Performance analysis of friction surfacing

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Abstract

Performance criteria regarding the material deposition rate and energy consumption per unit of deposited mass were established for the characterization of friction surfacing. These criteria were tested in the friction surfacing of mild steel, for a range of process parameters. The influence of forging force, consumable tilt angle, travel and rotation speeds on interfacial bond properties and process efficiency were investigated. Coatings were examined by optical microscopy, image processing techniques and hardness testing. The applied load on the consumable rod was found to be essential to improve joining efficiency and to increase the deposition rate. Higher rotation or travel speeds were detrimental for the joining efficiency. Tilting the consumable rod along the travel direction proved to improve the joining efficiency up to 5%. For the testing conditions under study, the material loss in flashes represented about 40–60% of the total rod consumed, while unbonded regions were reduced to 8% of the effective coating section. Friction surfacing was seen to require mechanical work between 2.5 and 5 kJ/g of deposited coating with deposition rates of 0.5–1.6 g/s.

Deposition rates are higher than for laser cladding or plasma arc welding with a specific energy consumption lower than for other cladding processes.

Highlights

Deposition rates are higher than in laser cladding or PAW. ► Specific energy consumption is lower than for other cladding processes. ► Mechanical work is between 2.5 and 5 kJ/g of deposit with rates of 0.5–1.6 g/s. ► Tilting consumable rod in 3° led to a 10% increase in the bonded width. ► Surface hardness increased up to 115% compared to consumable rod.

Introduction

Surface engineering has become a relevant research field for manufacturing industries, as it enables advanced component design and a selective functionalization of surfaces. Solid state processing technologies are now mature and reliable alternatives to conventional processes, as stated by Mishra and Ma (2005).

Nicholas and Thomas (1986) defined friction surfacing (FS) as a solid state technology used to produce metallic coatings. As depicted in Fig. 1, a rotating consumable rod is pressed under an axial load against a substrate. Heat generated in the initial friction contact promotes viscoplastic deformation at the tip of the rod. As the consumable travels along the substrate, the viscoplastic material at the vicinity of the rubbing interface flows into flash or is transferred over onto the substrate surface, while pressure and heat conditions trigger a inter diffusion process that soundly bonds the deposit. As the material undergoes a thermo-mechanical process, a fine grained microstructure is produced by dynamic recrystallization. However, the coating cross section presents unbonded regions on both sides, as referred by Vitanov and Voutchkov (2005).

According to Macedo et al. (2010), friction surfacing has been used in the production of long-life industrial blades, wear resistant components, anti-corrosion coatings and in the rehabilitation of worn or damaged parts such as, turbine blade tips and agricultural machinery. Other applications feature the hardfacing of valve seats with stellite and tools such as punches and drills.

Friction surfacing allows to deposit various dissimilar material combinations. Rafi et al. (2011), Batchelor et al. (1996) and Chandrasekaran et al. (1997) investigated the deposition of stainless steel, tool steel and nickel-based alloys (Inconel) on mild steel substrates, as well as, stainless steel, mild steel and inconel consumables on aluminium substrates. Friction surfacing was also used to produce aluminium metal matrix composites on aluminium and titanium substrates by Reddy et al., 2009, Reddy et al., 2011.

The influence of process parameters on deposit characteristics and bonding strength was addressed. Vitanov et al. (2000) developed a decision support system to correlate the resulting bond strength, coating thickness and width with forging force, spindle and travel speeds. The increase of forging force improved the bond strength and reduced the coating thickness. The undercut region was reduced at higher forging forces and increased at faster travel speeds. Vitanov et al. (2010) observed that higher ratios between the feed rate of the consumable and the travel speed resulted in superior bonding quality, as well as, lower to intermediate values of rotation speed. In the friction surfacing of low carbon steel with tool steel H13 consumables, Rafi et al. (2010) concluded that the coating width was strongly influenced by the rotation speed, while thickness was mostly determined by the travel speed.

Since the deposition results from severe viscoplastic deformation, friction surfacing presents some advantages over other coating technologies based on fusion welding or heat-spraying processes, that produce coarse microstructures and lead to intermetallics formation, thereby deteriorating the mechanical strength of the coatings. Other techniques can prevent this problem as shown by Lachenicht et al. (2011) using spray forming. Macedo et al. (2010) stated that apart from avoiding defects commonly associated to fusion and solidification mechanisms (porosities, hot cracking or slag inclusions), the heat input in friction surfacing is minimum and localized, preventing part distortion and minimizing the heat affected zone and dilution. Additionally, the absence of spatter, toxic fumes and emission of radiation makes this process cleaner and environmentally friendly.However, friction surfacing currently struggles with several technical and productivity issues which contribute to a limited range of engineering applications. Further work is needed to develop friction surfacing as a competitive alternative to other existing coating technologies. Comparison between coating processes must consider the quantification of deposition rates and energy efficiency, which have not been determined for friction surfacing.

This paper aims to quantify the mass transfer rate and the specific energy consumption in friction surfacing, contributing to establish a realistic comparison with other coating technologies and to present a deep study of the process parameters, specially the effect the tilt angle, which was not addressed in previous studies.

Section snippets

Materials and methods

Friction surfacing was performed using a ESAB LEGIO™ 3UL numeric control machine equipped with 10 mm diameter mild steel consumable rods with 30 mm length. Mild steel Plates 10 mm thick were used as substrates. This material was selected since it is soft and ductile, thus prone to flash formation, which is relevant if a performance analysis is to be addressed. Table 1 presents the base material chemical composition determined by X-ray fluorescence spectroscopy.

Consumable rods and substrates were

Metallurgical analysis

Fig. 2 presents a cross section macrograph of both rod and coating, depicting the gradual transformations of the consumable material into the deposit as it is plunged downwards. The fitting between the surfaces of the consumable and the coating can be observed, as well as, the flash developed. The consumable rod microstructure is depicted in Fig. 2a, presenting a α-ferrite and pearlite phases elongated along the rod extrusion direction. The hot working of the consumable rod tip and the deposit

Performance analysis

In order to analyze the performance of friction surfacing, the following quantitative variables were defined to support an effective analysis of the material deposition and energy efficiency.

Comparison with other coating processes

Table 3 presents typical performance values for several mainstream coating processes, proving that friction surfacing is a leaner technology in terms of specific energy consumption compared with laser-based and arc-welding techniques.

Conclusions

From the present work, the following can be concluded:

  • Friction surfacing enables intermediate mass deposition rates and higher energy efficiency in comparison with several mainstream laser-based and arc-welding cladding processes. It was seen to require mechanical work between 2.5 and 5 kJ/g of deposited coating with deposition rates of 0.5–1.6 g/s.

  • Forging force enhances joining quality and coating hardness, while contributing to a higher overall coating efficiency.

  • Faster travel speeds improve

Acknowledgments

The authors would like to acknowledge FCT/MCTES funding for the project ‘Technology developments of friction stir processing to produce functionally graded materials and improve surfaces for advanced engineering applications – FRISURF’ (PTDC/EME-TME/103543/2008). The authors wish to express their gratitude to MRA – Instrumentação for thermal imaging technical support.

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