لخّصلي

Abstract This study explores the viability of upcycling the metal residues from machining industry into powders for additive manufacturing (AM).Sample Microhardness AISI P20+Ni Metal Chips Powder Produced by PND 1 (38<x<212 um) - Disc Milling 532 +- 31 HV 0.01 532 +- 26 HV 0.01 The powder produced by disc milling (PND 1) exhibits a comparable Vickers microhardness to that of AISI P20+Ni metal chips.A scanning electron microscope (SEM) (FEI Quanta 400 FEG (ESEM, Hillsboro, USA) equipment, using secondary electron (SE) and backscattered electron (BSE) modes) equipped with energy-dispersive X-ray spectroscopy (EDS) (EDAX Genesis X4M, Oxford Instrument, Oxfordshire, UK) was utilized to perform comprehensive microscopic analysis as well as semi-quantitative chemical analyses.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).390 Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386-396 Author name / Structural Integrity Procedia 00 (2019) 000-000 5 Powder particles from PND 1 exhibited a distinct flake-like shape and irregularity, as seen in Figure 3, with an aspect ratio (AR) of 0.5 unlike spherical powders (with AR close to 1) commonly used for AM printings, which are typically produced through gas atomization process.This laser emitted a Gaussian distribution with a wavelength of 1070 +- 10 nm. It operated in continuous wave (CW) mode, providing a maximum power output of 3000 W. The employed parameters for producing depositions are presented in Table 2.%) for each milling procedure Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386-396 391 6 Author name / Structural Integrity Procedia 00 (2019) 000-000 The results of microhardness measurements of the original AISI P20+Ni metal chips and the powder particles produced by PND 1, via disc milling in size range 38-212 um are presented in Table 5.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).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 <0.0055 1.73 0.77 0.79 2.11 0.38 - AISI 4140 0.45 0.01 0.79 0.19 - 1.17 0.19 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.Table 2 - Parameters for the AISI P20+Ni printing Parameter type Parameters Variable Laser Power 500, 900, 1800 W Scanning Speed 120, 360, 450, 480, 852 mm.min-1 Powder Feed Rate 3, 4, 6, 8 g.min-1 Fixed shielding gas/carrying gas Argon, Grade 3 carrying gas flow rate 4 liters.sec-1 shielding gas flow rate 5 liters.sec-1 stand-off distance 13 mm inclination angle 5 o laser spot size 2.1 mm For each combination of parameters (laser power, scanning speed, and powder feed rate), single line printings were performed and evaluated.Keywords: upcycling; additive manufacturing; mechanical milling; powder characteristics; microstructure; mechanical properties Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386-396 387 2 Author name / Structural Integrity Procedia 00 (2019) 000-000 1.Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386-396 389 4 Author name / Structural Integrity Procedia 00 (2019) 000-000 2.5 Microstructural and Mechanical Characterization To study the characteristics of the powder and metal chips and the final microstructure of the printed samples via L-DED, the samples were prepared metallographically.In this manner, it is feasible to state that the chemical composition of the milled particles is similar to that of the chips, the slight differences is attributed to characteristics of EDS analysis that is a semi-quantitative technique performed only on a specific-tiny volume of the sample excited by the energy source.Figure 1 Milling conditions used out in the current study for disc milling in a non-controlled atmosphere 388 Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386-396 Author name / Structural Integrity Procedia 00 (2019) 000-000 3 Initially, for each procedure, the metal chips were submitted to an initial milling of 5 minutes to flatten them.Table 4 Loose & tap density, and flowability of powders of different particle sizes PPS (um) Loose density (g/cm3 ) Tap density (g/cm3) Flowability 53-150 38-212 2.96 3.35 3.95 4.14 19.40 15.02 Figure 3- SEM image of particles with a size between 38 um and 212 um resultant from PND 1 Figure 2- Particle size range-wise yield of milled powders (wt.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).While the conventional recycling method involves melting and casting of these waste materials within foundry industries, there exist more sustainable alternatives with considerable potential.It investigates the production of powders from AISI P20+Ni steel chips using two different techniques: vibratory disc milling (VDM) and planetary ball milling (PBM).These powders were then sieved to select specific size ranges for directed energy deposition (DED) and laser powder bed fusion (L-PBF) processes.Through careful visual analysis of the lines' top and cross-sectional views, the line exhibiting good adhesion, least defects, and optimal linearity were chosen to proceed with printing, using the same parameters.Afterwards, all cut samples were cold mounted in epoxy resin and ground usnig SiC abrasive papers to 4000 grit sequentially, and finally polishing them using diamond suspension of 6 and 1um respectively.To conduct metallographic observations, digital microscope Leica DVM6 A and optical microscope (OM) Leica DM4000 M was utilized.%) of metal chips and powder particles (except C and S) C S Si Mo Cr Mn Ni Fe Metal Chips Powder Particle PND 1 (38 um <PPS< 212 um) Powder Particle PNB 5 (20 um <PPS< 53 um) 0.51

