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Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour
MATIEIUALS SCIENCE & ENGINEERING ELSEVIER
A
Materials Science and Engineering A210 (1996) 102-113
Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour P. Peyre a, R. Fabbro a, P. Merrien b, H.P.
Lieurade b
aLaboratoire d'Application des Lasers de Puissance (LALP), Unitk Mixte ETCA, CNRS 114, 16 bis Avenue Prieur de la C6te d'Or, 94114 Arcueil Cedex, France bCentre Technique des Industries Mkcaniques (CETIM), Departement Materiaux, 52 Avenue Felix Louat, 60304 Senlis Cedex, France Received 25 May 1995; in revised form 31 October 1995
Abstract
Subjecting target metallic samples to a very short pulse (about 20 ns) of intense (GW cm-2) laser light generates, through a surface plasma, a high-pressure stress wave propagating to the first millimetre in depth, which is commonly called laser shock processing (LSP). The purpose of this work was to evaluate the role of this novel process on the cyclic properties of A356, AII2Si and 7075 aluminium alloys. Major contributors to the fatigue performance improvements were investigated in order to determine the optimum shock conditions. These were mainly compressive residual stress (RS) levels for which a large range of incident shock conditions was performed. We showed that stress levels were very sensitive to the laser fluence and the number of local impacts, and experimental RS measurements were found to be in good agreement with analytical modelling results. In comparison, a conventional shot peening (SP) treatment was found to lead to higher surface hardening and RS levels, but with a very detrimental roughening not observed after LSP. High cycle (107) fatigue tests carried out on laser- processed, shot-peened and untreated notched samples illustrated the efficiency of LSP as a new, promising method to improve the fatigue limits a D of structures, especially in comparison with enhancements displayed by SP ( + 22% vs. + 10%). According to crack detection electric measurements, fatigue performance improvements with LSP mainly occurred during the crack initiation stage.
Keywords: Laser shock processing; Shot peening; Aluminium alloys; High cycle fatigue behaviour
I. Introduction
The ability of high-energy pulsed lasers to generate shock waves and plastic deformation in metallic targets was demonstrated for the first time in 1963 [1] in the USA and extended in 1968 [2] with the development of confined ablation modes. Thereafter, the application to materials processing was initially investigated in the 1968-1981 period at Battelle Columbus Laboratories (OH, USA) with the application to fastener holes [3]. After the last publications of Battelle searchers, dated 1981, no further developments emerged because of the lack of laser sources to provide real industrial potential. In France, since 1986, and supported by the automotive industry (PSA) and other industrial partners, new studies have been started. Systematic investigations have centred on highlighting the main key points of the process at this time. These various studies have exam-
ined the confined interaction mode [4,5], breakdown factors limiting the amount of laser energy reaching the surface [6] and analytical modelling of the mechanical processes involved [7,8]. The original results reported here [10] follow from previous studies on the laser shock strengthening of steels [5,7,8] and nickel base superalloys [9]. Three commercial aluminium alloys were chosen on which to evaluate the effects of laser shock processing (LSP). The first two are AI-Si-type cast aluminium alloys (A112%Si (All2Si) and A1-7%Si (A356-T6)) often used in the automotive industry. The third is an aeronautical 7075-T7351 wrought alloy. This paper reports a series of experiments relating the influence of laser-induced shock waves on the surface modifications and high cycle fatigue properties of these aluminium alloys. The objective was to compare LSP effects with a conventional mechanical treatment (shot 0921-5093/96/$15.00 © 1996 - - Elsevier Science S.A. All rights reserved SSDI 0921-5093(95)10084-5
P. Peyre, R. Fabbro /Materials Science and Engineering A210 (1996) 102 113
103
(2 HEL amplitude) plastic waves Shock waves
1
I Peak Pressure
] -----~k elastic waves I • plastic waves
depth I T HEL Fig. 1. Laser shock processing (LSP) in the confinedablation mode. peening (SP)) which has been used for many years in the industry, except on mechanical parts when access to the treated surface is limited.
