LASER SURFACE ALLOYING (LSA) OF ALUMINIUM AA 1200 LASER SURFACE ALLOYING (LSA) OF ALUMINIUM AA 1200 WITH WITH

NICKEL POWDERNICKEL POWDERA.P.I. Popoola (1)*, S.L. Pityana (1,2), T. Rutlokoane (1)A.P.I. Popoola (1)*, S.L. Pityana (1,2), T. Rutlokoane (1)

1. Department of Chemical and Metallurgical Engineering, Tshwane University of Technology2. Center for Scientific and Industrial Research – National Laser Centre

 

INTRODUCTIONINTRODUCTIONAs the automotive industries addresses environmental concerns, the problem of fuel consumption and weight reduction has come to fore. Various test works have been done to introduce alternative aluminum alloy due to its low cost, high strength- weight ratio, low density and good corrosion resistance. Aluminum exhibit weak interatomic bonds, low abrasion resistance and low melting temperature does not suit the element in some engineering applications.

An intermediary element (Nickel) is introduced as an alloying substance to form a metallic layer on the surface of the alloy. Pure Ni is ductile, tough and possesses a face-centred cube crystal structure up to its melting point [1, 2, 6]. The alloying process was done by the use of high power Rofin Sinar ND: YAG laser machine with Ni as a coating element. Laser surface alloying is particularly efficient for producing surface layers with improved wear resistance. It combines the controlled modification of the microstructure and chemical composition to tailor surface properties [1, 2,5,].

PROJECT GOALS AND APPROACHESPROJECT GOALS AND APPROACHES The following work relates to surface hardening of aluminum alloy

AA1200 of known micro hardness of ~24HV. The test work is to be performed at four different laser scan speeds (0.6, 0.8, 1.0, 1.2m/min) to produced the best hardened alloy. The chemical composition of the aluminum substrate to be used, is indicated in table 1.

Particle size analyses of the Nickel powder determined using the Malvern Mastersizer 2000 indicated an average particle size of m. Figure 1 indicates size distribution of Nickel particles against the bulk volume.

Figure 1: Particle size distribution of Ni powder

Figure2: Scanning electron micrograph and EDS of Ni powder

The EDS indicated high peaks of Nickel, which indicates that the powder to be used for alloying is pure. Figure 3 indicates the X-diffractograph of the Ni powder used for alloying. The XRD analysis on the Nickel powder showed their patterns with only Nickel, powder purity confirmed.

Figure 3: Xray Diffractograph of the Nickel powder

Figure 6: Stereo micrograph of the laser-surfaced AA1200 with Nickel (sample A) at 0.6m/min laser scan speed.

A crackless, uniformly alloyed surface and good bonding was obtained from sample A as illustrated in figure 6. The depth of the alloyed surfaces decreases with an increase in laser scan speed. For sample A and sample D, the depth is 1.46mm and 1.20mm respectively.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTSThe author would like to thank Tshwane University of Technology and CSIR- National Laser Centre for financial support of this work.

REFERENCESREFERENCES

[1]S.L. Pityana, Hardfacing of aluminium alloys by means of Metal Matrix Composites produced by laser surfacing alloying, Proceedings of LIM-2009, Munich, Germany, 439-444.

[2] M.H. Staia, M. Cruz, N.B. Dahotre, Microstructural and tribological characterization of an A-356 aluminium alloy superficially modified by laser alloying, Thin Solid Films 377-378, (2000) 665-674.

[3] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Improvement of the wear resistance of 316L stainless steel by laser surface alloying, Surface ad Coatings Technology 80, (1996) 207-210.

[4] H.C. Man, S. Zhang, T.M. Yue, F.T. Cheng, Laser surface alloying of NiCrSiB on Al6061 aluminium alloy, Surface and Coatings Technology 148, (2001) 136-142.

[5] R.J. Davis, J. R. Davis & Associates, Aluminum and aluminum alloys, ASTM International. Handbook Committee, (1993) 574-579.

Position [°2Theta]

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Counts

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Ni

Ni

Ni

NIPOW.RD

The X-ray diffractograph of the AA1200 alloy can be seen in Figure 4. This shows the identified phases present in the Al; only aluminum peaks can be seen, an evidence of the purity of the substrate. The micro hardness value of the AA 1200 is 24.0 ± 0.4.

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2 Theta (degrees)

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nsity

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1 = Al

Figure 4: Xray diffractograph of AA1200 aluminum alloy

Sample A is the best improved sample on hardness value (~646.9Hv). The dispersion of HV obtained is related to the heterogenouity of the surface microstructure. The lowering of the hardness can be attributed to melting and mixing of more aluminium substrate into the coating, which is clearly demonstrated. The improvement in hardness (over 20 times the microhardness of substrate) was attributed to the formation of intermetallic phases/dendritic microstructure formed by alloying.

Hardness profile with depth of each sample

0100200300400500600700800900

0 500 1000 1500 2000Position (microns)

Har

dnes

s (H

v) sample A(0.6m/min)

sample B(0.8m/min)

sample C(1.0m/min)

sample D(1.2m/min)

Figure 5: Microhardness distribution of laser-alloyed surface under different laser beam energy.

