INFLUENCE OF LASER PEENING CONDITIONS ON SURFACE CORROSION RESISTANCE OF ALUMINIUM ALLOY

Transcription

1 INFLUENCE OF LASER PEENING CONDITIONS ON SURFACE CORROSION RESISTANCE OF ALUMINIUM ALLOY Prof. Janez Grum, Mr. Uros Trdan, J.L. Ocaña

2 OUTLINE Introduction Eperimental procedure Base material specifications Peening conditions Eperimental results: Conclusions Surface analsis (roughness, Autocorrelation function) SEM & EDS analsis Microhardness analsis Analsis of residual stresses Pitting corrosion analsis Surface analsis after corrosion tests 3D metrolog

3 Base material specifications Fig. 1. Stress-strain curve and SEM Microstructure with EDS analsis of the precipitates. For eperimental purposes aluminium allo ENAW 6082-T651 was chosen. The allo was subjected to homogenization at T=540 C,quenched, subjected to tensile loading with a 2% strain, and artificiall aged at T=160 C for 10 hours. Clindrical specimens of Φ40 mm 20 mm, with an average value of the roughness parameter Sa of 0.55 µm were prepared. The material mechanical properties were determined b the tensile test. Microstructure which was carried out with SEM JEOL JXA-8600M.

4 Base material specifications Chemical composition and mechanical properties of the allo are given in Table 1 and 2. Table 1: Chemical composition of the aluminium allo. Designation Chemical composition [%wt] Si Mg Cu Mn Fe Cr Zn Ti ENAW 6082 AlSi1MgMn Table 2: Mechanical properties of the treated aluminium allo. Chemical Rm R Designation p0.2 E [GPa] HV composition [%wt] [MPa] [MPa] 0.5 ENAW 6082 AlSi1MgMn

5 Peening conditions Laser Nd:YAG X-Y computer controlled table Basket with water Confined plasma Fig. 2. Photographic representation of Laser Shock Processing (LP) eperimental setup. LP was preformed with a Q-switched Nd:YAG laser wavelength 1064 nm power densit of GW/cm 2 two pulse densities were chosen, i.e. 900 and 2500 pulses/cm 2 laser pulse duration of 10 ns repetition rate of 10 Hz.

6 Surface analsis Alicona-Infinite Focus sstem Focus-Variation as Measurement Technique PARAMETERS: Ojective: 20 Vertical resolution: 100 nm No. of points: 7 miliion Size of imagefield: cca µm Size of measure. point.: 438 nm Cut off wavelenght: 200 µ m Fig. 3. Infinite Focus device (Alicona gmbh). The eamination of the surface roughness and the testing of the LP homogeneit was performed b 3D surface metrolog method using the Infinite Focus (Alicona Imaging GmbH) sstem. After LP with 900 pulses/cm 2, the size of the surface craters occurring ranges between 50 and 100 µm and after 2500 pulses/cm 2 the craters are in the range from 100 µm to 200 µm. Fig. 4. Topographic photos of the specimens surface. Initial state LP 900 pulses/cm 2 LP 2500 pulses/cm 2

7 Surface roughness ,65 0,52 3,04 2,39 5,12 4,13 No LSP 900 pulse/cm² 2500 pulse/cm² S Specimens after LP treatment with 900 pulse/cm 2 had the roughness values Sa=2.39 µm and Sq=3.04 µm. Specimen which was treated with 2500 pulse/cm 2 was less favorable and had higher roughness values, i.e. Sa=4.13 µm and Sq=5.12 µm. 1 L L a = L L / 2 L / 2 L / 2 Z(, ) dd / 2 Sa [µm] Sq [µm] Fig. 5. Surface roughness values under different LP conditions. S 1 L L q = L L / 2 / 2 L L / 2 Z / 2 2 (, ) dd Where Z(,) is sampled b a set of M points over the evaluation length L in the direction and N points over the evaluation length L in the direction.

8 Surface spatial characterization In order to characterize the homogeneit of LP surfaces and to determine the repetition of a surface areal auto-correlation function (AACF) was applied. Figure 6 shows the auto-correlation model of the specimens, with the preferential direction of the surface teture. The non-treated specimen after cutting (Fig. 6a) reveals a strong trend of the surface teture in the cutting direction whereas the laser treated specimen (Fig. 6b & 6c) shows no evident surface direction (homogeneous in and directions). Fig. 6. Auto correlation model. The initial state (a), LP treated specimen with 900 pulses/cm2 (b) and LP treated specimen with 2500 pulses/cm2 (c).