2. Process and experimental environment The generation of high-amplitude laser stress waves in metals requires irradiation with a short (tenths of nanoseconds), intense (greater than 1 GW cm-2), focused laser pulse. The laser-material interaction generates vaporization of the first atomic layers of the target, called an ablation phenomenon. Blowing off, the partially ionized gas (plasma) drives a shock wave by expansion from the irradiated surface. This shock wave propagates into the target, causes plastic deformation to a depth at which the peak stress no longer exceeds the Hugoniot elastic limit (HEL) of the metal (equivalent to the yield strength under shock conditions) and, in turn, induces residual stresses (RSs) throughout the affected depth. The experiment involves coating the surface to be treated with a sacrificial opaque material (black paint, metallic foil) followed by a dielectric material transparent to laser light (water, glass) (Fig. 1). As the laser beam passes through the water, the energy is initially absorbed by the opaque overlay, and only a thin layer (less than 10/~m) of this sacrificial material is ablated. Thus the material undergoes no thermal process or microstructural changes. Additionally, as the vapour continues to absorb the rest of the laser energy, it is readily heated and ionized into a plasma. Therefore the plasma expands and generates two shock wave systems,
one in the target and the other in the confining medium. As shown in Fig. 1, shock wave systems have two components: an elastic component below 1 HEL ((1 + 2/2/Z)ay) and an elastic-plastic component above 1 HEL. An attenuation in depth occurs when highvelocity elastic recoil waves come back from the surface and reduce the peak pressure amplitude of 2 HEL. As it is desirable that the laser beam is converted into a stress wave with the highest degree of efficiency, the plasma is trapped between the dielectric transparent material and the workpiece surface to retard its expansion and to achieve higher stress amplitude and duration. This configuration, which is termed the confined ablation mode, is the basis for the LSP of materials. It allows 5 GPa peak pressures vs. 1 GPa for the same incident laser power densities in direct ablation mode, and durations two to three times longer than the duration of the laser pulse. The resultant effect is the generation at the metallic surface of a higher compression wave, which causes enhanced deformations and higher compressive RS, provided that the metal has been covered with a sacrificial coating. Since this coating (usually black paint or metallic foil) protects the specimen from melting and vaporization, LSP is a pure mechanical treatment inducing plastic flow and negative RS. These compressive RSs are the key to the superior fatigue or stress corrosion performance of structures while reducing the applied surface stresses. A precise description of the confined ablation mode was given recently [4], with a three-step process for the pressure environment generation (heating, adiabatic cooling and final expansion of the plasma). Using this
104
P. Peyre, R. Fabbro / Mater&& Science and Engineering A210 (1996) 102-113
model, and considering the plasma to be a perfect gas, the scaling law of the pressure generation can be estimated by the following relationship
Pressure (GPa)
10
SRTgaussian I
P(GPa) =0.01 / ~ - ~
X/Z (g cm-2 s - l)x/Io(GW c m - 2)
DGI
(1)
o~ where I0 is the incident laser power density, P is the pressure, Z is the reduced shock impedance between the target and the confining medium and ~ is the efficiency of the interaction, where ~E contributes to the pressure increase and (1 - ~)E is devoted to the generation and ionization of the plasma (c~ = 0.1-0.2). All the experiments shown in this paper were performed with a water co.nfinement mode. In this kind of configuration, easily achievable in the industry, Eq. (1) gives P(GPa) = 1.02~/I0(GW cm - 2)
(2)
For a water confinement mode, the peak pressure is approximated as the square root of the incident laser power density. In Fig. 2, the laser and pressure pulses, monitored with a fast photodiode and an x-cut quartz gauge system respectively, are illustrated. It can be observed that the decay time is much slower for the stress pulse than for the laser pulse because of plasma confining effects. These higher stress wave durations are expected to increase the amount of shock-induced cold work, mainly for low-strength materials such as aluminium alloys. Fig. 3 shows the peak pressure measurements as a function of the power density for a water confinement mode and two different pulse shapes: a classical gaussian and a dissymmetrical short rise time (SRT) pulse [6]. It appears that pressures reach a limit over a power density of 4 GW cm-2 for the gaussian pulse. These saturations are assumed to reflect dielectric breakdown
GN• D []
[] •
•
BiD •
• •
•n
d~
I
I
I
I
I
•I
I
I
peak power density ( G W / c m 2 ) Fig. 3. Peak pressure levels as a function of the peak power density for the water confinement mode. Influence of the pulse shape (gaussian or SRT); piezoelectric quartz measurements [6].
phenomena in the confinement medium that limit the amount of energy reaching the metallic surface. As shown in Fig. 3, the breakdown thresholds can be increased (up to 5 GW cm 2) using SRT pulses. All our experiments were performed with the LALP pulsed Nd-glass prototype laser operating at 2 = 1.06 /zm and consisting of an oscillator followed by four amplifier stages. This system is capable of emitting up to 80 J in a pulse that is semi-gaussian in shape or has an SRT with a full width at half- maximum (FWHM) of 15-30 ns. Only 25 ns FWHM gaussian pulses were used since they allowed maximum peak pressures of 2.5 GPa, capable of treating any aluminium alloy. To provide a good energy reproducibility on the samples, the maximum repetition rate was limited to about one shot every 2 min. The laser energy was focused with a lens of 600 mm focal length to spots of 5-12 mm diameter on the targets. Distilled water (thickness, 2-5 mm) was used as confining medium. In this configuration, aluminium samples were submitted to 1-8 GW cm -2 irradiation resulting in 2.5 GPa maximum peak pressures. 3. Materials
Three industrial aluminium alloys were chosen to evaluate the effects of LSP. Both A1-Si alloys were chilled cast, solution treated, quenched and aged to the T6 condition. Age hardening occurred mainly by fine Table 1 Weight per cent compositions of the three aluminium alloys
I
-100
I~
pulse I
L
I
I
200
300
400
Si 0
100
time (ns) Fig. 2. Gaussian laser pulse and resulting pressure pulse submitted to the target.
