Material

The test object was X5CrNiCuNb16-4 (17-4PH) steel samples both conventional and additively manufactured (DMLS). The specimens of 17-4PH steel were in the shape of disks with dimensions of 30 mm in diameter and height of 6 mm. The first part was conventional, and the second part was printed from GP1 metal powder produced by EOS. The particles of the powder used are mainly spherical in shape and had an average size of 10-50 μm. Nitrogen was used as a shielding gas for fabrication. The 3D metal printing process was carried out through DMLS technology. The EOSINT M280 device (from EOS GmbH) was used, using the optimal printing parameters from the closed license of EOS software, including the laser power of 200 W, the laser spot size of 0.1 mm, and the sinter thickness of 0.02 mm. Scanning speed was 1000 mm/s, and hatch spacing at 0.1 mm. A parallel scanning strategy with alternate scanning direction was adopted, and for subsequent layers the scanning direction was rotated by 90° relative to the previous layer. The fabricated specimens were built horizontally orientated. Once the samples were obtained, their surface was prepared by abrading on papers of decreasing gradation. The processes listed below in in Table 1 were then applied. At the same time, however, it should be noted that precipitation hardening was applied first and the next step was peening of the prepared and cleaned surface. Cooling after the solution treatment process was carried out in oil, while after the aging process, the cooling was carried out in air.

It should also be noted that only the top surface of the specimens was treated by SP. Three different peening media was used, namely glass, steel and ceramic. Glass beads (Marbad) with a diameter of 200-300μm, steel shots (Amacast 30) with a diameter of 150-500μm and Ceramic (Marbad, CERAM 350) (250-425 μm). Distance from the nozzle was constant at 20 mm.

Methods

The chemical composition was tested using with a Magellan Q8 spark emission spectrometer from Bruker (Billerica, MA, USA). Five spark burns were performed for each specimen to calculate average values. Roughness parameters were calculated thru optical profilometry on Contour Countour GT-K1 (Veeco, USA) based on the VXI technique with the 5× magnification lens. Surface microgeometry maps were examined in the area of 1,26 mm × 0.946 mm. Obtained arithmetic mean deviation (Ra), root mean square deviation (Rq), maximum peak to valley (Rt) parameters from the maps were then tabularised and mean value was calculated. For each specimen, 8 measurements were taken. Measurements were taken according to ISO 4287 standard recommendations.

Hardness tests were carried out on a Future-Tech FM-800 microhardness tester at a load of 1,961 N and a dwell time of 15 s. Ten indentations were made for each sample on the top surface.

The structural characteristics of the surface were identified using a Phenom World ProX SEM microscope. In addition, an EDS attachment was also used to determine the surface's composition after shot peening. Above this, surface images were also taken in topographic mode at 1000x magnification.

Electrochemical tests were carried out on an ATLAS 0531 potentiostat system in a 3.5% NaCl solution at 22°C. A three-electrode electrochemical tank was used; the test area was 0.5 cm². The control electrode was a platinum electrode, the saturated calomel electrode (SCE) was the reference electrode. Polarisation curves were recorded with an automatic potential shift of 1 mV/s between -500 mV and +600 mV. Corrosion current density I_corr and corrosion potential E_corr were determined from Tafel curves by analysing potentiodynamic curves in AtlasLab from -25 mV to +25 mV. The corrosion rate was calculated based on ASTM G 102-89 according to the formulas 1 and 2 given below.

f_i – mass fraction of the i-th element in the alloy, M_i – atomic mass of the i-th element in the alloy [g*mol^-1]

Chemical composition

The chemical composition did not differ from that declared by the manufacturer and was in accordance with the requirements presented in ASTM and ISO standards. It is also in line with previous results as well as other relevant literature [18,19].

SEM-EDS Analysis

Surface of analysed referenced material showed visible grooves after grinding process (see Fig. 1 a, b). As can be seen in the Fig. 1 and Fig. 2, differences were evident between the groups depending on the pressing medium used. The PHPG and PHDG samples were characterised by visible grooves originating from fragmentation of the medium.

Fig. 1.

a) RP topographic mode, b) RD c) PHPG, d) PHDG, e) PHPS, f) PHDS topographic mode - SEM micrographs

Fig. 2.

g) PHPC, h) PHPC - SEM micrographs

The next group, PHPS and PHDS samples, are characterised by a smoother surface with visible, in places oval-shaped craters resulting from the impact of the spherical medium. This is also visible in the first group, but to a lesser extent. However, the greatest number of spherical craters are visible in the last group, the ceramic-impacted spherical craters (PHPC and PHDC). SEM-EDS analysis showed the presence of SP medium particles stuck on the surface of the tested materials. The presence of both glass and ceramic ball particles was verified. The presence of steel beads has not been detected using the EDS method due to the similar composition. Comparable results are reported in the previous authors papers [20] related to the shot peening of the different types of stainless steels.

Hardness

The analysis of the hardness plots given in fig. 3 indicates that precipitation hardening process increased surface hardness for the DMLS-produced 17-4PH steel of approximately 53% from 255 HV_0.2 for the reference material to 389 HV_0.2. Equally significant improvements were observed for the conventional 17-4PH steel (40%) from 322 HV_0.2 for the reference material to 421 HV_0.2. Precipitation hardening combined with shot peening with glass contributed to the same increase in surface hardness in both cases (596 HV_0.2) corresponding to increases of 85% (PHPG) and 134% (PHDG), respectively, compared to the reference material. The use of precipitation hardening combined with steel peening increased the hardness of the conventional material by 65% and that of 174PH DMLS steel by 92%, while ceramic peening contributed to increases in surface hardness of 64% (PHPC) and 136% (PHDC).

Fig. 3.