9 Surface spatial characterization The AACF is the areal auto-covariance function (AACV) normalized b the square of the root-mean-square height of the selected surface area: AACF( τ, τ ) = AACV ( τ, τ ) S 2 q AACV ( τ, τ ) 1 L L = lim L, L L / 2 L / L / 2 L 2 Z(, ) Z( + τ, + τ ) dd / 2 For further information of the surface teture spatial parameters, i.e. S al (the fastest deca autocorrelation length) and S tr (teture aspect ratio) was applied. Parameter S al is defined as the shortest horizontal distance of the AACF, which has the fastest decas to 0.2 in an direction. Parameter S tr is the parameter used to identif the uniformit of the surface teture (isotrop vs. anisotrop): Sal 2 2 ( τ + τ ) ( ) ( ) 2 2 min τ + τ = min S tr =, 0 < Str 1 R{ ( τ, τ ) : AACF( τ, τ ) 0.2} 2 2 ma τ + τ Table 3: Spatial parameter value of the auto-correlation analsis. Parameter S al [µm] S tr [/] No LP pulses/cm pulses/cm The results of the analsis confirmed major difference between the treatment parameters.

10 SEM & EDS analsis (a) (b) Fig. 7. Microstructure (a) and fractograph (b) of investigated aluminium allo. The microstructure shows aluminium matri with homogeneousl distributed fine precipitates. The latter formed due to cold deformation and precipitation annealing. The size of the precipitates was in the range from0.5µm to1µm, whereas larger precipitates were in the size range of 5 µm.

11 Microhardness analsis Microhardness HV Initial hardness value Depth [ µ m] 2500 pulses/cm² 900 pulses/cm² Fig. 8. Microhardness variation. Figure 8 shows through-depth microhardness variations at different pulse densities after LP. The hardening results indicate that the material microhardness increases with higher pulse densit, where the average hardness in the soft state amounts to approimatel 98 HV 0.2. The highest increase of micro-hardness was established after LP with 2500 pulses/cm 2 and it amounted to 118 HV 0.2. In comparison the treated material with 900 pulses/cm 2 which had the maimum microhardness value of 114 HV 0.2.

12 Microhardness analsis = A B AB A B 2 Fig. 9. Response surface and contour plot of micro-hardness with regard to the pulse densities level (dashed region). Due to considerable deviation of the measured microhardness results, the factorial analsis was used. The major microhardness dissipation is most likel attributed to the small size of an indentation calotte size, since the allo contains both the softer phase α and the harder intermetallic phase β.

13 Analsis of residual stresses 200 σ min [MPa] ,2 0,4 0,6 0,8 1 Depth z [mm] No LSP 900 pulses/cm² 2500 pulses/cm² Fig. 10. Variation of minimal principal residual stresses σ min. Minimum residual stresses in initial state of the specimen are ideal since the amount to around 0 MPa. Such a variation confirms that the heat treatment chosen (T-651) is adequate for the initial material state. The analsis of the principal residual stresses after LP with a power densit of GW/cm 2 confirmed the influence of pulse densities. After 900 pulses/cm 2 compressive residual stress of -242 MPa are obtained at the surface whereas after 2500 pulses/cm 2, the are higher, i.e MPa.

14 Pitting corrosion analsis Corrosion resistance of the aluminium allo was tested with potentiodnamic polarisation tests in a 3.5% water solution of NaCl. The potentiostatic polarisation tests were performed with device Voltalab 21 and corrosion cells CEC/TH, products of Radiometer Analtical. The data were established with a scan rate of 10 mv/s, where the electrode potential was being increased up to -500 mv SCE. Calomel refernce elctrode - SCE Counter electrode potentiostat CEC-TH cell specimen Fig. 11. Corrosion testing device. Corrosion of aluminium allo in water solution in general comprises two electrochemical reactions, i.e. an anodic reaction (Al oidation) and a cathodic reaction (hdrogen reduction ): 3 Al Al + + 3e or + 3 3H + 3e H 2 A sum of anodic and cathodic reactions equals: Al + 3H Al H 2 or Al + 3H 2O Al( OH ) 3 + H

