Comparative Analysis of the Eff ect of Machining with Wire and Ceramic Brushes on Selected Properties of the Surface Layer of EN AW-7075 Aluminium Alloy

In the cutting process, when the cutting edge leaves the material, burrs are often formed on the edges of the workpiece. Due to inconveniences associated with burrs, such as problems with properly fi xing the workpiece on the machine for further processing or problems with parts working in assemblies, deburring operations must be performed. In addition to that, burrs may pose danger for workers at the production stage and for customers at the stage of using the fi nished product. One of the popular deburring methods is brushing. Apart from deburring, brushing can be used to change surface layer properties. The aim of this study is a comparative analysis of the eff ect of machining with wire and ceramic brushes on microhardness and the surface quality of EN AW-7075 aluminium alloy. Results show that, for all defi ned brushing conditions, lower surface roughness parameters are obtained when using ceramic brushes rather than wire brushes. In contrast, the use of wire brushes leads to increased microhardness of the surface layer.


INTRODUCTION
Final operations on production lines where products are manufactured by machining are usually performed at the locksmith's workstation. These operations involve checking, rounding and deburring the edges of the workpiece. The choice of the edge processing method depends on the workpiece size [1]. Automated processing such as vibratory fi nishing is used for small parts [2]. Other methods are rotary-abrasive, abrasive blasting and abrasive fl ow machining [3]. Abrasive water jet machining can be used for deburring, particularly in hard-to-reach workpiece areas [4,5]. To automate most production processes, deburring methods without human intervention are being investigated, especially for large-size parts produced on CNC machines. Given that the workpiece is fi xed on the CNC machine table, the deburring process can be conducted using dedicated tools such as brushes. The main area of industrial application of the brushing treatment is deburring. However, brushing can also play other functions, e.g. it can be used for surface cleaning [6], removing corrosion and old varnish coats, hole fi nishing [7], surface layer modifi cation and strength improvement [7,8]. Examples of the above-mentioned applications and eff ects of brushing are shown in Figure 1.
The studies [9][10][11][12] showed that the brushing treatment induces changes in surface layer properties, in terms of both surface roughness and microhardness. It is estimated that up to 80% of damage to machine components occurs at the surface or in the surface layer directly below the surface. One of the popular ways to improve strength and fatigue life is to introduce residual stresses into the surface layer of an object. The study [9] showed that benefi cial compressive stresses in the surface layer were induced after brushing with a cylindrical wire brush. Brush fi bres hitting the Comparative Analysis of the E ect of Machining with Wire and Ceramic Brushes on Selected Properties of the Surface Layer of EN AW-7075 Aluminium Alloy surface can produce a similar eff ect in the surface layer as shot peening or shot blasting [13].
Compressive residual stresses have a signifi cant impact on improving fatigue strength [14][15][16]. The study [17] investigated the eff ect of brushing process on the surface roughness, residual stresses, surface layer work-hardening, and fatigue strength of AA 5083 aluminium alloy. It was established that the wire-brush hammering of notched samples led to increased fatigue life.
In addition to surface cleaning and corrosion product removal, the brushing treatment can increase corrosion resistance. This is possible due to the grain refi nement that occurs after brushing. Infl uence of brushing on corrosion resistance of AZ31B magnesium alloy was analysed in [18]. It was shown that the corrosion resistance of the brushed magnesium alloy sheet was about four times higher than for the non-brushed workpiece. Corrosion resistance is correlated with the formation of structural defects. The study [19] proposed a novel method of detecting defect using recurrence plots and recurrence quantifi cations. In addition, continuous surface nanocrystallization (SNC) of rebars was obtained through the brushing process, which signifi cantly improved corrosion resistance [20]. The process of nanocrystallization was also investigated in [21]. It was shown that the surface nanocrystallization of mild steel could be obtained by wire brushing and that an ultrafi ne-grained surface layer with an average grain size of 77 nm could be produced through this treatment.
Ceramic fi bre brushes are relatively new tools. The problem of surface uniformity after ceramic brush machining aimed at automatic lapping of a large work surface was studied in [22]. Variable revolution speed, feed rate, preload and protrusion were used in the study. The numerical model took into account diff erent values of fi bre projection length from the sleeve. It was found that elastic deformation of the brush tip had a signifi cant eff ect on the surface profi le.
The study [23] investigated the infl uence of ceramic brush machining on the edge state and surface quality of aluminium alloy after AWJ cutting. It was shown that in the water jet cutting process burrs were formed primarily at the exit side of the jet. A desired deburring eff ect was obtained for all tested fi bre types yet the edge state (chamfering or rounding) depended on the fi bre rigidity.
The objective of this study is a comparative analysis of the eff ect of treatment with wire and ceramic brushes on microhardness and the surface quality of EN AW-7075 aluminium alloy. These tools are dedicated to fi nishing both edges (deburring, rounding) and surfaces.