Figure6: (a) Scanning electron micrograph and EDS of Sample A (0.6m/min), (b)Optical micrograph of sample A

The optical micrograph, the scanning electron micrographs with the EDS of the cross section of the polished samples laser alloyed can be seen in Figures 6 and 7. The microstructure is dendritic in nature for both samples alloyed.

Figure 7: (a) scanning electron micrograph and EDS of sample D (1.2m/min) (b) optical micrograph of sample D

Single track lines were made on the AA 1200. Cross-sections of the alloyed layers were cut and polished. The polished surfaces were etched using Keller’s reagent. The microstructures of the new phases were characterized by optical and SEM. The characteristics of the phases were studied by means of X-ray diffraction. It was analysed with a PANalytical X’Pert Pro powder diffractometer with X’Celerator detector and variable divergence- and receiving slits with Fe filtered Co-Kα radiation. The phases were identified using X’Pert Highscore plus software.

Hardness was determined using the Vickers hardness tester.100 μm spacing between corresponding indentations with an applied load of 200 g and a holding time of 5 seconds was used. The hardness/depth profiles were plotted for all samples.

This indicated high possibilities of a uniform reactivity between the powder and aluminum substrate during laser alloying. Figure 2 indicates the scanning electron microscopy and the EDS of Nickel powder. The shapes of the Ni particles are round and irregular, which assist in providing constant, frictionless movement when fed onto the molten surface of the substrate during alloying.

Sample label A B C DSystem composition Al-Ni Al-Ni Al-Ni Al-NiLaser Power (kW) 4.0 4.0 4.0 4.0Beam diameter (mm) 3.0 3.0 3.0 3.0Scan speed (m/min) 0.6 0.8 1.0 1.2Powder feed rate (rpm) 2.0 2.0 2.0 2.0Shielding gas Argon Argon Argon ArgonShielding gas flow (l/min)

4.0 4.0 4.0 4.0

Table 2: Laser processing parameters on all four samples produced

RESULTSRESULTS

Position [°2Theta] (Cobalt (Co))

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PopoolaP_Al-Ni_7

Peak List

Al; Aluminum, syn; Cubic; Fm-3m

Al1.1 Ni0.9; Cubic; Pm-3m

Al4 Ni3; Cubic; Ia-3d

Al3 Ni2; Hexagonal; P-3m1

Figure 8: X-ray diffractograph of a typical alloy layer; laser processing parameter: laser power 4 kW, scan speed 0.6 m/min and powder feed rate 2rpm.

The X-Ray diffraction results of sample A shown in figure 8 reveal that the laser treated surface is composed mainly of pure aluminum and secondary phases (Al4Ni3, Al3Ni2 and Al1.1Ni0.9). Some of the phases have overlapping peaks. The EDS analysis of the samples confirmed the presence of Ni, Al and O. Very slight trace of C elements were shown to be present in the EDS analysis of samples A.

CONCLUSIONCONCLUSION1. Laser surface alloying of AA 1200 pure aluminium with Nickel

reinforcement using a Rofin Sinar continuous wave Nd: YAG solid-state laser was successfully carried out. The best alloyed surface with the highest HV was obtained at the lowest Laser scan speed. The hardness of the newly produced surface is 20 times higher than the original substrate. The average HV of the alloy was improved from ~24 to ~646HV.

2. The alloyed surfaces microstructure formed consisted of dendritic structure and different intermetallic phases which includes: Al4Ni3, Al3Ni2, Al1.1Ni0.9. A crackless surface was achieved.

Element Al Fe Cu Si

Composition (wt.%) Balance 0.59 0.12 0.13

Aluminium and Nickel powder verificationAluminium and Nickel powder verification The substrate material, was cut to dimensions 100 x 100 x 6 mm, sand blasted to clean the surface of substrate and to improve absorptivity of the laser beam.

The Malvern Mastersizer was used for the analysis of the Ni powder particle size distribution. The powder particle morphology and size distribution were analyzed using a scanning electron microscope SEM. A Philips PW 1713 X-ray diffractometer fitted with a monochromatic Cu Kα radiation set at 40 kV and 20 mA was used to determine the phase composition of powder. The scan was taken between 10 and 80 two theta (2Θ) with a step size of 0.02 degree. Phase identification was done using Philips Analytical X’Pert HighScore® software with an in-built International Centre for Diffraction Data (ICSD) database.

Laser surface alloying and material characterisationLaser surface alloying and material characterisation

Laser surface alloying was carried out with a high power Rofin Sinar Nd: YAG solid-state laser fitted with off-axes nozzle used for powder feeding. The laser is delivered to the substrate through fibre optics. A Kuka robot is used to move the alloying head. Argon gas was used to shroud the molten pool from the atmosphere to prevent oxidation during the alloying process. Laser parameters are shown on table 2.

Table 1: Chemical composition of the alloy AA1200