<0.0055

0.77 0.71 0.65 0.38 0.83 0.56 2.11 2.33 2.14 1.73 1.93 1.87 0.79 0.91 0.90 Rem.Figure 4 illustrates the cross-sectional views of each line, such as bead, dilution and heat affected zone (HAZ) and Table 7 presents the relevant measurements corresponding to each line's characteristics made using the Image J software.Utilizing these parameters, design of experiments (DOE) Taguchi L9 method was implemented to generate eleven distinct combinations of parameters that will be further discussed.In conclusion, the study found that upcycled AISI P20+Ni feedstock can be used in DED, but strict atmospheric control during milling and printing is necessary.To determine the milling procedure with maximum productivity for L-DED's feedstock, powders in size range of 38 to 212 um were sieved and weighed.(Castanheira 2022) conducted a study on the production of sustainable powders from stainless steels, successfully producing powders for L-DED.The method involved the use of three standard sieves from Retsch with apertures of 212 ,150 and 38 um for further insights into the productivity of the powder particle size (PPS) for each milling procedure.Metal chips and powder specimens underwent 10 indentations with 25 gf (HV0.025) indentation load, while L-DED manufactured samples had indentations with 10 and 100 gf (HV 0.01 and HV 0.1) indentation loads.Moreover, the cold-welding phenomenon characterized by the bonding of smaller particles during the milling process can be observed in magnified section of Figure 3.Microstructural and microhardness analyses were conducted to evaluate particle consolidation and work-hardening effects.The deposit bead evaluation and dilution analysis were conducted, and subsequently correlated with the energy density. The chemical composition of these steels is shown in Table 1 (obtained by optical emission spectroscopy technique using SPECTROMAXx equipment).Additionally, it was connected to a Fraunhofer IWS powder splitter, enabling the separation of the powder material into four distinct channels.This way, after determining the optimized parameters, the printing of a multi-layered bulk was initiated.Table 3 presents the chemical compositions obtained via SEMEDS analysis for initial metal chips and the powders produced in the current study.PND 1 only needs 75 minutes of milling time to obtain 63 wt.% yield of powder particles in size range of 38-212 um. Consuming only 56% of the milling time of that of PND 2, PND 1 can achieve almost same yield as of PND 2 for the metal powder in 38-212 um range.Consequently, PND 1 showcased lower energy consumption and economic viability while yielding comparable results.As mentioned earlier, the typical size range for metal powders utilized in L-DED falls between 50 and 150 um. However, for this study it was decided to use metal powders with a size range between 38 and 212 um after carefully considering the density and flowability measurements of either particle size ranges (Table 4).This similarity is due to the insufficient plastic deformation during both the chip production and milling processes, which did not induce significant work-hardening in these samples.The study further aimed to optimize milling parameters to improve process efficiency and powder characteristics.The powders produced by VDM had a flaky morphology, while those produced using PBM had a rounded shape.A multi-layered volume was printed for further microstructural, chemical, and hardness analyses.Further optimization of process is recommended to ensure chemical composition stability in the printed alloy.AISI P20+Ni is a medium carbon low alloy tool steel from mold steels family (Steel 2023).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.2.4 Powder Deposition For the deposition process, a COAX12V6 nozzle head manufactured by Fraunhofer IWS, Germany, was used.The energy source employed was FL3000, a fiber laser manufactured by Coherent, USA.Results and Discussion 3.1 Chemical Composition The chemical analysis enables a comparison of the semi-quantitative and qualitative chemical composition of the material, both before and after the milling process.3.3 Printings through Direct Energy Deposition Table 6 outlines the parameters applied for the eleven lines, considering the process variables like energy density and powder deposition density.Procedure number 2 for disc milling (PND 2) was a continuation of PND 1, the final batch of 120 grams was milled further for another 60 minutes.To supply the necessary powder, the equipment featured two independent Medicoat AG Disk powder feeders from Magenwil, Switzerland.These feeders operated smoothly within a range of 0.5 g/min to 100 g/min, ensuring a pulsation-free powder flow.Mechanical characterization was performed through Vickers hardness tests following ISO 6507-1:2023 standard (ISO 2023).The weight % powder yield (in size ranges of interest) of each milling procedure was calculated after sieving analysis (Figure 2).This evaluation reveals that the use of a larger size rang can provide a better flowability and densification.Despite the non-spherical shape of VDM powders, they were successfully used in the DED process.For the specific case of the metal chips, the C & S analysis was aided with carbon-sulphur technique.The milling process was limited to a continuous operation of 5 minutes, followed by a mandatory 10-minute pause.The milling of procedure number 1 for disc milling (PND 1) was performed with two separate batches of 60 grams each for 30 minutes.Subsequently, both batches were combined, resulting in a total of 120 grams, and they were milled again for 10 minutes.Procedure number 3 for disc milling (PND 3) consists of 80 minutes milling of a single batch of 120 grams.2.3 Powder Characterization The sieving of the powder particles was carried out using in accordance with ASTM B214 standard (ASTM 2022).Both the carrying and shielding gases were provided by an inert gas supply regulated at 6 bar.For the depositions conducted in this project, argon (Ar) Grade 3 was used as the inert gas.With this analysis it is possible to conclude that PND 1 is a potentially suitable procedure.The machine was operated at its maximum rotation speed, 1407 +- 3 rpm.For the current study, three different milling procedures (as shown in Figure 1) were tested to find the optimized milling parameters.This size range is wider than the one commonly used for LDED technology (i.e., 50 and 150 um), however the reason to this choice shall be presented and discussed in Section 3.2.The procedure PND 1 in this work is adopted from the optimized milling procedure developed by (Castanheira 2022).This preparation was initiated with the cutting of cross section aided by Mecatome T260 cutting machine.Table 3- Semi-quantitative chemical composition (wt.3.2 Powder Production for L-DED Since the goal was to obtain the maximum amount of powder particles in the size range of 38-212 um from the metal chips.This behaviour is attributed to the accommodation of particles of a larger size range.Table 6- Printing parameters combined considering the Taguchi L9The substrate for printing was AISI 4140 steel plate.Furthermore, after every total 30 minutes of milling, a 30-minute break was incorporated for the safety of the machine.Loose and tap density was determined following ASTM B212 (ASTM 2020) and ASTM B527 standard (ASTM 2022), respectively .The flowability of the powder was assessed as per ASTM B213 standard (ASTM 2020).The process parameters used for laser processing are given in Table 2.Nital 2% was used as an etchant to reveal microstructure.Microhardness tester FALCON 300 was used to perform these tests.Approximately 1500 grams of powder was produced using PND 1. A total 18 hours of total milling time was required to produce this powder, accounting for nearly 57 hours including break times (a 10-minute pause after every 5 minutes of milling and a 30-minute pause after every 30 minutes of milling).Furthermore, the carrying gas could be modified as required.Table 5- Microhardness of the chips and powder particles.2.3.

