur during each of these three phases since most tasks are manually performed by the AM operator. To date only one study has investigated particle emissions from and exposure to metals during PBF while there is no published literature on emissions or exposure from DED. PBF and DED are applied in South Africa and therefore, the investigation of particle emissions from and personal exposure of AM operators to hazardous chemical substances (metals) is needed. Aims and objectives: The general research aim of this thesis was to assess the emissions of and occupational exposure to hazardous chemical substances (particles and metals) associated with two metal powder based AM process categories (powder bed fusion and direct energy deposition) at three South African institutions utilising AM. The specific objectives were: (i) to establish the physico-chemical characteristics (particle size, shape and elemental composition) of virgin and used metal powders, used during powder bed fusion and direct energy deposition, and the relevance thereof to AM operators’ health. (ii) to assess particle emissions and respiratory metal exposure of AM operators when using three titanium powders and maraging steel during the pre- processing, processing and post-processing phases of AM at three AM facilities utilising PBF. (iii) to assess particle emissions and respiratory metal exposure of AM operators when using two stainless steel powders during DED at an AM facility. Methods: Samples of virgin and used (recycled) titanium and maraging steel as well as used stainless steel powders, along with their respective safety data sheets (SDSs) were collected from three AM facilities in South Africa utilising PBF and DED. Powder samples were characterised in terms of particle size distribution (PSD), particle shape and elemental composition. Real time monitoring devices, namely a Condensation Particle Counter (0.01 to > 1 μm) and an Airborne Particle Counter (0.30 to 10.00 μm) were used to investigate particle emissions [particle number concentrations (p/m3)] during PBF and DED process phases. The concentration of metals in the workplace air was established by means of static area monitoring according to the National Institute for Occupational Safety and Health (NIOSH) 7300 method using a closed-faced cassette (CFC). Personal (AM operator) respiratory exposure to metals used the same NIOSH method. Ethics approval for this study was obtained from the Health Research Ethics Committee of the North-West University (NWU-00004-16-A1). Researchers followed well know occupational hygiene methods, and other state of the art methods nor currently used in occupational hygiene compliance monitoring. Personal exposure to metals < 300 nm in size was assessed by using a nanoparticle respiratory deposition sampler. Results: Only one of the three titanium powers PSD was in accordance with the SDS. From the SEM images, thoracic sized (< 10 μm) and respirable sized (< 4 μm) particles were observed in powders from all three facilities. The elemental composition analysis (XRD analysis) of the investigated powders differed from the composition stated in the respective SDSs. Particles ≤ 1 μm were present in the workplace air. Increases in particle number concentrations were observed during specific pre- and post-processing tasks such as cleaning and powder sieving which led to an increase in particle emissions with the highest particle number concentration of 6.12 x 1010 p/m3 (0.01 to > 1 μm) measured during the post-processing phase of PBF with titanium. Tasks such as unloading of the AM machines led to particle concentrations (0.01 to > 1 μm) as high as 5.98 x 1010 p/m3 during PBF with maraging steel and 5.75 x 1010 p/m3 during DED of stainless steel. Static area monitoring indicated low concentrations of metals in the work place atmosphere during PBF and DED. During both PBF and DED, AM operators were exposed to detectable concentrations of metals including aluminium, chromium, copper, iron, nickel, titanium and vanadium, including metals < 300 nm in size. Full shift metal exposure was calculated for comparisons with occupational exposure limits (OELs) of all detected metals. AM operators’ personal respiratory exposure was < 1.04% of the time weighted average OELs (TWA-OELs) when using titanium powders, and < 3.60% of the respective TWA-OELs and < 9.01% of threshold limit value (TLVs®) during the use of maraging steel. Exposure to Ni, a human carcinogen, was the highest. Conclusion: There is a shortage of studies reporting particle emissions from and AM operators’ respiratory exposure to metals resulting from use of titanium and metals powders used in PBF and DED. Inadequate information in the SDSs may mislead employers and AM operators’ regarding the protection of AM operator health, and therefore, powder characterisation should form an integral part of risk assessments at each facility. During the pre-and post-processing phases of PBF and DED, particles were emitted during phase specific tasks such as cleaning, sieving, and unloading of the AM machine. When particle number concentrations during the processing phase from PBF and DED were compared to FDM™ printers, it was found that particle number concentrations emitted from FDMs™ were approximately five to nine times higher. The maximum particle number concentrations emitted from DED were 3% lower than that of PBF. Low concentrations of metals were present in the workplace atmosphere throughout the use of titanium, maraging steel and stainless steel powders. Although no OELs were exceeded, AM operators were exposed to detectable concentrations of metals, including metals < 300 nm in size. The findings of this study serve as a starting point to create awareness of AM operator exposure associated with metal AM, and to assist industrial AM facilities in identifying hazards and implementing phase and task specific control measures during PBF and DED using titanium powders and metal powders. Eleven recommendations are made for the attention of all role players including AM powder manufacturers/suppliers, employers (AM facilities) and AM operators/employees in an effort to reduce particle emissions from and AM operator’s respiratory exposure to metals. Along with the recommendations, specific limitations experienced during the study were also identified along with recommendations for future studies.