IMPROVEMENT THE CAVITATION EROSION RESISTANCE OF Al-Mg ALLOYS BY TIG SURFACE REMELTING

Aluminum-based alloys (Al-Mg, Al-Si-Mg, Al-Zn-Mg, etc.) are intended for the manufacturing of parts subjected to intense stresses by cavitation erosion. This complex phenomenon includes both the hydrodynamic factors of the liquid and the microstructure, hardness and ductility characteristics of the material. The present paper describes a method of increasing cavitation erosion resistance by using the local TIG remelting technique of the AlMg3 alloys surface. The experimental tests were performed according to ASTM G32-2016 standard. The response of the material to each value of the heat input was investigated by measuring the mass loss as a function of the cavitation time and by analysing the damaged surfaces using the optical and scanning electron microscopy. It has been shown that the TIG surface modification treatment increases the resistance to cavitation erosion of the alloy, as a consequence of the higher chemical and microstructural homogeneity and finishing of the granulation.


INTRODUCTION
Surface degradation of engineering components by cavitation erosion is due to the appearance, development and collapse of bubbles in liquids, as a result of pressure fluctuations, from values below the vaporization to values lots above that are inside the bubble. By the collapse of the bubbles, near the surface of the material, shock waves are generated whose pressure can reach 1 -1.5 GPa and micro-jets whose speed reaches approx. 130 m/s [1][2][3][4]. They repeatedly hit the surface of the piece leading to plastic deformations, microcracks and pinches, respectively cavities, by removing the material from the impacted area. Generally surface properties have a significant influence on the performance of components exposed by cavitation erosion. To improve the cavitation erosion behaviour of metals and metal alloys and to extend the lifetime of the components made from them, a series of surface engineering techniques are used to modify surface properties, such as thermal spraying, local remelting, thermochemical nitriding treatments, hard layer deposited by welding, etc. [3,4].
Al alloys are frequently used in the manufacture of components that operate under cavitation erosion conditions, such as cylinder liners, pistons, pumps, valves and combustion chambers. They usually have poor performance in cavitation erosion. This paper analyses a method of improving the cavitation erosion resistance by local surface remelting using the TIG electric arc. The local surface remelting was performed with the help of a device to which the TIG welding installation was attached (Figure 1).

Figure 1
The experimental stand used The remelting parameters used are given in Table 1. Other process data: • tungsten electrode diameter: 2.4 mm; • electrode distance -piece: 2 mm; • the step between two successive passes: 3 mm; • temperature between passes: 25 ºC; • argon flow rate used as shielding gas: 8 l/min.
Subsequently, surface remelted samples for cavitation tests and microstructural studies were taken.
Cavitation tests were performed in accordance with the international standard G32-2016 (the stationary specimen method) [5]. The test equipment contains a high frequency generator of 500 W, a transducer with piezoceramic crystals, an amplifier for mechanical vibrations and a water vessel with cooling oil, in which the test specimen is inserted (Figure 2). The vibration frequency was 20 ± 0.2 kHz, and the vibrations amplitude was 50 µm. The distance between the sample and the tip of the sonotrode was 1 mm. The test medium was distilled water maintained at 22 ± 1 ° C.

Figure 2 Image of the experimental cavitation test stand
For each structural state of the material, 3 samples were cavitated which testing surface was rectified and polished to remove any mechanically hardened layer that may form during their preparation. The total duration of each test was 165 minutes and was divided into 12 intermediate periods (one of 5 and 10 minutes and 10 of 15 minutes each).
At the beginning and at the end of each testing period, the samples were washed under tap water, distilled water, alcohol, acetone, dried under hot air and weighed.
Before beginning and ending of each intermediate test period, the surfaces exposed to cavitation were examined with the naked eye and photographed with the high resolution Canon Power Shot A480 in order to reveal the degradation of the surface exposed to the cavitation attack. Weighing was done with an analytical balance whose accuracy is 5 decimals (up to 0.00001 grams).
At the end of each intermediate test period "i", the corresponding weight loss mi, was determined. The analysis of the morphology of the damaged surfaces, after the completion of the cavitation tests, was performed by scanning electron microscopy. The explanation is based on the structural changes generated by the remelting process using TIG electric arc which is materialized by finishing the granulation and microstructure of the surface layer accompanied by an increase in hardness to values of 60…72 HV0. 3. It is known that the finer the granulation, the larger the surface boundary between the grains, the higher the deformation resistance and the lower the cavitation erosion rate. Scanning electron microscopy analysis, after finishing the cavitation test (165 minutes), of the remelted samples surface at a current of 150 A. Figure 6 demonstrates that this alloy suffers a degradation similar to metals with crystalline face-centred cubic lattice, meaning ductile fracture on the entire surface. Material losses  From the point of view of the rate erosion value towards which the curve v(t) tends, Figure 4, at 165 minutes, the decrease achieved by using a remelting current of 150 A is about 85% compared to the use of a current of 110 A and double by using a melting current of 190 A.

Topography of surfaces eroded by cavitation
Typical topographies of the cavitated surface highlight a degradation specific to ductile fracture with the formation in isolated material areas of striated craters with a flat bottom.