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Tribology OnlineJapanese Society of Tribologists

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Vol. 15, No. 3 (2020) 126-135.ISSN 1881-2198

DOI 10.2474/trol.15.126

Article

Tribological and Mechanical Characterization of Nickel Aluminium Bronze (NAB) Manufactured by Laser Powder-Bed Fusion (L-PBF)

Fathia Alkelae* and Shinya Sasaki

Sasaki Laboratory, Department of Mechanical Engineering, Tokyo University of Science,6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan

*Corresponding author: Fathia Alkelae ([email protected])

Manuscript received 07 February 2020; accepted 14 April 2020; published 15 June 2020

Abstract

To cope with the increasing demands of high-performance materials, efforts were made using Laser Powder Bed Fusion technique (L-PBF), taking advantage of its extremely short solidification time to fabricate copper alloy with highly refined microstructure. Copper and Copper alloys are reputed for their high reflectivity resulting in low laser absorption, which makes their 3D fabrication process very challenging. This study aims to provide high density samples in order to obtain high mechanical and tribological properties. This objective is realized through the optimum energy density value which is obtained by monitoring laser power P, scan speed v, hatch space h and layer thickness t. Upon mapping of optimum parameters, the best combination is adopted to manufacture the samples of this study. Mechanical and tribological characterization of nickel aluminium bronze alloy confirmed the efficiency of laser powder-bed fusion (L-PBF) in providing high performance materials, higher properties such as wear resistance, tensile strength and hardness were obtained compared to other manufacturing techniques.

Keywords

additive manufacturing, copper alloy, nickel aluminium bronze, tribology, friction, wear, laser powder-bed fusion (L-PBF)

Copyright © 2020 Japanese Society of TribologistsThis article is distributed under the terms of the latest version of CC BY-NC-ND defined by the Creative Commons Attribution License. 126

1 Introduction

Copper is the third widely used metal behind iron and aluminium, having a density of 8.93 g/cm3, an elastic modulus of 128 GPa and a melting point of 1083°C. Its high electrical conductivity guaranties its extensive use in electrical wirings, its high corrosion and oxidation resistance, excellent formability, strength when alloyed, wear resistance … all these characteristics allow copper alloys to be used in a wide range of industrial applications (electrical contacts, bearings, sea water environment, heat exchangers, propellers).

Copper can be manufactured using many techniques such as casting, forging, powder metallurgy and additive manufacturing (AM). This latter technique has been in a flurry of activity in the last 25 to 30 years, although it still encounters many challenges up to date especially with copper alloys due to their high reflectivity. L-PBF is based on incremental building of a 3D part using laser source as the energy input to selectively melt the powder bed according to a 3D CAD model inserted into the machine. Using this technique, attention must be given to porosity parameter since it leads to degradation of the part properties (physical, mechanical and tribological). Pure copper and some copper alloys have been successfully manufactured

using different 3D printing techniques, whereas other copper alloys are still struggling with low density levels due to the lack of full physical understanding of the manufacturing process.

Among studies concerned with L-PBFed copper/copper alloys, Scudino et al. [1] succeeded the process of Cu-10Sn bronze with a relative density of 99.7% using a laser power of P = 271 W. Shasha et al. [2] produced high density Cu-10Zn (99.97%) using high laser power (1800 W), Zhang et al. [3] produced Cu-15Ni-8Sn with an energy density of 142 J/mm3 and obtained parts with a relative density of 99.4%, Cu-Cr alloy was produced with 99.98% relative density using 2000W laser power [4]. Popovich et al. [5] manufactured Cu-Cr-Zr-Ti alloy using 310 W laser power and obtained parts with a relative density higher than 99%. High-strength Cu-12.5Zn-2.9Si silicon brass processing was successful and parts with 98.8% relative densities were obtained using 395 J/mm3 energy density [6]. These alloys provided parts with higher mechanical properties than the conventionally manufactured counterparts. On the other hand, other copper alloys such as C18400 [7, 8] or 84.5Cu, 8Sn, 6.5P, 1Ni [9] alloys are still missing the industrial requirements (Fig. 1).