Abstract
This study explores the viability of upcycling the metal residues from machining industry into powders for additive manufacturing
(AM). It investigates the production of powders from AISI P20+Ni steel chips using two different techniques: vibratory disc milling
(VDM) and planetary ball milling (PBM). These powders were then sieved to select specific size ranges for directed energy
deposition (DED) and laser powder bed fusion (L-PBF) processes. The study further aimed to optimize milling parameters to
improve process efficiency and powder characteristics. The powders produced by VDM had a flaky morphology, while those
produced using PBM had a rounded shape. Microstructural and microhardness analyses were conducted to evaluate particle
consolidation and work-hardening effects. Despite the non-spherical shape of VDM powders, they were successfully used in the
DED process. The deposit bead evaluation and dilution analysis were conducted, and subsequently correlated with the energy
density. A multi-layered volume was printed for further microstructural, chemical, and hardness analyses. In conclusion, the study
found that upcycled AISI P20+Ni feedstock can be used in DED, but strict atmospheric control during milling and printing is
necessary. Further optimization of process is recommended to ensure chemical composition stability in the printed alloy.
Keywords: upcycling; additive manufacturing; mechanical milling; powder characteristics; microstructure; mechanical properties
Mariana Cunha et al. / Procedia Structural Integrity 53 (2024) 386–396 387
2 Author name / Structural Integrity Procedia 00 (2019) 000–000

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