PSI - Issue 53

Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386–396 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

387

2

1. Introduction The conventional subtractive manufacturing industries generate approximately 14.6% (by weight of input raw material) metallic scrap in form of turnings, milling or drilling chips, and metal swarf (Cullen and Allwood 2013). While the conventional recycling method involves melting and casting of these waste materials within foundry industries, there exist more sustainable alternatives with considerable potential. One promising approach that has garnered substantial attention is mechanical milling of these low-cost metal chips , thereby transforming them into fine metal powder (Dhiman, Joshi et al. 2021) which could be used for the additive manufacturing via laser directed energy deposition (L-DED).This methodology holds the promise of revolutionizing the AM sector, offering the prospect of substantially lower raw material costs in a sustainable fashion (Batista, Fernandes et al. 2021). 2. Materials and Methods 2.1 Alloy Powder and Substrate For this study, metal chips of AISI P20+Ni were provided from a metalworking industry, to produce powder feedstock for Laser directed energy deposition (L-DED). AISI P20+Ni is a medium carbon low alloy tool steel from mold steels family (Steel 2023). The substrate for printing was AISI 4140 steel plate. The chemical composition of these steels is shown in Table 1 (obtained by optical emission spectroscopy technique using SPECTROMAXx equipment). For the specific case of the metal chips, the C & S analysis was aided with carbon-sulphur technique.

Table 1 Chemical composition of the steel powder and substrate plate

Material

weight %

C

S

Mn

Si

Ni

Cr

Mo

P

AISI P20+Ni

0.51 0.45

<0.0055

1.73 0.79

0.77 0.19

0.79

2.11 1.17

0.38 0.19

-

AISI 4140

0.01

-

0.02

2.2 Powder Production for L-DED

The RETSCH's Vibratory Disc Mill (VDM) RS 1, with a drive power of 400-700W (variable speed drive), was utilized as the milling equipment to produce powder for L-DED technology. The machine was operated at its maximum rotation speed, 1407 ± 3 rpm. The milling process was limited to a continuous operation of 5 minutes, followed by a mandatory 10-minute pause. Furthermore, after every total 30 minutes of milling, a 30-minute break was incorporated for the safety of the machine. This intermittent approach aimed to maintain optimal conditions during the milling process and mitigate potential issues related to the rise in temperature and subsequent material oxidation. For the current study, three different milling procedures (as shown in Figure 1) were tested to find the optimized milling parameters. To determine the milling procedure with maximum productivity for L- DED’s feedstock, powders in size range of 38 to 212 µm were sieved and weighed. This size range is wider than the one commonly used for L DED technology (i.e., 50 and 150 µm), however the reason to this choice shall be presented and discussed in Section 3.2.

Figure 1 Milling conditions used out in the current study for disc milling in a non-controlled atmosphere