Elsevier

Surface and Coatings Technology

Volume 200, Issue 8, 24 January 2006, Pages 2602-2609
Surface and Coatings Technology

Laser surface alloying of a marine propeller bronze using aluminium powder: Part I: Microstructural analysis and cavitation erosion study

https://doi.org/10.1016/j.surfcoat.2004.12.021Get rights and content

Abstract

In a previous study laser surface melting (LSM) was employed to improve the cavitation erosion resistance of manganese–nickel–aluminium–bronze (MAB)[9], [10][C.H. Tang, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 182 (2004) 300; C.H. Tang, F.T. Cheng, H.C. Man, Mater. Sci. Eng. A 373 (2004) 195]. To further enhance the improvement, laser surface alloying (LSA) using fine aluminum powder has been attempted in the present study. By employing appropriate laser processing parameters, a homogeneous alloyed layer of thickness about 1 mm, free of cracks or pores, was obtained. The alloyed layer was composed of a single phase, the bcc β-phase, with a Knoop microhardness higher than 300 HK. Cavitation erosion test in deionized water of the alloyed layer recorded a 30-fold improvement in the cavitation erosion resistance compared with as-received MAB. The resistance achieved in LSA was more than 3 times that by LSM. The relatively low-cavitation erosion resistance of as-received MAB was attributable to its heterogeneous and multi-phased structure. Surface-alloyed MAB, on the other hand, was characterized by a homogeneous microstructure which was single-phased. Apart from microstructural homogenization, the enhancement in cavitation erosion was also related to the increase in microhardness. Morphological evolution monitored over a period of cavitation erosion test revealed that brittle fracture mode prevailed, with material being chipped away from weak triple-junctions and grain boundaries. Such a mode of erosion damage was similar to the case in laser surface-melted MAB, but at a much milder degree, consistent with a higher erosion resistance in the case of LSA. The higher Al content in the LSA samples which resulted in a harder β phase could be the major reason for the higher resistance. In addition, the relatively larger grains in the Al-alloyed samples resulted in less grain boundaries, which were vulnerable sites for erosion initiation, hence also contributing to higher cavitation erosion resistance compared with the laser-melted samples.

Introduction

Manganese–nickel–aluminium–bronze (MAB) is an alloy commonly used for marine propellers, with popularity just next to its competitor nickel–aluminium–bronze (NAB). Both the MAB and the NAB are composed of the same component elements, but with MAB having a high Mn content (10.8 wt.%) and NAB, a low Mn content (1.3 wt.%). The two bronzes are similar in many aspects, but there are important differences [1]. In brief, MAB possesses better welding, hot-working, casting, and foundry properties, but its resistance to certain types of corrosion and cavitation erosion is inferior compared with NAB. It is the cavitation erosion problem that we intend to address in the present study.

Cavitation erosion is a common problem in hydraulic machinery such as propellers and in liquid-handling equipment [2], and repair of cavitated area in propellers is a common maintenance service in dockyards. Cavitation refers to the generation and collapse of bubbles in a liquid due to pressure fluctuations arising from changes in flow or to vibrations [3]. When the bubbles collapse near a solid surface, intense stress pulses are exerted on the surface. Repetitive attack from these pulses causes fatigue and fracture, leading to material loss, i.e., erosion [4]. Owing to the unique mode of attack in cavitation, which is in particular characterized by its localized nature, resistance to cavitation erosion of a material should be regarded as an independent property of its own [5]. In addition, owing to the localized nature of cavitation attack, microstructure also plays an important role [6].

The selection of materials for engineering components is always a compromise among a pool of factors, some of which being related to bulk and surface properties while others, to fabrication and cost. In this respect, the employment of surface modification or treatment has greatly widened the repertoire in material selection due to various possible combinations of bulk and surface properties. It also provides an economical way of prolonging the service life of engineering components since only the surface layer is treated. Among the various types of surface modification techniques, laser technique has a unique position. Laser surface modification is capable of producing a layer or treated area with microstructure and composition not obtainable by other techniques, together with a number of other desirable characteristics [7], [8].

In a previous study, laser surface melting of MAB to improve the cavitation erosion resistance was attempted, with the erosion resistance in deionized water increased by about 8.9 times [9], [10]. The improvement in resistance was attributed partly to homogenization in microstructure and partly to an increase in hardness. Although simple to perform, the effect brought about by laser surface melting is limited because the composition of the treated surface remains unchanged. The potential of laser surface treatment for improving the cavitation erosion resistance of MAB and other bronzes is far from being fully explored. According to the original developer, the composition of MAB has been compromised with regards to a set of performance and fabrication/processing properties. In particular, an Al content of 7–8% is chosen to optimize bulk mechanical properties [11]. However, in surface modification, only the surface properties are of concern and the restriction on composition can be relaxed. For surface layer intended to resist cavitation attack, hardness rather than bulk mechanical properties is a more important factor to consider. Thus laser surface alloying (LSA) of MAB with Al to increase the hardness is a possible way of enhancing the cavitation erosion resistance. It is the aim of the present study to investigate the effect of LSA of MAB with Al. In Part I, the microstructure and the cavitation erosion behavior in deionized water will be reported, and in Part II, the corrosion and cavitation erosion–corrosion behaviors together with the synergistic effect will be reported.

Section snippets

Materials and laser processing

Rectangular samples of dimensions 30 mm×15 mm×7 mm were spark cut from a propeller blade made of manganese–nickel–aluminium–bronze (MAB) conforming to UNS 95700 or B.S. CMA 1. The composition of MAB in weight percentage as determined by EDS analysis is shown in Table 1. Aluminium powder of average size of about 4 μm was mixed with a binder (4 wt.% polyvinyl alcohol, PVA) to form a slurry and then preplaced on sand-blasted sample surface by painting. The preplaced layer was mechanically grinded

Metallographic and microstructural analysis

Preliminary trials indicated that when the laser fluence was too low owing to a high scanning speed (sample MAB-Al-1), the melt pool was shallow and the surface layer cracked (Fig. 1) due to an excessively high cooling rate. On the other hand, when the fluence was too high (sample MAB-Al-6), the dilution was high and the resulting hardness was low. With appropriate laser processing parameters (fluence lying in the range 20–30 J mm−2), the preplaced Al powder together with an appropriate amount

Conclusions

Laser surface alloying of a marine propeller bronze (Cu–Mn–Al–Ni–Fe, designated as MAB in the present study) using aluminium powder has been attempted by means of high-power Nd:YAG laser. The cavitation erosion behavior of the laser treated samples in deionized water has been studied. The following conclusions are drawn.

  • (1)

    With a preplaced Al powder layer of 100 μm and laser fluence in the range 20–50 J mm−2, an alloyed layer of about 1 mm thick and free of cracks was obtained.

  • (2)

    The alloyed layer

Acknowledgments

The authors would like to acknowledge the Research Committee of the Hong Kong Polytechnic University for the provision of a research grant (Project No. G-9051).

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