The low density obtained is caused by defects generated during manufacturing, interaction between the heat source

Tribology Online, Vol. 15, No. 3 (2020) /127Japanese Society of Tribologists (http://www.tribology.jp/)


and the powder may result in many instabilities monitored by different driving forces such as recoil pressure, hydrostatic pressure, Plateau-Rayleigh instability and Marangoni effect, all result from temperature gradients in the melt pool. Competition between these factors in order to set the equilibrium in the melt pool yields several forms of irregularities known as pores, keyhole effect, denudation zone, spattering, balling…Processing parameters play a decisive role in these defects production. For instance, low laser power and high hatch space yield keyhole-related pores, high laser speeds causing weak wettability produce interlayer pores, high laser power and high scan speed yield denudation zones…etc. this subject is beyond the interest of this paper which is mainly concerned about the ability of additively manufacturing hard Ni-Al bronze alloy and the resulting properties, further details can be found in other studies [10-12].

2 Material and methods

Aluminium bronze alloys (Cu-Al) consist of a copper matrix alloyed with aluminium, called binary system, when another alloying element is added (typically iron, nickel, manganese, silicon) they are called ternary system, beyond two added elements (as its the case in this study) generally complex alloys nomenclature is used. Nickel-Aluminium-bronze (NAB) consists of copper matrix alloyed with aluminium (8 to 12%) as highlighted by the phase diagram's orange area in Fig. 2, nickel and iron (3 to 6%) and manganese (small addition). They are widely used in applications requiring high corrosion resistance, however increasing aluminium content above 8.2% make the alloy sensitive to corrosion. Depending on elements content, alloys with different phases (Fig. 2 a) yielding a wide range of properties may be obtained. Alloying with nickel and iron allows delaying the formation of the corrosion prone phase γ2 observed in aluminium bronze phase diagram (Fig. 2

Fig. 1 Comparison between conventionnally manufactured and L-PBFed copper alloys

Fig. 2 Comparison of NiAl-Bronze and binary system phase diagrams; reproduced with permission from [13] Copyright (1978) Springer Nature.

a) b)


Fathia Alkelae and Shinya Sasaki

b). Our alloy's chemical composition is shown in Table 1. It is categorized as complex alloy (Al, Fe and Ni principal elements). 99.9% of powder particles have an average size below 45 µm as highlighted in Fig. 3. From 8 to 11% of Al, hardness, strength and fatigue strength increase whereas elongation decreases, adding 4% of Fe increases tensile strength of the alloy compared to Cu-Al alloy. Due to its refining behaviour, iron improves hardness, toughness, fatigue and corrosion resistance. Nickel addition delays the formation of the eutectoid phase γ2 prone to corrosion and improves tensile strength, wear resistance and hardness but has a negative effect on elongation. Addition of manganese improves mechanical properties of Cu-Al, small addition of Mn is used to deoxidize copper and improves fluidity and castability [13]. NAB is widely used in applications such as bearings, propellers, gears due to its attractive properties. In order to define the L-PBF (Prox DMP200, 3D systems, US) optimum parameters for Ni-Al Bronze alloy, 25 cubes were manufactured with various parameters values (Laser power, scan speed and hatch space). After wire cutting,

the cubes are polished for density measurements based on Archimedean technique in order to reveal L-PBF optimum parameters allowing high density obtention. Measurement of Vickers hardness is performed on these cubes. Afterwards, ANOVA (analysis of variance) analysis is conducted considering different parameters combinations leading to the densest condition. P = 260 W, v = 600 mm/s, h= 120 µm and a layer thickness of t = 30 µm (E =120.37 J/mm3) is the best combination allowing obtention of higher densities for this material (ρ ≥ 94%). In the following, all samples will be manufactured according to this combination for mechanical and tribological characterization.

Tensile samples (Fig. 4 a) were manufactured with respect to ISO standard for strength measurements, with a width of 3.5 mm, a thickness of 3.5 mm, a round section of 20 mm radius and a grid section length of 12 mm, they were tested in the as built condition. For friction tests, disc specimens were manufactured according to their maximum density and hardness values as shown in Fig. 4 b, the manufactured discs were polished prior

Al‐Bronze Al Fe Ni Zn P Pb Sn Mn O Cu Mass % 9.56 4.2 3.39 0.28 0.02 0.012 0.082 0.843 0.09 balance

Table 1 Chemical composition of NiAl-Bronze

Fig. 3 Grain size distribution

a) b)Fig. 4 L-PBFed NiAl-Bronze parts (dimensions in mm): a), tensile specimen and b) disc for friction test



to testing. To reveal the microstructure of the as-built specimens (with no heat treatment applied), etching with ferric chloride and hydrochloric acid solution was applied prior to microscopic observations.

