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63ELIMINATION OF SURFACE DEFECTS OF SLM PRODUCTS DUE TO A SYNTHESIS OF NiAl INTERMETALLIC AT ELECTRO-SPARK DEPOSITION OF Al-CONTAINING PRECURSOR
S. K. Mukanov*", M. I. Petrzhik", A. E. Kudryashov", and E. A. Levashov"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia *e-mail: sam-mukanov@mail.ru
DOI: 10.24411/9999-0014A-2019-10103
Selective laser melting (SLM) is a kind of additive manufacturing, which consists in the obtaining parts of complex shape by layer-by-layer melting of a metal powder using a laser beam scanning [1]. A characteristic feature of SLM alloys is an increased surface roughness due to a formation of surface defects as pimples, microcracks, pores, etc. [2-5]. Pulsed electrospark deposition (ESD) is successfully used for surface modification to restore damaged or weared surfaces and working edges of the used parts due to simplicity and efficiency of the technology [6-8]. Thus, new prospects are opening up applying an electrospark modification of SLM parts to improve their roughness and geometry.
In this work, SLM parts based on alloys having in structure nickel aluminides which are prone to surface cracking at the production process [3, 4] were used. To develop the combined SLM + ESD technology it is necessary to tailor novel electrode materials and modes of ESD to secure a reactive formation of desired phases at modified surface layers free of all undesirable surface defects and having high performance properties.
The aim of the study is to apply electrospark deposition for surface modification of nickel-containing SLM parts using precursor-electrodes made of AK9 hypoeutectic aluminum alloy. The idea of the work is to create on the surface of SLM parts a modified layer formed by the local chemical interaction of the elements contained in both the precursor (Al) and SLM (Ni) products. It is expected that the precursor will locally melt at ESD, the molten near-eutectic Al-Si-Ni alloy will spread, interact with Ni of SLM substrate and fill in near-surface pores and cracks, leveling the surface relief and form reinforcing intermetallic phases during solidification.
An object of study was SLM alloy [1, 9, 10] Ni41Al41Cr14Co4 with a dimension of 7 x 9 x 7 mm. To ensure the formation of low-melting drops at ESD process, a specially prepared precursor cut off from the AK9 alloy [11] was quenched from the melt to suppress the formation of primary crystals and coarse eutectics to obtain homogeneous structure cast rod.
ESD was performed on the ALIER-303 METAL experimental stand using high-frequency discharge mode f = 3200 Hz, I = 120 A, t = 25 |s, U = 20 V). The formation of the surface layers was carried out under protective atmosphere of argon by repeated scans of the precursor on the surface of conductive substrate in 120 s. Microstructure of the surface layer and its cross-section was studied using a S-3400N scanning electron microscope (SEM) ("Hitachi", Japan). To determine the surface roughness, a WYKO NT1100 (Veeco, USA) optical profiling system was used. X-ray diffraction analysis performed using a DRON-3 (Russia) diffractometer with monochrome Cu radiation was used to estimate the phase composition of samples. The study of hardness and Young's modulus near surface of polished cross-sections was carried by instrumented nanoindentation using a Nano-Hardness Tester (CSM Instruments, Switzerland).
Figures 1a and 1b show the morphology of the surface of SLM specimen. One can see defects in the form of microcracks and non-melted spherical particles of the original NiAl micropowder. In addition, there are areas containing Al and O in the ratio of 37:63 (Table 1, region 2), possibly based on aluminum oxide AhO3.
After the electrospark modification with an aluminum contained precursor using high-frequency processing mode, a decrease in the width of surface cracks was observed, the presence of defects in the form of spherical particles was not detected (Figs. 1c, 1d). The thickness of the modified layer was 10.0-13.5 p,m as shown in Fig. 1c.
Fig. 1. The microstructure of the surface of the SLM sample: (a) x 100; (b) x 500; (c) the surface of the layer x 100; (d) cross-section x 2500.
Table 1. The composition of the regions of the EDX for the SLM sample.