All2Si A356 7075
Cu
Fe
12.2 1.48 0.16 7.05 0.05 0.2 0.08 1.48 0.17
Mg
Mn
Ni
Zn
1.3 0.33 2.6
0.03 0.03
1.25 0.03 0.03 5.95
Cr Ti
0.2
0.01 0.15 0.04
105
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102 113 Table 2 Monotonic and dynamic mechanical properties of the alloys Material
E (GPa)
HV (25 gf)
a v (compressive monotonic) (MPa)
crv (compressive dynamic) (MPa)
gv (tensile monotonic) (MPa)
A (tensile)'¼,
A356 T6 All2Si T6 7075--T7351
70 77 72
110 120 170
-218 - 238 -450
-260 - 310 - 550
225 170 415
8 0.7 11
precipitation of the coherent Mg2Si phase. The compositions are given in Table 1. Each cast alloy exhibited grain size coarsening to about 300/zm levels so that the RSs induced by LSP could not be measured by conventional X-ray diffraction. The A356 alloy was solution treated for 6 h at 480 °C, water quenched (20 °CA and aged for 6 h at 160 °C to the T6 condition. Tensile tests performed on 8 mm diameter samples revealed (with scanning electron microscopy (SEM) observations) ductile-brittle failure aspects. All2Si was obtained by solution treatment for 6 h at 480 °C, water quenching at 70 °C and aging for 6 h at 220 °C to the T6 condition. The microstructure of as-received All2Si-T6 consisted of the Mg2Si phase (like A356) with A12Cu partially coherent precipitates. In alloying A1 with 12% Si the material was found to lose most of its plasticity behaviour (to less than 1% elongation values). The starting 7075 material was 4 cm thick sheet oriented with the direction parallel to the hot rolling direction. It was delivered in the T7351 thermal state which involved quenching, tensile stress relieving and two-stage aging (8 h at 135 °C and 1 h at 190 °CA. Optimization of the shock conditions and characterization of the mechanical effects induced by LSP on the three alloys were performed using 15 mm thick plane specimens. It has been known for some time that the resultant effects of LSP are primarily mechanical, since thermal effects are confined to the surface layers of the sacrificial coating. Therefore LSP is usually considered as a pure high strain rate (106 s ~) uniaxial compression, creating surface layer stretching during interaction. The mechanical properties of the alloys were determined by monotonic tensile and compressive tests (strain rate, 10- 3 s 1) and by dynamic compressive tests (103 s - i ) using the Split-Hopkinson pressure bar (SHPB) system. However, as SHPB monitors information only over 2% plastic strain, dynamic yield strengths were extrapolated from the first 2%-5% of plastic strain. The resulting mechanical properties are summarized in Table 2. Compressive measurements on cylindrical samples (radius, 5 mm; height, 15 mm) show that strain rate sensitivities between 10 3 s J and 10 3 s 1 lead to + 20%- + 30°/,, increases in the elastic limits o-y of each material.
4. Mechanical effects induced by LSP
4.1. General trends As fatigue cracks mostly originate at the surface of materials, the fatigue behaviour of mechanical parts depends strongly on their mechanical and geometrical surface states. For instance, a compressive surface layer prevents crack opening and growth and therefore has a beneficial effect on the fatigue life, whereas a high surface roughness generates local stress concentration factors detrimental to crack initiation. Therefore a large part of our work was devoted to the experimental description of surface changes due to laser shock waves, mainly in terms of the compressive RSs induced within the materials. Our interest was stimulated by the need to investigate a large range of power densities in order to optimize the shock conditions and to generate maximum RS levels at the surface of the target materials. The experimental results were correlated with analytical predictions [7] of the optimum shock conditions and RS fields induced. In the normal configuration (thermoabsorptive overlay + confining medium), the generation of RSs by LSP may be summarized as follows: during laser material interaction, the pressure pulse generated by the blow off of the plasma crushes the treated area and creates pure uniaxial compression along the direction of shock wave propagation and tensile stretching in the plane parallel to the surface. After reaction of the surrounding zones, a compressive stress field is generated within the affected volume (Fig. 4(b)), while the underlying layers are in a tensile state.
4.2. Analytical modelling An elastic plastic model was developed recently [7] to predict the amount of plastic strain and RS induced by any fast impact considered as a pure uniaxial deformation. This model is based on the calculation of the elastic-perfectly plastic response of a material to longitudinal and planar shock wave systems. The following assumptions were used for the calculations: (1) the shock-induced deformation is uniaxial and planar; (2) the pressure pulse is uniform in space; (3) materials obey a Von Mises plasticity criterion.
106
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
Treating the elastic and plastic loadings and unloadings separately, the model comes to the following definite conclusions: (1) plastic deformation increases linearly between 1 HEL and 2 HEL (HEL < P < 2 HEL); (2) above 2 HEL, plastic deformation reaches a maximum limit; (3) above 2.5 HEL, surface release waves focus and amplify from the edges of the impacts thus modifying the RS field, regardless of the impact shape (circular or square). As a consequence, according to the model, the optimal peak pressure corresponds to 2-2.5 HEL (Fig. 4(a)). Thereafter, calculation of the RS field was performed using the case of a parallelepiped or circle inclusion in a semi-infinite body. The analytical formulation of surface RS developed in Ref. [7] was recently extended [8] to the case of a prestressed material (with an initial stress field of %). The main analytical calculations from Refs. [7] and [8] are summarized in Table 3.
® HEL
0 el~¢ deformation
reverse straining with surface release waves
N plastic deformation bounding
3~+2g E p~
(~
5.1. Experimental techniques On the 7075 alloy, RS measurements were performed with X-ray diffraction using a 20 = f(sin2¢) law which allows easy determination of the multiaxial deformations and stresses with a 1.5 mm spot size, 2 ~b angles and 11 ¢ angles. The incremental hole drilling destructive method [11] was applied on cast A356 and AI12Si alloys because of grain size coarsening up to 300 /~m which is detrimental to X-ray measurements (X-ray diffraction patterns were not obtained). As reported in Ref. [11], hole drilling to D diameter (1-3 mm) allows RS determination over D/2 depth. Strain variations during layer removal are thus measured at the surface of the part with three strain gauges in a rosette. Finite element software allowed the calculation of the principal RSs from the strain measurements.
5.2. One-impact LSP
2 HEL
-2 HEL
5. RSs induced by LSP: experimental results
1. during the interaction : tensile stretching of the impacted area
P
2. After the switch-off of the laser pulse : reaction of the surrounding matter induces a compressive stress field
~q Fig. 4. (a) Plastic deformation induced by LSP as a function of peak pressure [7]. (b) Compressive residual stresses induced by LSP: general principles.