3 Characterization

The microstructures of the as-built specimen and after testing were characterized using scanning electron microscope (FE-SEM Supra40, Carl Zeiss, Germany) equipped with energy dispersive X-ray spectroscopy (EDS-Quantax Esprit 1.9, Bruker, Germany) for chemical composition analysis. For phases analysis, X-Ray diffraction was used (SmartLab 9Kw, Rigaku, Japan). Friction tests were performed using SRV frictionmeter machine (SRV4, Optimol instruments, Germany), wear volume was measured by a Laser microscope (VK-X150, Keyence, Japan) Tensile strength was analysed by tensile test machine (AG-10 kNX plus, Shimadzu, Japan) at a speed of 1 mm/s. 2D Hardness mapping was conducted using unidirectional mode of the nano-indentor (i Micro, KLA, USA), with a Bercovitch type indentor, a load of 25 mN was applied for hardness mapping of the top surface: an area of 200 µm x 180 µm, according to the microstructure. Vickers hardness of the cubes after polishing and density analysis was measured with micro Vickers Hardness Tester (HMV-G-FA-D, Shimadzu, Japan) in order to evaluate the bulk material's hardness.

4 Results and discussion

4.1 Mechanical characterization4.1.1 Hardness

The microhardness mapping allowed visualization of hardness distribution on the top surface as depicted in Fig. 5 a). High and homogeneous hardness distribution is recorded. The surface high Hardness may be explained as shown in Fig. 5 b) by the very fine microstructure generated due to the high solidification rate consisting mostly of the hard martensitic β' phase (dark etched) but also the needle-like α phase (white etched). The bulk material's hardness was also investigated by micro Vickers hardness tester, 25 indents were performed on the densest cube and a mean of 393 ± 29.5 Hv is obtained, thus

assuming the highest hardness for NAB compared to other studies (please refer to Table 2 for further details).

4.1.2 Tensile strengthNickel aluminium bronze alloy L-PBF manufactured shows

a ceramic-like behaviour as we can see in Fig. 6, two tensile specimens manufactured with the same L-PBF parameters were tested and showed the same behaviour with an averaged strength of 479 MPa, the ultimate tensile strength is the same as the strength at the break-up, with a weak elongation (averaged around 1.49%). This behaviour is maybe due to the very fine grains of the martensitic microstructure (Fig. 5 b) generated due to the very fast cooling during L-PBF process thus imparting the alloy a high strength and hardness.

4.1.3 Comparative study of NAB properties regarding manufacturing techniques:

Properties of nickel aluminium bronze conventionally manufactured using techniques such as traditional (gravity) and centrifugal casting [14, 15], hot forging [16], friction stir processes [17] for further improvement of materials properties, hot forging, cold working (used especially for high ductility materials to improve materials performance, on the other hand, high strength materials are hard to cold work due to their brittleness aspect) are highlighted in Table 2. Comparing with these manufacturing techniques and others [18-20], we can see that L-PBF has proved its efficiency in providing good combination of properties.

4.2 Tribological characterization4.2.1 Friction test

Reciprocating sliding tests were conducted on the as-built discs after polishing using a counterpart of JIS SUJ2 steel cylinder (ϕ = 15 mm, L = 22 mm, HRC = 60 ~ 62) as shown in Fig. 7, engine oil FG-5 was used as a lubricant assuming flooded conditions. The surface roughness was controlled to Ra ≈ 0.2 µm. The conditions shown in Table 3 were applied and results are depicted in Fig. 8. The friction coefficient of our material is stabilized after a short running-in period around an average of µ = 0.2 for the three tests.

a) b)Fig. 5 a) 2D Mapping of the microhardness and b) the revealed microstructure of the L-PBFed sample



reference Chemical composition μ Wear volume

Yield strength (MPa)

UTS (MPa)

Elongation (%)

Hardness

(VH)

Manufacturing technique

[18] NAB‐A (non‐communicated) ‐‐‐ ‐‐‐ 460 700 ‐‐‐ 249 non‐communicated

[18] NAB‐B (non‐communicated) ‐‐‐ ‐‐‐ 415 760 ‐‐‐ 168 non‐communicated

[17] Cu9.5Al4.2Ni4Fe1.2Mn ‐‐‐ ‐‐‐ ‐‐‐ ‐‐‐ ‐‐‐ 290 Casting ‐ Friction stir process

