PSI - Issue 42

Michael P. Milz et al. / Procedia Structural Integrity 42 (2022) 830–837 Michael P. Milz/ Structural Integrity Procedia 00 (2019) 000 – 000

832

3

2. Materials and methods 2.1. Substrate, material and manufacturing

Thermal sprayed ZnAl4 coating systems on S355 J2C + C (1.0579) substrate were investigated. The substrate has a tensile strength R m of 672 MPa, and an elongation at fracture A5 of 12.7%. The chemical composition is shown in Tab. 1. The manufacturing process can be divided into three steps: machining, thermal coating, and MHP. Four conditions were tested: • (I) Sand-blasting: Sandblasted substrate material • (II) ZnAl-coating: ZnAl4 coated as-sprayed substrate material

• (III) MHP 1: ZnAl4 coated and machine hammer peened (parameter set 1) • (IV) MHP 2: ZnAl4 coated and machine hammer peened (parameter set 2).

Tab. 1. Cast analysis of S355 J2C + C according to the datasheet of the manufacturer.

Element

Fe

C

Si

Mn

P

S

Cu

Al

V

wt.-%

Bal.

0.1510

0.2235

1.2646

0.0134

0.0060

0.3131

0.0200

0.0043

The samples were machined according to the geometry in previous investigations: Length of 260 mm, gauge length of 13 mm with a diameter of 10 mm (Milz et al. 2022). After machining they were sandblasted in the tapered area. Sandblasting ensured sufficient adhesion of ZnAl4 coatings and was performed with alumina powder with EKF 24 (600 – 850 µm) size fraction, 4 bar blasting pressure, 100 mm stand-off distance, and 45° blasting angle. The specimens were cleaned with an ultrasonic Ethanol bath. Machining is followed by the thermal spraying process, twin wire arc spraying (TWAS), to apply the ZnAl4 coating. Chemical composition of the feedstock is given in Tab. 2. Prior to spraying, the specimens except the tapered area were masked. TWAS process was performed in the tapered area using a spray unit Durum Duraspray 450 (Durum, Germany) with 3.2 m/min wire feed rate, 22 V arc voltage, and 5 bar dry and compressed air as atomization gas. In order to achieve a coating as uniform as possible, a rotating unit and an industrial robot ABB IRB 4600 were used with 120 mm stand-off distance between spray gun and substrate surface, v s = 18,000 mm/min axial gun velocity, and s = 4 mm meander spacing. Machine hammer peening was performed on the high-performance turn-mill center G250 (Index-Werke, Esslingen, Germany) with a FORGEFix Air MHP tool (Pokolm Frästechnik, Harsewinkel, Germany) using a spherical solid carbide tappet with diameter d p = 16 mm, Fig. 1. While the workpiece rotates, the MHP tool blasts the surface, constantly moving to the side to process the gauge length. Parameter settings were used based on former studies (Tillmann et al. (2021), Milz et al. (2022)) using a track distance l p of 0.25 mm (MPH 1 (III)) and 0.67 mm (MHP 2 (IV)), resulting in an impact density ρ i of 12 mm -2 (MPH 1 (III)) and 34 mm -2 (MHP 2 (IV)), respectively.

Fig. 1. Setup and parameters of the machine hammer peening (MHP) process using turn-mill center Index G250.