Formation of the surface roughness during grinding with flap wheels after shot peening

Shot peening is widely used in forming long panels and sheaths. Due to impact by shot on the processed surface, a specific microgeometry is formed, the characteristic feature of this microgeometry are the numerous dimples as the traces of shot impact with different diameters and depths. A presence of these dimples causes deterioration of the surface roughness parameters. Therefore, after shot peening the mandatory requirement is the implementation of surface grinding with flap wheels for partial removal of the dimples. The size of the assigned allowance for grinding depends on the quality requirements of the part surface. At the same time, the depths of the remaining dimples are determined by the part surface roughness requirements. After grinding, the new surface microgeometry is formed, as a combination of micro-roughness from previous types of processing and the remained dimples in result of shot peening. In this work the microgeometry formation of surface layer of the samples after shot peen forming and subsequent grinding with flap wheels was analysed. The parameters of surface roughness were measured by the method of three-dimensional optical scanning. In the measurement result, the mathematical model of the surface micro-profile formation was formulated, the analytical dependences of the position of the center plane and the arithmetic mean deviation of profile were obtained.

where kk is the surface roughness factor before and after grinding (kk = 1...1,1 when the height of the original micro-roughness decreases by 1-2 times, kk = 1,2...1,25 -by 3-4 times); Rz org is the height of the original micro-roughness. It follows from (1) that the allowance value for grinding is equal to or greater than the average height of the original micro-roughness. In this case, the dimples with the depth exceeding the value of a, will be partially removed.
Regarding to the surface structure after shot peen forming, the allowance value a, characterizing the value of removal of the dimples, is determined by the depth of the dimples (Fig. 1), [7].

Fig. 1.
Schematic representation of the part surface treated by shot peening, where hmax is the maximum depth of dimple; a is the allowance value, which will be removed during grinding.
The maximum value of the depth of the shot dimple hmax on the investigating sector of the surface at grinding stage must be reduced by the allowance amount a for compliance with the requirements of design documentation and tolerance of the part. In this case, a removal of all traces of shot is not necessarily and in the result after grinding a large number of shot dimples are remained.
Thus, the traces of shot peening largely determine the surface roughness of the grinding part before and after grinding.
Due to a chaotic distribution of dimples on the treated surface [1], similarly to a method for determining a height of micro-roughness within a base length (Rz), a height of profile micro-roughness within a base area (Sz) [6,8] can be determined by the following formula (2): where ηpi, ηvi -respectively 5 the highest peaks and 5 deepest valleys of the surface profile within the base area.
Analysis of the topography of the milled surfaces of the samples with subsequent shot peen forming showed the following: in general, the highest peaks of the profile are formed from previous milling process, and the deepest valleys are the deep dimples from shot peening process, herewith the bottom depths of these dimples is significantly higher than the deepest valleys from milling. Thus, the average height of the micro-roughness of the resulting surface profile is more dependent on the depth values of the largest dimples. Figure 2 shows the scanning result with the optical profilometer «Bruker Contour GT-K1» of the milled surface area of the sample followed by shot peen forming. At the top of figure shows the top view of scanned surface, at the bottom -the profile of surface in the mutually perpendicular planes, passing through one of the largest dimples on the study area. In the result after grinding with flap wheels, from the processed surface of part the required metal layer was removed. Figure 3 shows the scan result for the sector area of 10×10 mm of the cleaned surface of the sample shown in figure 2, so that the scan area for shot peen forming coincided with the scan area for grinding. This figure also shows the surface profile in mutually perpendicular planes passing through the same deepest dimple. In result of data review on the figures 2 and 3 we can conclude that even in this slight removal of the allowance by flap wheels, on machining part a new surface micro-relief is formed, as a combination of micro-roughness formed by successive stages of processing, including mechanical machining, shot peening and grinding. That is, after grinding, there are still many large dimples that are directly involved in forming the surface roughness of the machining part. It should be noted that the parameter Ra during grinding is not exceeded the allowable value during machining of panels and sheaths (in our case, the allowable value Ra = 3,2 microns).
Thus, the original center plane of the surface profile formed by milling, is shifting down after shot peening [9]. In subsequent grinding the center plane also moves down when the material layer thickness within the allowance is removed. That is, the center plane of profile after grinding is below the center plane of profile after shot peening. Figure 4 presents the graphical diagram of the location of main parameters of surface profile after milling, shot peening and grinding with flap wheels. At the same time, the basic parameters of surface roughness after grinding are determined on the basis of the location of the center plane Pk. The figure 4 includes the following designations Rs -radius of shot; P0 -original center plane after milling; Pi -center plane after shot peen forming; Pp -center plane after grinding without shot peen forming; Pk -center plane after grinding; hi -bottom depth of the k-th dimple; hp -allowance removed during grinding; hk -dimple depth after grinding; hk' -bottom depth of dimple from center plane after grinding, including shot peen forming; hk" -distance between center plane after grinding without taking into account shot peen forming (Pp) and center plane after grinding (Pk); ri -dimple radius in center plane after milling (P0); rk -dimple radius in terms of center plane Pp; rk' -dimple radius of in terms of center plane Pk; V'k -volume of voids in dimple after grinding under center plane Pk; Vk'' -volume of voids in dimple after grinding over center plane Pk.
If there are still a lot of dimples remained after grinding, then similarly to a method of determining a center plane after shot peen forming [9], the final position of center plane after grinding is determined by the following form (3): where t -is number of dimples remained after grinding with depths exceeded a level of original micro-relief; Vk -volume of voids of a k-th dimple under center plane Pp; Fbsurface area under study as a reference.
Here with, (4) Similarly to the method proposed in [9] for determining the roughness after shot peen forming, the arithmetic mean deviation of the profile within the base area after grinding is determined by the formula (7). The obtained dependence determines the value of SaSPF after shot peen forming without taking into account an original surface roughness.
The final formula for calculating the arithmetic mean deviation of the profile within the base area after shot peen forming and grinding, taking into account the original roughness, is determined as follows (8): where Sagri -is the arithmetic mean deviation of the profile within the base area after grinding, excluding shot peen forming.
Formula (8) is only valid with the condition of availability of sufficient number of the remained dimples from shot peening processing. Otherwise, when the value of the allowance is close to the value of depths of the greatest dimples, the remained dimples do not have a significant impact on roughness formation of treated surface after grinding with flap wheels. That is, the formation of surface roughness during grinding with flap wheels can be considered as a process of roughness formation during traditional grinding processing.