[16] CuAl10Ni5Fe4 ‐‐‐ ‐‐‐ ‐‐‐ ‐‐‐ ‐‐‐ 290 Hot‐forged

[14] 79Cu5Fe10Al5Ni0.25Mn ‐‐‐ ‐‐‐ 370 4.66 250 Centrifugal casting

[15] Cu9Al4Ni4Fe ‐‐‐ ‐‐‐ 442 602 17.7 227 Centrifugal casting

[15] Cu9Al4Ni4Fe ‐‐‐ ‐‐‐ 436 577 19 194 Gravity casting

[20] 81.1Cu9.5Al4.2Ni4Fe1.2Mn ‐‐‐ ‐‐‐ 620‐630 1480 255 gas‐atomzt + spark plasma

sintering

[19] Cu8.8Al5.2Ni4.4Fe1.1Mn ‐‐‐ ‐‐‐ 332 689 25.3 188 Wire arc additive manufacturing

Our work Cu9.56Al4.2Fe3.39Ni0.28Zn 0.2 0.131 mm3

479

‐‐‐ 1.49 393

L‐PBF

Table 2 Comparative study of NAB properties regarding manufacturing techniques

Fig. 6 Tensile test results



4.2.2 Wear analysisWear loss of the whole wear scar was measured and the

mean of friction coefficient was also calculated for the whole test duration, the results are shown in Fig. 9 a. A magnified area

of the wear scar is depicted in Fig. 9 b using FE-SEM in order to reveal its morphology, we can see a homogeneous worn area, which is a feature of the mild wear regime characterized by low wear loss and friction coefficient. The investigation of the

Fig. 8 Friction coefficient evolution

Parameters Values

load (N) 50

Frequency (Hz) 50

Time (h) 1

Temperature (°C) 80

Stroke (μm) 1000

Table 3 Test conditions

a) b)Fig. 9 a) Wear volume quantification and mean friction coefficient _, b) FE-SEM micrograph of a magnified area of the wear scar

Fig. 7 Cylinder on disc configuration used



wear scar using EDS (Fig. 10 a) shows that copper, nickel and relatively iron are evenly distributed in the whole area, whereas aluminium is clearly lowered and oxygen is concentrated, this leads to imagine that formation of aluminium oxides occurs in the wear scar area. X-ray diffraction analysis is conducted to prove this hypothesis, the result is depicted in Fig. 10 b).

Aluminium oxides (also called alumina) are identified, they play the role of a protective tribofilm [21-23] leading to mild wear regime transition (Fig. 9 b) by preventing the contact between counterparts, lower friction and wear are registered. This tribofilm is also important for corrosion passivation which is of utmost interest especially for seawater applications.

4.3 Microstructure analysisAfter etching, FE-SEM was used to reveal the microstructure

as seen in Fig. 11, various shapes are present with intercellular spacing such as needle-like (white lines inside grains in Fig. 11 c and 11 e) and globular morphologies (small grains as in Fig. 11 f), areas with ultra-fine grains along with relatively coarse grains exist. Different spots in the microstructure exhibit the

same chemical composition regardless the shape difference (Fig. 11 d). Knowing that the time required to melt the powder particles is of microseconds order [11] and that the cooling rate in L-PBF process is of the order of 106 K/s [24], a very fast solidification process takes place leading to a very fast freezing of the microstructure in the β' martensitic phase.

Culpan et al. [25] stated that the hot β cannot exist at room temperature and it should be described as β', this can also be assumed from hardness distribution shown in Fig. 5. From various studies ([13, 25-30]), it is shown that Nickel Aluminium Bronze exhibits different phases depending on manufacturing techniques and heat treatments applied, α phase, martensitic β', retained β phase, anodic γ2 phase and κ phases, each of them having a shape, hardness and chemical composition different from others, since the profile analysis in Fig. 11 d) and the chemical compositions are similar for different shapes, we assume that the martensitic β' is formed due to the extremely fast solidification as mentioned above with no presence of κ phases for the as-built material.

Fig. 10 Wear scar analysis, a) EDS mapping, b) XRD pattern

a)

b)



Fig. 11 Microstructures obtained after etching: same sample with various locations



5 Conclusion

Our endeavours from this study were fulfilled thanks to L-PBF process, production of high performance nickel-aluminium-bronze was successfully achieved, comparison with conventionally manufactured NAB was in favour to our L-PBFed NAB thanks to the very fine microstructure generated.

In a future investigation, the focus will be given to heat treatment for further improvement of our alloy performance, our objective will be the precipitation of the κ phases (κII, κIII and κIV), some are reputed for their high strength and hardness, others for their high wear resistance, combination of both is proved to be difficult to get using the same heat treatment condition. The challenge this time will concern determination of the heat treatment allowing obtention of phases assuming the best performance (mechanically and tribologically).

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