RS fields induced by single impacts were investigated first. In general terms, the amplitude of the surface stress increases with the magnitude of the pressure pulse, which is related to the incident power density ~b. When ~b exceeds a given value, RS increases with depth but decreases at the surface of the materials (mainly at the centre of the impacts) because of surface waves previously examined in Ref. [9]. For instance, for the A356-T6 alloy, surface stresses increase up to - 1 4 5 MPa for 1.3-1.5 GPa pressure levels (1.5-2 GW c m 2), but further power density enhancements (up to 3 GW cm -2) tend to reduce the stress level to - 1 0 0 MPa, whereas in-depth levels continue to increase (Fig. 5(a)). Consequently, it is expected that all materials have an optimal shock condition. Fig. 5(b) shows the in-depth RSs induced by optimized shock conditions in A356, AI12Si and 7075 alloys with one circular spot. A comparison between experiments and calculations is presented in Table 4 in terms of the optimum shock conditions and the surface-induced RS. As predicted by the model (see Table 3), the maximum surface stress field induced ar~sSurris dependent on the yield strength ay of the unshocked material. The experimental data agree closely with the analytical predictions, except for the 7075 alloy where the magnitudes of the surface stresses differ by about 30%. For both AI12Si and A356 cast alloys, the results are in agreement with many previous studies [7-9]: surface stresses reach a maximum level of - 0.6ay, while 7075 only exhibits a stress magnitude of - 0.35o-v. For 7075, it has been suggested that shock-induced plastic deformation may not be saturated by only one impact, even under severe shock conditions (4 GPa impact only achieves an RS magnitude of - 175 MPa).
P. Peyre, R. Fabbro / Materials Seience and Engineering A210 (1996) 102-113
107
Table 3 Analytical calculations of the mechanical effects induced by a fast laser shock impact on an elastic perfectly plastic material [7,8] Calculated value
Analytical formula
Plastic strain condition PH
HEL = (1 + + ) ( 6 y - - 0 " o ) =
Plastic deformation
cp = - 3k + 21l ~ -
Optimal pressure
P = 2 2.5HEL
Plastified depth Superficial residual stresses (circular impact)
Comments
1.75 (O'y-- O'o)
2HEL(P )
Starts at HEL, saturates at 2HEL and depends linearly on P
1
C~,Cp, r ( P - HEL'] L = Cel-- Cpl \ 2HEL /
For a pure uniaxial deformation increases with cr~ < 0
Drives to c p saturation
triangular pulse
a""':¢ = 6°--I/t~P '1~ +-' 1 v + aoJ7 [1 - 4 ~
The plastified depths, which are almost linearly dependent on the pulse duration, are better than 1 mm at the optimized shock conditions. Moreover, very small surface stress gradients are found after LSP. This latter point is beneficial because it is known to be the key to little or no cyclic stress relaxation.
5.3. LSP with repeated impacts In the future, application of LSP to extended areas will necessitate the repetition or at least overlapping of impacts in order to generate large homogeneous compressive zones. Therefore our investigations have centred on repeated impacts. On cast alloys, as predicted by the model described above, no surface stress increases can be achieved with two or three impacts at the same point, but in-depth RS and plastified depths are strongly enhanced. For 7075, a different behaviour is observed: cumulative impacts have a very beneficial effect on the stress levels in the superficial layers as shown in Figs. 6(a) and 6(b). For instance, a 4 G W c m - 2 treatment generates - 170 MPa at first impact, - 2 4 0 MPa with a second impact and - 340 MPa with a third (Fig. 6(a)). Moreover, X-ray diffraction peak broadening observed in 7075 ( + 7% to + 15%) reveals that this particular material, submitted to numerous impacts, seems to undergo local cyclic hardening leading to much higher RS levels (which depend mostly on o-v). Bauschingerlike effects have also been proposed as a mechanism
(1 + v)
Depends linearly on the pressure duration r Increases with eP Decreases with L Increases with a o < a Increases with the size of the impact
which would tend to reduce the plastic flow level during the first impact and allow much more deformation and RS during the following impacts.
6. Comparison of LSP and SP surface effects A356 and 7075 were selected to explore the influence of LSP and SP on the surface modifications and fatigue properties. The issues relevant to RS, hardening and surface roughening are reviewed and discussed. SP involves the impact of high-speed small beads with about 100%-200% covering rates. Contrary to LSP, SP mechanical effects generally result from the influence of surface hammering by b e a d - m e t a l contact, which creates tangential stretching of the surface layers and Hertz-type uniaxial loading with a maximum shear stress in the subsurface. Thus shot-peened surfaces are submitted to more multiaxial, intense loadings than laser- shocked materials. A quantitative comparison of the loading conditions is presented in Table 5. Perhaps the most distinctive change in the impact conditions involves the peak pressure durations which are 10-20 times longer in the case of SP. As shown by recent investigations on steels [7,8], shot-peened specimens generally display much higher RS levels than laser- shocked materials, mainly in the first 100-200/~m of depth. Our experimental results on A356 cast alloy [12] show that surface stresses of - 210 MPa are achievable with SP against only - 150 MPa
108
P. Peyre, R. Fabbro /Materials Science and Engineering A210 (1996) 102-113
with LSP. A t first glance, these results c o u l d indicate t h a t the plastic d e f o r m a t i o n i n d u c e d will be lower with L S P t h a n with SP, a l t h o u g h c a l c u l a t i o n s give o p p o s i t e q u a n titative d e s c r i p t i o n s (calculated values o f ε p are n e a r 0 . 2 % - 0 . 3 % for SP a n d 0 . 5 % - 1 % for LSP). H o w ever, as s h o w n in Fig. 7(a), in 7075, L S P c a n d e v e l o p an a l m o s t equal R S level in the first 1 0 0 / ~ m by m u l t i p l e impacts. M o r e o v e r , in all the m a t e r i a l s investigated so
0
I
-20 -4 0
] I I '
-60
I
[] JL
I
1,5 GW/cm2 -- 1,3 GPa 0,9 GW/cm2 = 1 GPa 3 GW/cm2 = 1,9 GPa I
I I i I •
-50
t
-
-12
Calculation Experiment
AI12Si-T6
7075-T7351
Poptimum
O'surface Poptimum °'surface Poptimum
°'surface
(GPa)
(MPa) (GPa)
(MPa) (GPa)
(MPa)
1.2 1.3-1.4
- 150 1.3 - 145 1.6-1.8
- 120 2 - 125 2-2.2
-280 - 160
S u r f a c e r e s i d u a l stresses (MPa) 0 I F I I I
-
-,o -80
A356-T6
far, the stress field has been shown to extend two to five times d e e p e r t h a n is typical for SP ( 0 . 2 - 0 . 3 mm).
