VOL. 19 March ISSUE YEAR 2018
in Vol. 19 - March Issue - Year 2018
Feasibility Study of Waterjet Fine Particle Peening
Figure 1: Test setup for waterjet fine particle peening
Figure 2: Machining of Almen strips and the resulting deformation
Figure 3: Almen intensity for shot peening and fine particle peening
Figure 4: Almen intensity for waterjet fine particle peening
Shot peening is a cold working process to improve the mechanical properties such as fatigue strength and corrosion resistance of metal parts. Applications using these improvements are found primarily in the aerospace and automotive industries, as these component parts are subject to high alternating stresses. Exemplary components are springs, drive shafts, crankshafts, gears, connecting rods, steering and transmission parts as well as turbine blades [1-6]. To further increase the dynamic strength of components, fine particle peening was developed using particles with a size of about 100 µm in diameter, which are about one order of magnitude smaller than particles used in conventional shot peening processes . Especially for components with limited accessibility such as small gear wheels, fine particle peening is of great advantage. In addition, when using smaller particle sizes, the machined surface qualities are better than for surfaces machined with conventional shot peening [4, 7]. This fact is especially advantageous for applications where finishing processes like finish grinding or vibratory finishing follow the peening process to remove the generated dimples e.g. for the manufacturing of gear wheels.
Due to their low particle mass, their kinetic energy is limited using pressurized air for acceleration resulting in an insufficient plastic deformation in the boundary layer for some applications. An alternative possibility is the acceleration of the particles using a waterjet with high pressure, which results in a much higher kinetic energy. In a feasibility study, the waterjet fine particle peening (WJFPP) and its effect on the Almen intensity was analyzed and compared to shot peening using pressurized air.
In this study, a waterjet cutting machine was used for WJFPP. A six-axis robot was used for jet manipulation with a maximum feed velocity of up to vf = 2.5 m/s. The water pressure was realized with a hydraulic high pressure intensifier delivering up to p = 600 MPa and a flow rate of up to VW = 2.5 /min. The particles were added into the waterjet via injection. A combination of orifice with a diameter of dD = 0.3 mm and focus with a diameter of dF = 1.0 mm and a length of lF = 76 mm were selected. Cast shots with different particle sizes were used for a comparison of shot peening (dP = 1.0 mm) and waterjet fine particle peening (dP = 0.2 mm). In order to cover a selected area, several paths with a lateral distance of b = 0.5 mm and a feed speed of vf = 500 mm/min were machined. The angle of impact was kept constant at α = 90° at a stand-off distance of s = 20 mm. The test setup is illustrated in Fig. 1.
Almen strips were used to quantify and compare the intensity of the peening processes for shot peening and waterjet fine particle peening. Compressive stress introduced by the peening process causes the strip to deform with an arc height h, Fig. 2. The arc height h is proportional to the transmitted energy, which is furthermore proportional to the compressive residual stress. The Almen intensity is defined as the first point of the curve that, if the exposure time is doubled, the arc height h increases by only 10%. The unit of the Almen intensity is given in mm followed by the type of Almen strip type. In this study, type A was used resulting in the unit mm A.
In a first test, a comparison of shot peening with pressurized air with different particle sizes was conducted. Cast shots with diameters of ds = 1.0 mm and ds = 0.2 mm were used and their effect on the Almen intensity was detected. In case of peening case-hardened steel used for gear wheels (20MnCr5), a suitable Almen intensity between I1 ≈ 0.2 mm A (middle intensity) and I2 ≈ 0.4 mm A (high intensity) is reported  as leading to good fatigue behavior and wear resistance. The results show that for conventional shot peening with ds = 1.0 mm and an air pressure of p = 0.5 MPa, a suitable Almen intensity can be achieved. In case of small gear wheels with a very limited accessibility in the tooth root, commonly used particle sizes are not able to impact onto these small geometries, which necessitates the use of smaller particle geometries. However, reducing the particle size to ds = 0.2 mm leads to an insufficient effect regarding the achieved Almen intensity, Fig. 3. Due to the low density of pressurized air, not enough kinetic particle energy can be produced to achieve the demanded intensity for gear wheels.
The use of water as acceleration medium leads to a drastic increase of kinetic particle energy. Even with small particle diameters, the whole bandwidth of suitable Almen intensities can be attained by varying the water pressure, Fig. 4. Varying the water pressure between p = 60 to 200 MPa led to a linear increase of the Almen intensity. Further pressure increase was showen to have no beneficial effect, indicating that the maximum values of compressive residual stresses in the Almen test strips may have already been achieved.
Conventional shot peening using pressurized air is a widely-used process for increasing the dynamic strength of a number of components and is suitable as long as the necessary accessibility is given. For the peening of small geometries such as small radii, the particle size has to be reduced. For these applications, the use of pressurized air for acceleration in some cases does not achieve the requested particle energy. The use of a liquid as an acceleration medium was investigated in this work and its effect on the Almen intensity. The waterjet fine particle peening seems to be a promising technology to induce very high compressive residual stresses in parts with limited accessibility due to the much more effective particle acceleration. Be varying the water pressure, the particle energy can be controlled in a much wider range and can easily be adapted to the requested values.
Although the Almen intensity gives a good estimation of the induced energies, the compressive residual stresses should be investigated in further experiments for a more detailed comparison to fine particle peened parts using pressurized air. Also, the static and dynamic behavior of waterjet fine particle peened parts needs to be further examined to validate the advantages of this process.
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Prof. Dr. h. c. Dr.-Ing. Eckart Uhlmann
Technische Universität Berlin
Institute for Machine Tools and Factory Management
Tel: +49.30.314 23349
Dipl.-Ing. Fabian Faltin
Technische Universität Berlin
Institute for Machine Tools and Factory Management
Tel: +49.30.314 23624