Electro-Spark Coating with Special Materials - nonconventional ...

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The operator has full control over the process, during its run time. It is a cold coating process, without heating the base metal, so that no distortion occurs. The coating rate is 17,4... 18,6 g / h. B. Electro-spark coating with an aluminium electrode On a sample with the sizes 60 mm x 50 mm x 25 mm of the same base metal, some coatings were also performed, using an 1.6 mm diameter aluminium electrode [6]. The applied parameters were in the ranges mentioned in the previous example. In 20 min, a coating of approximately 8mm x 12mm with continuous aspect is made on the base metal sample. The thickness of the coating is estimated at 0.05...0.15 mm. The deposition rate is 18... 25 g / h. C. Electro-spark depositing with tungsten carbides In an experiment [7,8], the coatings were realized by the electro-spark process, using a WC-Co (97% WC and 3% Co) electrode with a cross-section of 3 x 4 mm (connected to the anode), onto rings made of carbon steel C45 (connected to the cathode) [7,8]. The EIL-8A model was used as equipment for ESD, with the following parameters: · voltage U = 230 V, · capacitor C = 300 μF, · current intensity I = 2.4 A. The quality of electro-spark deposition depends mainly on the shape, duration, and average value of current or pulse power. Then, the coatings were treated with an Nd:YAG laser (impulse mode), model BLS 720, with the following parameters: · laser spot diameter: d = 0.7 mm, · laser power: P = 20 W, · traverse speed: v = 250 mm/min, · nozzle-workpiece distance: Df = 1 mm, · pulse duration: ti = 0.4 ms, · pulse repetition frequency: f = 50 Hz, · laser beam shift jump: S = 0.4 mm. 5. RESULTS AND DISCUSSION 5.1. Analysis of the coating morphology A. In the case of the electro-spark coating with a stainless steel electrode, Fig.5 presents the structure of the stainless steel coating (magnification 250 X). The structure is relatively uniform. The structural constituents are austenite and delta ferrite Nonconventional Technologies Review – no. 1/2011 60 (4...6% content), spread relative uniformly in the coating. There are white and black zones, coresponding to these structural constituents. No defects, as cracks, lack of material, inclusions of other materials. This means, at this magnification of the microscope, that the material is uniformely overlayed on the base metal surface. Fig. 5. Electro-spark coating with stainless steel on carbon steel (250 X) [6] Fig. 6. Electro-spark coating with aluminium on carbon steel (250 X) [6] B. In the case of the electro-spark coating with an aluminium electrode, as presented in the microstructure of fig.6, the following structural costituents are detected: metal matrix of poly-crystalline aluminium, with poly-crystalline silicon grains and aluminiumsilicon eutectoid at the grain boundary. Inclusions of silica (silicon oxide) and alumina (aluminium oxide) are observed, which arised from the elaboration of the aluminium batch, in the continuous casting of the wire, that then was cold drawn to 1.6 mm diameter wire, of which the electrode applied here was
delivered. No defects were seen, of the categories cracks, lack of metal, macroscopic non-metalic inclusions with elongated or round shape, etc. By informative scratch tests, under experimental conditions, it was estimated that the adherence of the coating is satisfactory. C. A micro-structure analysis was conducted for the tungsten carbides (WC-Co) coatings [7], using a scanning electron microscope Jeol JSM-5400, before and after the laser treatment. Fig. 7. WC-Co coating microstructure after the electro-spark coating process [7] Fig. 8. Microstructure of the electro-spark WC-Co coating after treatment with a Nd:YAG laser [7]. The thickness of the ESD obtained layers was 20 to 30 µm, whereas the heat affected zone (HAZ) ranged 15 to 20 µm into the substrate. There is a clear boundary between the coating and the substrate, where pores within micro-cracks are observed, as shown in the fig. 7. The laser treatment leads to the homogenization of the chemical composition, structure refinement, and crystallization of supersaturated phases, due to the occurrence of both increased temperature gradients and high cooling rate. The laser- Nonconventional Technologies Review – no. 1/2011 61 modified outer layer has no micro-cracks and pores and no discontinuity of the coatingsubstrate boundary, as presented in the fig. 8. Its thickness was in the range of 40 to 50 µm. The HAZ thickness was 30 to 40 µm. Its content of carbon was higher. 5.2. Hardness tests The average micro-hardness of the WC- Co coating [7] was 617 HV0.04, that is a 335% increase compared to the substrate. The HAZ micro-hardness after the laser treatment has increased by 185% in relation to the substrate. A slight decrease of 21% of the coating micro-hardness after the laser treatment occurred, that may cause an improvement of the elastic properties. This is important for operation under high loads, e.g.: drilling tools in the extractive industry, press elements for ceramics, etc. [7-10]. 5.3. Tribological tests The tribological tests [7-10] were carried out with pin-on-disc testers. The pin of 4x20 mm was made of tool steel. The tests [8] were conducted at the following parameters: · rotational speed: n = 637 rev/min, · number of revolutions: i = 5305 rev, · test duration: t = 500 s (up to stabilization determined in the initial tests), · range of load changes from 5 to 15 N. Dry friction and status stabilization of the anti-wear layer was observed. The average friction force was 35% lower after the laser treatment. Seizure resistance was also measured using a pin-on-disc tribo-tester. The laser processing caused an increase in the seizure of the coatings and C45 steel. 5.4. Adhesion tests A scratch test was conducted to measure the adhesion of the WC-Co coatings [7,8] before and after laser treatment. A CSEM Revetest scratch tester was used. The mean value of the critical force was 5.99 N; after laser treatment, it increased to 7.88 N. The treatment caused a 32% improvement in the adhesion of the WC-Co coating, probably due to the lower porosity and higher sealing [7]. Other research on electro-spark deposition is conducted by NASA and US Navy [3]. 6. CONCLUSIONS 1. The electro-spark coating is a technological proces for realizing 0.01 ... 0.5
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