Requirements for Processing High-Strength AlZnMgCu Alloys with PBF-LB/M to Achieve Crack-Free and Dense Parts

Abstract

Processing aluminum alloys employing powder bed fusion of metals (PBF-LB/M) is becoming more attractive for the industry, especially if lightweight applications are needed. Unfortunately, high-strength aluminum alloys such as AA7075 are prone to hot cracking during PBF-LB/M, as well as welding. Both a large solidification range promoted by the alloying elements zinc and copper and a high thermal gradient accompanied with the manufacturing process conditions lead to or favor hot cracking. In the present study, a simple method for modifying the powder surface with titanium carbide nanoparticles (NPs) as a nucleating agent is aimed. The effect on the microstructure with different amounts of the nucleating agent is shown. For the aluminum alloy 7075 with 2.5 ma% titanium carbide nanoparticles, manufactured via PBF-LB/M, crack-free samples with a refined microstructure having no discernible melt pool boundaries and columnar grains are observed. After using a two-step ageing heat treatment, ultimate tensile strengths up to 465 MPa and an 8.9% elongation at break are achieved. Furthermore, it is demonstrated that not all nanoparticles used remain in the melt pool during PBF-LB/M.

Correlation between Differential Fast Scanning Calorimetry and Additive Manufacturing Results of Aluminium Alloys

“…Exactly the same nanoparticle-modified powder batches as in DFSC were used in PBF-LB/M processes. PBF-LB/M results and parameters are given in the references [ 5 , 11 , 12 , 13 ]. Additionally, the two powder variants 7021/TiC and 7021/TiB 2 have been investigated with both methods in this work.…”

“…The specimens were fabricated under an argon atmosphere with a residual oxygen level of approximately 2000 ppm. Own data from [ 5 , 12 ] has also been re-evaluated regarding the above parameter set, i.e., all results on 7075 and 7021 shown below originate from identical PBF-LB/M parameters.…”

“…As-build samples have been analysed regarding grain size and crack characteristics by metallographic methods. The metallographic methods are also described in the references [ 5 , 11 , 12 , 13 ]. In these experiments, cracks have been described by different measures, i.e., crack density determined on cross sections by light microscopy [ 5 , 12 ], crack volume determined by X-ray microtomography [ 13 ] and total crack length per area determined on cross sections by light microscopy [ 11 ].…”

“…The metallographic methods are also described in the references [ 5 , 11 , 12 , 13 ]. In these experiments, cracks have been described by different measures, i.e., crack density determined on cross sections by light microscopy [ 5 , 12 ], crack volume determined by X-ray microtomography [ 13 ] and total crack length per area determined on cross sections by light microscopy [ 11 ]. To compare these different crack measures, we suggest a crack characteristic value C, which is defined by the ratio of the crack measure with nanoparticles to the maximum crack measure without nanoparticles, equations (1–3).…”

Dry powder coating in additive manufacturing

“…A promising approach to obtain crack-free parts is grain refinement by addition of nanoparticles, such as carbides, nitrides or borides (see Table 2) to aluminum alloys (Kusoglu et al, 2020;Sun et al, 2021;Minasyan and Hussainova, 2022). Heiland et al (2021) dry coated Al7075 with 2.5 wt% TiC nanoparticles in a ball mill and successfully produced crack-free parts by PBF-LB/M due to the action of TiC as a nucleating agent. TiC lead to a refined microstructure and suppressed columnar grain growth (Zhuravlev et al, 2021).…”