15 Pitting corrosion analsis Potential [VSCE] -0,4-0,6-0,8-1 -1,2-1,4-1,6-1,8-2 Epitt 2500 = mv Epitt 900 = mv Epitt 0 = mv Increase of passivation potential E pass with higher overlapping level Current densit [ma/cm 2 ] Table 4: Pitting potentials from corrosion polarization tests Pulse densit [pulses/cm 2 ] E pitt [mv SCE ] E pitt [mv SCE ] No LSP No LSP 900 pulses/cm² 2500 pulses/cm² Fig. 12. Potentiodnamic polarization curves The potentiodnamic polarisation curves measured prior to and after LP showed an increase in the pitting potential after LP. After LP with 900 pulses/cm 2 an increase in the pitting potential of 62mV and after 2500 pulses/cm 2 of 120mV was established, compared to the same material in the as delivered state which had the pitting potential Epitt= mvsce.

16 Pitting corrosion analsis Deep pits Pit sites Pit site Shallow pits (a) Initial state after cutting (b) 900 pulse/cm 2 (c) 2500 pulse/cm 2 Fig. 13. SEM micrographs of pit sites at the surface of ENAW From the SEM surface images it can be assessed that the largest number of corrosion damages (pits) at the specimen surface occurs in the as-delivered state of material. The macrosection images of the specimen surfaces, which were subjected to a preliminar LP, confirm that the number of pits will reduce at higher pulse densit.

17 Surface analsis with 3D metrolog To validate the improved post-lp corrosion resistance even further, the surface condition of the specimens was checked b using the 3D surface metrolog method using the Infinite Focus (Alicona Imaging GmbH) sstem. Prior to surface analsis after electrochemical corrosion tests (ECT) the specimen surface was cleaned (according to ASTM standard) in nitric acid (HNO 3 ) for 120 seconds for the preparation of specimens after the corrosion test. In surface condition analsis, the choice of parameters was identical for all specimens, i.e. a magnification of 20, seven million captured points, each point size being 438 nm. The site of the selected surface analsis area of individual specimens was approimatel 1320 µm 980 µm, with a optical lateral resolution 800 nm vertical resolution of 100 nm.

18 Surface analsis with 3D metrolog Fig D digital elevation model of the surface after ECT. Initial state (a), LP-900 pulses/cm 2 (b), LP-2500 pulses/cm 2 (c).

19 Surface analsis with 3D metrolog Table 6: Surface roughness comparison before and after ECT. Prior to ECT After ECT Comparison Specimen S a [µm] S q [µm] S a [µm] S q [µm] S a [%] S q [%] Comment No LSP Not acceptable 900 pulses/cm Ver good 2500 pulses/cm Ecellent The highest roughness increase was recorded in the non-treated specimen, where roughness Sa increased b 111 % compared to the roughness of the specimen immediatel after the metallographic cut. Roughness increase of the LP-treated specimens was minimal and was even reduced as the pulse overlapping rate went up. The furrowed surface of the LP-treated specimens is onl a reflection of the laser pulse interaction with the specimen surface. After LP with 900 pulses/cm 2, the roughness Sa increased b Sa=9.20 %, compared to the specimen values before the ECT. At a higher pulse rate of 2500 pulses/cm 2, the increase in roughness values was even less, i.e. Sa=1.38%.

20 Conclusions The research results indicate that laser peening produces favourable microhardness and residual stress profiles in the thin sub-surface laer, with increased corrosion resistance. The analsis of residual stresses confirmed that the specimens after LP show higher compressive residual stress profiles at the higher pulse densit. After LP with 900 pulses/cm 2, compressive residual stresses of -242 MPa at the surface and after the higher pulse densit, i.e pulses/cm 2, residual stress, of -317 MPa, are obtained. Potentiodnamic polarization tests confirmed a growth of the pitting potential with a higher pulse densit: a 62 mv higher pitting potential at 900 pulses/cm 2 and a 120 mv higher pitting potential at 2500 pulses/cm 2 compared to the non-treated specimen. The analsis of the pitting corrosion attack performed on SEM showed a decrease with a higher pulse densit. A comparison between the roughness values Sa and Sq with a 3D surface metrolog method prior to and after the corrosion testing confirms big differences. In the laser-treated specimens the increase in roughness was minimal and even reduces as the pulse overlapping rate increases.