Shot peening is one of the finishing methods in which the shot peening elements exert a dynamic impact on the treated surface. A widely used method of shot peening is jet shot peening, which is characterized by the fact that the shot peening elements are ejected from the peening device and hit the workpieces [1, 2]. Steel [3], glass [4] and ceramic [5] balls are used as shot peening elements. Another variation of shot peening is vibratory shot peening. In the vibratory shot peening process, workpieces and shot peening elements (usually steel balls) are placed in a working chamber which performs oscillating motion [6, 7].
In both jet and vibratory shot peening processes, the peening elements move in a "disordered" manner, therefore it is difficult to determine parameters such as the impact energy of the shot peening elements and the number of impacts per unit area (impact density). These parameters can however be determined in impulse shot peening, in which the shot peening elements hit the workpiece with a known energy. The distances between successive dimples of hits can also be determined, which makes it possible to calculate impact density [8, 9].
Shot peening is mainly used to improve the properties of the surface layer of workpieces. As a result of shot peening, the geometric surface structure is increased, the surface layer is hardened and compressive residual stresses are induced [10 ÷ 12]. The practical result of changes induced in the properties of the surface layer by shot peening is an increased fatigue life of the workpiece [13, 14]. Shot peening also affects wear resistance [15, 16] and corrosion resistance [17]. Changes in the adhesive properties of treated surfaces as a result of shot peening were also observed, which led to increased strength of adhesive joints [18].
Surface layer properties that are favourable in terms of service life can also be obtained by burnishing. Burnishing involves impacting the surface of the workpiece with a smooth and hard burnishing element, with the force of this impact maintained at an approximately constant value [19, 20].
Effects similar to those obtained by shot peening and burnishing can also be produced by brushing. In brushing, a brush with metal or ceramic fibres is rotated at high speed to exert impact on the treated surface. In addition to changing the properties of the surface layer, brushing results in the removal of post-machining burrs and in the shaping of edges of produced parts [21, 22].
Shot peening, burnishing and brushing are used as finishing processes for, among others, magnesium alloy components. Owing to their properties such as low density, low coefficient of friction and ability to damp vibration, these alloys are an attractive construction material. The main areas of application of magnesium alloys include the aviation and automotive industries. Shot peening and burnishing increase the service life of components made of magnesium alloys and also make it possible to eliminate finishing, during which there is a risk of chip ignition [23].
Previous studies have mainly studied the effect of jet shot peening on the surface layer properties and service life of magnesium alloys. Wang et al. found that cold spraying shot peening of magnesium alloy AZ91D caused a significant increase in the microhardness of the surface layer and its wear resistance [24]. A study [25] investigated the effect of shot peening materials (glass, Ce-ZrO2, Zirblast B30) on the surface roughness, microhardness distribution, residual stresses and fatigue life of magnesium alloy AZ80. The impact of Almen intensity on the properties of the surface layer and fatigue life of this alloy was studied in [26 ÷ 28]. Fouad et al. conducted a comparative study of various surface treatments (burnishing, shot peening) on the wear rate of magnesium alloy AZ31 [29].
An important functional property of magnesium alloys is their corrosion resistance. Research by Liu et al. showed that shot peening resulted in a significant improvement in the corrosion resistance of magnesium alloy AZ31, while for the AZ91 alloy the improvement in this resistance was insignificant [30]. Mhaede et al. studied the effect of shot peening on the surface roughness, microhardness distribution and corrosion properties of the biocompatible magnesium alloy AZ31 samples coated with dicalcium phosphate dihydrate [31]. A study [32] investigated the effect of severe shot peening on the surface roughness, microhardness and residual stress distribution, fatigue properties and corrosion resistance of magnesium alloy AZ31.
Favourable changes in surface layer properties were obtained as a result of ultrasonic shot peening treatment and cavitation peening. The ultrasonic shot peening treatment of magnesium alloy AZ31caused a very large increase in the microhardness of the surface layer, which increased the wear resistance and reduced the friction coefficient [33]. Fatigue tests of ZK60 samples after ultrasonic peening treatment (UPT) showed a beneficial effect of this treatment on the residual stress distribution and fatigue strength. Changes in crack initiation sites were observed in the samples after UPT [34]. Zagar et al. found that cavitation peening of heat treatable magnesium alloy AZ80A resulted in an increase in the microhardness of the surface layer and in the formation of compressive residual stresses, as well as led to a several-fold increase in the Ra parameter of surface roughness [35].
Burnishing generally produces a lower surface roughness compared to that obtained by shot peening. Research by Jagadeesh et al. allowed the determination of the ball burnishing parameters for magnesium alloy Ze41A that ensured the lowest surface roughness [36]. A study [37] investigated the influence of burnishing force, feed rate, the number of passes, and medium type in the ball burnishing process for magnesium alloy AZ91D on the roughness of the machined surface of this material. The results of the study investigating the stereometric structure of magnesium alloy AZ91 after slide diamond burnishing were reported in [38]. On the other hand, the use of deep surface rolling as a treatment for magnesium alloy AZ91 made it possible to obtain a very large increase in hardness [39].
A study [40] presents the results of surface roughness tests of AZ91HP and AZ31 magnesium alloys after brushing with brushes with steel and brass fiber. The effectiveness of deburring and shaping the edges of brushed objects was also studied.
Previous studies on shot peening magnesium alloys have focused on jet and vibratory shot peening. The technological parameters determined in these types of shot peening processes related to shot peening devices such as air pressure and the distance of the nozzle from the workpiece (in jet shot peening), as well as the amplitude and frequency of vibrations of the vibrator (in vibratory shot peening). Parameters directly related to the shot peening process, such as impact energy and impact density, can be determined via impulse shot peening. The aim of this study is to determine the effect of impulse shot peening process parameters on the surface roughness and microhardness distribution, stress S11 of magnesium alloys AZ31 and AZ91HP.