stresses ( M P a )
residual
Table 4 Optimum laser shock conditions on aluminium alloys in a water confinment mode. Comparison of experimental and calculated values.
o [] •
[] []
-100
%D
-150
I
1 impact 2 impacts 3 impacts
[]
[]
D []
-200
D []
,
-16 0
0.2
.250
.
0.4
0.6
depth
(mm)
0.8
1
-300 -350
• I
-400 0
1
7075-T7351
I
I
[
2
3
4
power
i
i
5 6 (GW/cm2)
density
-
I
7
8
Residual stresses (MPa) 0
I
i
t
-40 -
I
•
R e s i d u a l stresses ( M P a )
-
E'o'5-T'351i 50 0,
-80
-120 -160
t-
I
/
~
.200 8 0
m
I
-
I
~ 2 , 5
]
&
7075-4,2
GW/cm2
-100 -
~
-15G
GW/cm2
A356- 1,5 GW/cm2
I
I
I
I
0.2
0.4
0.6
0.8
depth
I
, , , , , , , , , I ' ' ' ' ' ' ' ' ' i ' ' ' ' ' ' ' ' '
-20G 1
-250
1 impact 3 impacts
-
1
(mm) -300
Fig. 5. Optimization of the residual stress fields induced by LSP in aluminium alloys in the water confinement mode. Experimental measurements (incremental hole drilling method). (a) A356-T6 at various shock conditions. (b) Maximum surface stress fields induced With single impacts.
'
0
~
J
,
J
,
,
,
I
r
:
J
,
,
,
,
,
I
0.5
I
1 depth
I
I
s
,
t
I
III
.5
(mm)
Fig. 6. Residual stresses induced by repeated impacts on 7075-T7351 alloy: (a) surface effects; (b) in-depth effects.
109
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102 113
Table 5 Comparative loading conditions induced by laser shock processing and shotpeening with industrial configurations (LSP in a water confinment mode) Peak Diameter Pressure Mechanical Strain pressure of impacts duration impulse rate (GPa) (ram) (as) (GPa/is) (s 1) LSP 0 6 Shot peening 3 10
1 15 0.2-1
0.05 0.5 1
0 0.3 1-10
106 10 4
The second comparative factor investigated was the surface hardening. Fig. 7(b) reveals that SP induces double the surface hardness increase ( + 20%) generated by LSP ( + 10%), even when LSP is performed under severe shock conditions (with a glass confinement mode for instance). This may be attributed to differences in the pressure duration, which result in higher dislocation generation and motion, and to the number of slip planes activated by multiaxiai surface loading such as SP. In the same way, the X-ray diffraction peak broadening induced by LSP remains inferior to SP, even for multiple laser impacts, suggesting that SP is capable of achieving greater lattice distortion and work hardening in the alloys. However, as some fundamental questions concerning these mechanisms still remain unanswered, transmission electron microscopy (TEM) may be helpful in future investigations. Surface geometry modifications were also investigated. They are observed as a general metal depression for LSP and an important roughening for SP. Table 6 shows the comparative roughening effects induced in A356 and 7075 alloys. In both cases, while the lasershocked surface remains partially unchanged, SP generates a detrimental roughened surface with large increases in the mean and peak roughnesses R~ and Rt. By comparison, laser-peened surfaces generally exhibit a 5--15 /~m homogeneous depression with little roughness modification. As shown above, the advantages of LSP include the greater affected depths and the preservation of the surface state, whereas SP is found to generate higher RS and hardness enhancements. This further demonstrates the differences in the two processes in terms of the mechanical and geometrical changes generated on the metallic surfaces. Regarding fatigue limit modifications, the interesting work of Wang et al. [13] on shot-peened 7075 samples showed that the important contributors reviewed above can be quantitatively separated as follows: + 50% and + 20% enhancements of the fatigue limit aD due to RS and hardening beneficial effects respectively and - 3 5 % decrease in 0"D due to roughening effects.
7. Application
to fatigue
behaviour
The cyclic behaviour of mechanical parts has previously been shown [13] to depend strongly on the metallurgical, mechanical and geometrical surface states of the materials involved. Since many materials usually display pronounced improvements in fatigue limits with SP or cold rolling, LSP was envisaged as a substitute to conventional treatments for materials displaying a reduced fatigue limit increase. As reported in Section 6, the beneficial effects of LSP may originate from the large affected depth and surface state quality, which are (HV-25g)
Hardness 190
....