Content of elements, at %
Region of analysis Ni Al Cr Co Hf O
1 (matrix) 44.4 41.9 7.4 6.3 - -
2 - 36.7 - - 1.2 62.2
3 18.6 10.4 10.7 4.5 20.3 35.4
4 372 52.1 5.0 5.7 - -
5 34.3 57.6 3.8 4.3 - -
Figure 2 shows an image of a cross-section with a healed microcracks in an SLM sample subjected electrospark modification. EDX analysis showed that the concentration of elements in the composition of the surface layer (Table 2, spectrum 1) and the filled crack (Table 2, spectrum 2) are almost the same. The ratio of the atomic concentrations of Al to Ni is ~ 3 and has the same ratio with the intermetallic compound NiAb, i.e. during ESD the synthesis of nickel aluminide with an excess of aluminum was done. ES processing leads to filling in surface cracks of the initial sample with Ni-Al-Si melt. The depth of filled in cracks reaches 34 |im, and then the molten drop solidifies, having not enough time to completely fill in the microcrack.
Fig. 2. (a) Image of the cross-section and (b) element map.
Table 2. Composition of the regions of the EDX for the ESD layer._
_ . r i • Content of elements. at %
Region of analysis Al Ni Si Cr Co
1 61.8 24.8 2.9 6.4 4.0
2 67.8 19.9 4.3 5.3 2.7
3 41.3 44.2 9.4 5.2
Studies of the original SLM sample by XRD analysis showed (Fig. 3a) that it consists of two phases: intermetallic NiAl and chromium-based solid solution [9]. It should be noted that both these phases have similar lattice periods, which gives a significant error in the analysis of the spectra [1, 9]. As can be seen in Table 3, ESD leads to the formation on the surface of 100% NiAl intermetallic (Fig. 3b). The spectra of the surface layer completely coincide and correspond to the NiAl phase with a cubic crystal system and lattice periods of 0.2879 nm, which is consistent with literature data of 0.2887 nm [12]. Additional phases which is characteristic of the substrate material were not detected.
I,a.u.
I, a.u.
20
ESD layer ■ NiAl
L
_/V " JV __Л 20,
1 О 20 30 40 SO 60 70 SO ЭО 1 OO 110
(a)
>(lOO)
(HO)
(HO)
-NiAl •Cr
— SLM part
.(2П)
• uu)
• (200) -(200)
II A -(2l0) A
> (220)
■ (220)
30
50
(b)
Fig. 3. X-ray diffraction spectra of samples: (a) SLM part; (b) ESD layer.
Table 3. Phase composition of the electrospark deposited layer._
Phase_Structure type Volume fraction, % Weight fraction, %
Period, nm
NiAl
cP2/l
100.0 ± 0.0
100.0 ± 0.0
0.2879
So, the hypothesis that during the ESD process a molten Al-enriched precursor would interact with an Ni-enriched SLM substrate resulting in reactive formation of NiAl intermentallic was confirmed.
The surface roughness of the SLM sample was Ra = 8.37 ± 0.22 p,m. ESD leads to a decrease in the surface roughness due to the filling of cracks (Fig. 2). which leads to the leveling of the surface and is Ra = 3.41 ± 0.13 p,m (Fig. 4). The 2D profiles shown in Fig. 4c clearly show how irregularities in the form of depressions are present on the sample surface along the baseline. the synthesis of Al-containing precursor leads to a decrease in such irregularities.
0,0 0,5 1,0 mm
Fig. 4. Topography of the sample surface: (a) SLM part; (b) ESD layer; (c) 2D profilogram.
The study of mechanical properties was carried out in several areas of the SLM sample and the modified surface layer. The test showed that during the ESD stimulated synthesis of NiAl intermetallic a harder surface layer was formed. Its hardness value was 10.9 ± 0.5 GPa, whereas the hardness of the original SLM alloy was 6.2 ± 0.2 GPa.
Thus, the possibility of using ESD for surface modification of nickel-containing SLM parts was tested. A modified layer based on nickel aluminide was obtained by electrospark deposition of a rapidly quenched Al-containing precursor on the defective surface of an SLM part from nickel alloy. This work demonstrates that ESD allows to improve the surface quality of SLM products by means reducing the surface roughness and filling the microcracks, as well as increase the surfase strength.
The reported study was funded by Russian Foundation for Basic Research (RFBR) and Bulgarian National Science Fund (BNSF) according to the research project no. 19-58-18022.
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