TEST METHODOLOGY
Overall methodology of the study is schematically shown in Figure 2. The experiments were performed using two types of tools for surface fi nishing: an XEBEC ceramic brush and an end wire brush. Surface topography and roughness were measured with the T8000RC120-400 profi lographometer. Visual assessment of the surface after brushing was performed using the Keyence VHX 5000 digital microscope. Surface microhardness was analysed with the Leco LM700 tester. The load on the indenter was 50 g for a period of 15 seconds.  Table 1 shows the physical properties and chemical composition of this alloy.
Before brushing, all specimens were milled using an Iscar carbide end mill with a diameter of D = 20 mm. This cutter is characterized by geometry dedicated to machining of light alloys. Fixed milling parameters (a p = 0.5 mm, v c = 500 m/min, f z = 0.1 mm/tooth) were set to ensure that surface fi nish would have constant roughness. The mean value of the surface roughness parameter Ra was 1um. Eff ects produced with the brushing treatment were compared to the roughness obtained by milling.
To compare machining eff ects produced with the two types of brushes, tools with the same diameter, D = 15 mm, were used. In addition, the experiments were conducted using three diff erent feed rates. Details of the experimental parameters are given in Table 2.

Surface roughness
An example of the surface topography after the milling process is shown in Figure 3. The characteristic marks on the surface are the result of the shape of the blade mapping onto the surface layer, and the distance between the individual marks equals the feed per tooth f z . Table 3 contains sample images showing the surface topography after treatment with ceramic and wire brushes. At low feed rates, the brushes have longer contact with the workpiece, which leads to the removal of machining marks produced during milling. The ceramic brush treatment resulted in the removal of sharp peaks of micro-inequalities and reduced roughness compared to the surface after milling in the entire range of applied feed rates. In contrast, the wire brush treatment led to increased roughness in relation to the initial treatment.
The eff ectiveness of removing the sharp peaks of micro-inequalities is shown in the surface topography images in Table 4. It can be observed that the wire brush removed and deformed the material more intensively than the ceramic brush.
However, for the whole range of applied feed rates, the wire brush treatment led to higher values of the roughness parameter Ra compared to milling (Fig. 4). If the wire brush contact with the workpiece is longer (lower feed rate), greater deformation (increased roughness) of the surface layer can be observed.  Ceramic brushes ensure better finishing properties. Compared to milling, the ceramic brush treatment leads to improved surface roughness in all analysed cases. The lowest roughness value (Ra = 0.49 um) was obtained for the feed rate vf = 140 mm/min.

Surface microhardness
Brush fibres hitting the surface can induce a similar strain hardening effect as shot peening or shot blasting. Figure 5 shows the influence of feed rate and brush type on surface microhardness. The ceramic brush treatment has an insignificant effect on microhardness (its value is within standard deviation). This indicates that ceramic fibres have a predominantly abrasive effect on the workpiece surface. However, after machining with the wire brush, the microhardness increases significantly compared to its value before brushing (average microhardness HV0.05 after milling is 175). The maximum average microhardness HV0.05 after wire brushing is 201 for a feed rate of 140 mm/min. This indicates a shot peeing effect caused by the brush filaments hitting the workpiece surface.

CONCLUSIONS
The experimental results of the study investigating the impact of treatment with wire and ceramic brushes on the microhardness and surface quality of EN AW-7075 aluminium alloy lead to the following conclusions: • brushing can be used for surface fi nishing to impart specifi c stereometric as well as physical properties; • the Sa parameter is lower for a longer contact time of the ceramic brush with the surface (the lowest value of Sa = 0.504 µm was obtained in brushing with a feed rate of 140 mm/min); • wire brushing leads to almost complete removal of milling marks; • the wire brush treatment leads to an increase in the roughness parameter Sa; • for the whole range of applied feed rates, lower values of the Sa and Ra roughness parameters could be observed after ceramic brush machining, when compared to roughness after milling; • wire brushing leads to surface degradation and increased roughness parameters; • microhardness increases signifi cantly after wire brushing compared to milling (the maximum average value HV0.05 after wire brushing was 201 for a feed rate of 140 mm/min), which indicates a shot peeing eff ect caused by the brush fi laments hitting the workpiece surface.
It should be noted that the wire brush treatment does not always lead to increased surface roughness and that its eff ects strongly depend on initial roughness. For higher roughness values after milling, improved surface roughness can be observed after wire brushing.