....
180 -:~',
I ....
I ....
I'
'--7
' ' I'''~l
[ _~._~ / 7075
Shot-Peening
~ 27A
17C
-~
1613-
LSP
- ~
150.4GW/cm2
~
~
-
d
(glass)
140-~t-Peening
F15-20
!
A
1 3(~ ~ ~ 1 . ~ "
l-A-35 ~
12d
.~
1 10 -
LSP ~ L l 4 ~ ~ l [ ~ - ~ - ~ GW/cm2 (glass) ~ " ~ 100 ,, L ,~,F,,~ N,,,~I, , b ,, I, ~ 0
100
Residual 50
0
,
stresses ,
...........
-35
, 0
200
,
!
,
300
400
500
depth
(~tm)
700
(MPa) i
,
!
,
~ ,
!
. . . . . .
i
~ I, 0.2
I
600
.
,
,
, 0.4 depth
I ~ ~ 0.6
.
.
.
.
.
.
0.8
(mm)
Fig. 7. Mechanical effectsinduced by LSP and SP: (a) comparison of the residual stress fields induced in 7075 (LSP: 3 impacts, 4 GW c m 21; (b) Vickers hardness measurements with a 25 gf load (A356T6, 7075-T7351).
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
110
Table 6 Comparative roughening effects of laser shock processing and shot peening on A356 and 7075 alloys Material and Processing
R a (pm)
R t (#m)
A356 as milled A356 + SP F 3 8 - 5 0 N (0.3 m m beads) A356 + LSP 2 G W cm 2 (2 impacts) 7075 as milled 7075 + SP F20-23A (125%, 0.6 m m beads) 7075 + LSP 4 G W cm - 2 (3 impacts)
0.7 5.8 1.1 0.6 5.7 1.3
6.2 33 7.5 5.2 42 11
expected to influence favourably the initiation and cracking stages. High cycle bending tests were performed on the three alloys in order to demonstrate the efficiency of LSP and to distinguish between the fatigue behaviour of unshocked, shot-peened and laser-processed aluminium alloys. 7.1. L S P of fatigue samples
Notched rather than plane samples were selected in order to reduce the number of impacts per sample and to localize crack initiation at the notch root with stress concentration factors (Kt = 1.6-1.7). With such conditions, fatigue cracks are initiated in the laser-shock-processed zones thus restricting the failure cycle dispersion. The specimen geometries are presented in Fig. 8. The LSP conditions were close to those previously optimized through RS measurements (Section 5). Each notch was covered with a sacrificial aluminium scotch r
foil and immersed in 5 mm water confinement medium. A computer-controlled X - Y table was used to move the water tank containing the specimen in the required direction during processing. Different aspects of the treatment are discussed. For instance, on 7075, since it has been demonstrated that repeated impacts may have a beneficial effect on the surface stresses (see above), particular attention was focused on the overlapping rate (up to 67%). As a consequence, 7075 notches were submitted to three local deformations against only two on cast alloys (Table 7). For A356, square- and ellipse-shaped impacts were investigated so that the role of the impact shape in achieving fatigue life enhancement could be studied at constant incident power density. 7.2. Fatigue tests
High cycle (up to 107) fatigue experiments were performed using a three-point bending test in a 100 kN servo-hydraulic material testing system machine (stress ratio R = 0.1; test frequency, 40-50 Hz) in an ambient air atmosphere. Despite the small number of fatigue data points (about 10-14 points for each configuration with four levels of the maximum stress magnitude), statistical regressions were performed to yield the regression representations of W6hler curves a = f(N) and to determine the fatigue limits aD of the unshocked, laser-processed and shot-peened specimens. Regression laws were calculated with Esope software derived from the Bastenaire-type formula U=
A exp[ - (a - CrD/B) c]
(3)
o - - o-D
"t"-I
I I
I
I
d
i"
H
"1 ~ e
I i ~ ~'lll L
I
I
7.3. Experimental results on cast alloys
i~i~iii!i AS12UNG : LSP with 50 % overlapping
of 3 squared shape impacts
A356
where A, B and C are parameters adjusted with the software, aD is the fatigue limit at 107 cycles and N is the number of cycles to failure for a maximum applied stress o-.
AS12UNG
7075
L (mm)
86
86
90
H (mm)
24
24
22
, h (mm)
21
22
19,5
e (mm)
12
12
11
r (mm)
6
6
5
Fig. 8. Notched fatigue specimens.