SEM-EDS spot analysis after SP process a) PHPG, b) PHDG c) PHPS, d) PHDS, e) PHPC, f) PHDC

Fig. 4.

Vickers hardness (HV_0.2) of specimen’s top surfaces

In summary, the greatest increase in surface hardness was obtained for the material after additive manufacturing subjected to precipitation hardening and ceramic bead peening from 255 HV_0.2 to 599 HV_0.2. After supersaturation, the hardness decreased for the conventional material, while it increased for the DMLS material, a fact that underlines the importance of research in this area. However, these results obtained are in line with other studies in this area for both 17-4PH conventional steel [21] and AM steel [22,23]. The greatest increase in hardness may have also been due to the size of the ceramic particles and their hardness because, at constant pressure and the highest density, theoretically, it is the steel shot that should provide the greatest energy during impact on the surface significantly increasing hardness through deformation and the introduction of favourable compressive residual stress (CRS) [24].

Roughness

The surface roughness increased significantly in all cases of surface peening analysed (Fig. 5-7). The smallest increase in roughness Ra was observed in the case of surface peening with steel beads. This can be explained by the higher ductility of steel compared to ceramic materials, including glass beads. By changing part of the energy into plastic deformation of the steel pellets, part of the energy that should deform the peened surface is lost.

Fig. 5.

Comparison of Ra roughness parameters of 17-4PH steel

Fig. 6.

Comparison of Rq roughness parameters of 17-4PH steel

Fig. 7.

Comparison of Rt roughness parameters of 17-4PH steel

This phenomenon may also explain the lower increase in hardness after peening compared to other materials. Most studies suggest that the bead material does not deform significantly. However, this is possible if only because of the high hardness of the shot peened substrate (389 HV_0.2, 421 HV_0.2) and the low hardness of the steel shots of 200 HV.

Significantly lower roughness was achieved when compared to 17-4PH DMLS steel without preparation by grinding that achieved Ra 4.5-5.5 μm, Rq 5.8-7 μm, Rt 22-28 μm some differences can be noted [8]. Similar condition Ra 5.04±0.56 μm for reference material, for SP with steel shots Ra 5.71±0.26 μm and 4.39±1.43 μm for ceramic is described in [25].

Fig. 8.

Characteristic of the surface- 2D map of the surface of the investigated materials and surface features: a) RP, b) RD, c) STP, d) STD, e) PHP, f) PHD, g) PHPG, h) PHDG, i) PHPS, j) PHDS, k) PHPC, l) PHDC

Fig. 9.

Characteristic of the surface- 2D map of the surface of the investi-gated materials and surface features: g) PHPG, h) PHDG, i) PHPS, j) PHDS, k) PHPC, l) PHDC

Corrosion resistance

As previously noted, corrosion resistance in this case study results from the three main factors of grain size, chemical and phase composition and development, and, therefore, surface roughness. A link was noted between roughness and corrosion current density, which can be identified as a decisive parameter giving information on corrosion resistance. In the analysed case, the best coefficient for assessing corrosion resistance is CR (see Fig. 14) directly related to Icorr (Corrosion current density). In the case of the RP to PHD specimens, disregarding the STD with obtained a low roughness after surface grinding. However, the STD sample deviates slightly, and this translates into a significantly worse corrosion resistance. In the case of the RP sample, the influence of the microstructure is mainly visible, compared to the rest of the samples it is a material that has not been heat treated or mechanically processed.

In addition, it differs from the DMLS-made material in terms of its thermal history, as compared to the DMLS reference material it was only heated once while the RD went through a cycle of remelting and sintering successive layers while heating the material around it. The corrosion resistance of the glass-peened materials (PHRG and PHDG) is also related to the roughness. An increase in roughness resulted in a decrease in corrosion resistance in this group and a slight difference between the PHRG and PHDG samples tested was also evident in the I_corr parameter (see Fig. 11). Its increase led to a decrease in corrosion resistance in this group and a slight difference between the PHRG and PHDG samples tested was also seen in the I_corr parameter. The situation is similar for the next group of steel specimens, but in this case, the effect of the medium on the corrosion resistance is also shown by the present and previous studies and the model proposed by Kameyama and Komotori [26]. While the presence of steel particles located on the surface is sometimes difficult to confirm using, for example, EDS, the presence of glass and ceramic particles is easy to verify. Again, however, the difference between PHRS and PHDS is apparent due to the small difference in roughness. Reference specimen made using DMLS was characterised by smallest E_corr on the other hand the highest E_corr was observed for PHDS. As can be seen corrosion potential seam to generally lower after treatment for conventional material and increase for DMSL (see Fig. 12). Discontinuities or irregularities in the surface resulting from SP indentations cause local variations in surface morphology and roughness.Differences in the E_corr can be correlated to spots where corrosion attack start to create usually in the top of SP inducted dents [27].

Fig. 10.

Potentiodynamic polarization curves in 3.5% NaCl solution – Tafel plot

Fig. 11.

Corrosion resistance parameters I_corr - Corrosion current density

Fig. 12.

Corrosion resistance parameters E_corr - Corrosion potential

An important parameter in the context of corrosion protection is Epit (see Fig. 13). The highest E_pit value and at the same time the most favourable one was RD (0.539 V). The lowest E_pit value and, at the same time, the material with the greatest potential for pitting corrosion was PHDC (-0.153 V). As can be seen in Fig. 13, there is a visible decrease in pitting corrosion resistance after the precipitation hardening process. However, it is not possible to clearly indicate whether conventional steel or DMLS is more resistant. In the group of materials that underwent SP, the highest Epit value was found in PHPG (0.0366 V).

Fig. 13.

Corrosion resistance parameters E_pit - Critical Pitting Potential

Fig. 14.

Corrosion resistance parameters-Corrosion rate