Fig. 9 shows the fatigue data and their regression representations for as-machined and laser-shock- processed A356 and All2Si specimens. The fatigue data were plotted using maximum stress vs. fatigue life. As presented in Fig. 9, W6hler curves show much better performance for laser-treated samples than for as-machined specimens. For a fatigue life equal to 10 million cycles (at which the fatigue limit determination was made), the improvements in o'D are approximately + 36% for A356-T6 (110-150 MPa) and +22% for AII2Si-T6 (103-126 MPa). A large dispersion is found in the number of cycles to failure N for a given applied stress a. This can be attributed to casting defects due to dilation gradients between the Si primary phase and aluminium matrix
111
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113 Table 7 Summary of experimental LSP conditions on fatigue samples in a water confinement mode Material
0 (GW cm
,~356
2.3
~Xll2Si 7075
2.5 3.8
Impacts
Overlapping rate (%)
5 squares (6 mm) or 3 ellipses 3 squares (10 mm) 4 circles (11 mm)
50
2)
during the quenching stage. Indeed, it is clear from the examination of the fracture surfaces that fatigue cracks mostly initiate on porosities or surface Si inclusions. Therefore, for the particular case of cast aluminium alloys, LSP can be considered as a means to reduce the notch sensitivity of surface defects. Similar, earlier investigations of Forget [9] on Ni base superalloys also demonstrated that a laser-induced RS field could prevent fatigue cracks from initiating on surface inclusions as long as the local applied stresses remained inferior to the macroscopic plastic flow limit. The effect of LSP on the dispersion of the results was investigated statistically, but no real dependence was found since fatigue cracks initiate over a very large range of cycles. However, increasing the fatigue limits using LSP is very efficient for cast aluminium alloys. This is somewhat surprising since these materials are known to be very sensitive to work hardening effects that are not generated by LSP. Concerning the impact shape effects on A356, no real influence was found, thus indicating that the greater fatigue lives are attributable to the peak power density and pressure achieved at the workpiece surface. We have demonstrated that LSP can provide large increases in the fatigue limits of both A356 and All2Si cast aluminium alloys. These improvements are ascribed to a combination of compressive RSs and surface integrity effects. 7.4. L S P and S P effects on 7075
7075 notched specimens were laser treated or shot peened. As shown in Fig. 10(a), fatigue limit increases of about + 22'/o for laser-impacted specimens and only + 10% for shot-peened specimens are achieved. The surface roughness is considered to be the main detrimental contributor to the fatigue behaviour of shotpeened samples, compared with LSP which maintains a safe surface geometry and an almost equal RS level. Nevertheless, for shot-peened specimens, core initiations are detected at a stress amplitude of 220 MPa and at depths between 0.35 and 0.4 mm from SEM observations (below the shot-peened layer). Using a local fatigue strength concept taking into account the RS field [14], we confirm that the local fatigue strength is lower
Number of cyclic deformations
50 67
than the applied stress (calculated with a Creager approach) only beyond a depth of 0.35 mm. For higher applied stresses (greater than 220 MPa), initiation always occurs at the surface. According to these considerations, it can be concluded that with a deeper SP treatment or a different notch geometry (with a higher K,), LSP may display higher fatigue improvements. 7.5. Detection o f crack initiation on 7075 using an a.c. potential drop method
An a.c. potential drop method was developed to detect crack initiation and to investigate at what stage fatigue life increase occurred. Experimentally, an electric crack detector (ANS) connected to both sides of the notch was used. Current pulses (2 A, 0.02 s) were delivered to the samples during fatigue tests at a frequency of 30 Hz, and crack initiation (limit of detection, 50-100 /zm) was considered as the number of cycles corresponding to a change in potential down the notch root. Fig. 10(b) shows that, for a stress of 260 MPa, fatigue life improvements induced by LSP can be separated into a sevenfold increase in the crack initiation stage and only a threefold increase in the propagation stage. In contrast, SP provides a homogeneous two to threefold increase in both initiation and propagation durations. In addition, the ratios between initiation and propagation are changed from 85%-15% for shotpeened samples to 90°/,7 10% for laser-processed samples. This different behaviour can be ascribed to surface embrittlement and surface roughening due to SP, which will tend to reduce the beneficial effects of compressive RSs, whereas the electric detection results are indicative of delayed initiation stages in the case of laser-processed materials.
8. Combination of SP and L S P
LSP is generally envisaged for high-cost, fully localized mechanical parts, whereas SP can be used on larger parts with a smaller cost. In this context, preliminary studies were performed to investigate the combination of the two treatments. Fig. 11 shows that laser
112
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
shocking a pre-shot-peened zone combines and enhances the advantages of the two treatments with both in-depth and superficial RS increases. Therefore SP beneficial effects are preserved (hardening) or increased (RS field) by LSP treatment. Further fatigue studies should provide interesting information on the mechanical improvements generated by S P + L S P treatment which seems to be a very promising process.
m a x (MPa) 320 ....................................................................................... ~.............................
........................... i............................i............................ i ..........~---..-D .i............................i............................ i............................ ~ 280 ".. . 3i8 G W / c m 2 i i
260..............
oi
2 o o ............................~ n t r ; ~ d ~
-.:::: .~91
;ii;~
1 8 0 ...................................................... t~ ...................... -2-O-~ ..................... G
max
160 10000
(MPa)
105
. . . . . . . . . . . . .
160
........O N - • I ' I - I - - - - ~ -
140
...........• . . . ~ . . . . . .
120
.....u.n: ho ck e d - - - ~ ~ . - l ~ : ~ l 0 M P a :
[]
[ 107
10~
N
2 2 0 ................................................................................................................ ~._.. I A356"T6 I 2 0 0 ............13 ............ !.....................................i....................................i ::LSP 2,3 GW/cm2 ': 180
[ 11~
D-----D
..........
.l~(~-----=----;50--~p a
• ...................L ~ . . . . . ! )
N cycles
..~.............
Laser-peened
-
-
3105 -
100 .................................... !.......... • .................... --~-~---P--- ................. : 2 80 I 10 5
10 6
10 7
10 8
• []
Cracking + failure Initiation
2105 -
N 1105
Shot-peened
-Untreated 0 --
Fig. 10. (a) O ' m a x - N c u r v e s for unshocked, shot-peened and lasershocked 7075 alloy. (b) Crack detection tests on 7075 alloy with a.c. potential drop method. Comparison of initiation and cracking stages at O'max = 260 MPa.
G max (MPa)
[ 140
" ~ N
0 ....................,
100
:
~
LSp- 2,5 GW/~cm2
10 5
i
". . . . . . . . . . . i....... [] ........ O - i ........................... i
~
I
2
6
A
M
p
~ - - ~ - - - - - ~ I - I ) 3 - - - - ~ i - P
60 10000
It
:
2
::
2
I 10 6
10 7
a
a
10 8
N Fig. 9. High cycle fatigue tests on A356-T6 and ASI2UNG-T6 cast aluminium alloys, amax-N curves for unshocked and laser- shockprocessed specimens.
9. C o n c l u s i o n s
LSP is a very attractive, promising, new surface treatment for aluminium alloys yielding large fatigue improvements in A356, All2Si and 7075 alloys. In our study, mechanical surface modifications were investigated in terms of the RS fields induced, surface hardening and surface roughness modifications. In all three alloys investigated, the RS fields extend to a greater depth than is typical o f other techniques, with affected depths better than 1 mm. Hardening is limited to + 10% o f the initial value, which is half the increase achievable by conventional SP treatment. The main difference between the two treatments has been
P. Peyre, R. Fabbro /Materials" Science and Engineering A210 (1996) 102 113
113
Appendix A. Nomenclature
::::::: :::::::i:i:i..... ...............................iiiiii!
i
II.
1
.........
i .400
, II I
0
0.2
~_I_
shot-peen,ng
li
a - - +LS-I,50Pa li , , +LS-2,1GPa li ,
0.4 0.6 depth (ram)
'
,
0.8
=i
1
Fig, 1I. Residual stresses induced by a combination of shot peening and laser shock treatment on 7075 alloy.
demonstrated to originate from the surface roughening: LSP generates a general depression of the material with no roughness changes; SP strongly affects the surface and creates potential initiation sites. Therefore much better fatigue performance is observed with laser-shocked 7075 than with conventional shotpeened specimens. This is attributed, through fatigue crack detection, to larger increases in the crack initiation stage for LSP than for SP specimens. It has also been shown that a combination of LSP and SP treatments is capable of achieving an optimum surface layer. Whether a high level of fatigue behaviour improvement of LSP + SP materials is possible has not yetbeen demonstrated, but this combination is expected to provide better fatigue resistance than any single process alone. Despite the interesting potential of LSP, no real industrial applications could be found, mainly because of the low repetition rates of the laser sources presently available. However, the newly developed excimer gas laser [15] can deliver output characteristics similar to those required in the industry (with a repetition rate of better than 10 Hz). In the near future, with the use of such laser sources which will enable the problems inherent in the process to be overcome, LSP may replace conventional methods for many applications such as fatigue or stress corrosion behaviour.
a L P r0 ep 2, /~ v a~,rr av a0 r
square-shaped impact edge (mm) plastically affected depth (mm) shock pressure circle-shaped impact radius (mm) plastic deformation rate induced by LSP Lam6 elastic constants (GPa) Poisson's ratio (0.3) surface residual stress (MPa) static yield strength (MPa) initial residual stress (for unshocked material) (MPa) pressure pulse duration (ns)
References [1] R.M. White, J. Appl. Phys., 34 (1963) 2123. [2] N.C. Anderholm, Bull. Am. Phys. Soc., 13 (1968) 388. [3] A.H. Clauer, B.P. Fairand and J. Holbrook, in L. Murr (ed.), Shock Waves and High Strain Phenomena in Metals-~ Concepts and Applications, Plenum, New York, 1981, pp. 38, 675. [4] R. Fabbro, J. Fournier, P. Ballard, D. Devaux and J. Virmont, J. Appl. Phys., 68 (1990) 775. [5] J. Fournier, Thesis, Ecole Polytechnique, France, 1989. [6] D. Devaux, R. Fabbro, L. Tollier and E. Bartnicki, J. Appl. Phys., 74 (1993) 2268. [7] P. Ballard, Thesis, Ecole Polytechnique, France, 1991. [8] C. Dubouchet, Thesis., University of Orsay, France, i993. [9] P. Forget, Thesis, Ecole des Mines de Paris, France, 1993. [10] P. Peyre, Thesis, University of Technology of Compi6gne, France, 1993. [11] J. Lu, A. Niku Lari and J.F. Flavenol, J. Mech. Work. Tech., 11 (1985) 167. [12] P. Peyre, P. Merrien, H.P, Lieurade and R. Fabbro, Materiaux et Techniques, 6 7 (1993) 7. [13] R. Wang, X. Li and H. Wu, Proceedings of the 3rd International ConJerence on Shot-Peening (ICSP3), Elsevier, GarmischPartenkirchen, 1987, p. 563. [14] R. Lin and T. Ericsson, in Residual Stresses, DGM-Hauk-Hougardy-Macherauch, Francfort, February 1993, p. 505. [15] B. Godard, M. Stehle, P. Murer, J. Bonnet and D. Pigache, Proceedings of ConJerence on Lasers and Electro- Optics, Baltimore, USA, June, 1993.
A
Materials Science and Engineering A210 (1996) 102-113
Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour P. Peyre a, R. Fabbro a, P. Merrien b, H.P.
Lieurade b
aLaboratoire d'Application des Lasers de Puissance (LALP), Unitk Mixte ETCA, CNRS 114, 16 bis Avenue Prieur de la C6te d'Or, 94114 Arcueil Cedex, France bCentre Technique des Industries Mkcaniques (CETIM), Departement Materiaux, 52 Avenue Felix Louat, 60304 Senlis Cedex, France Received 25 May 1995; in revised form 31 October 1995
Abstract
Subjecting target metallic samples to a very short pulse (about 20 ns) of intense (GW cm-2) laser light generates, through a surface plasma, a high-pressure stress wave propagating to the first millimetre in depth, which is commonly called laser shock processing (LSP). The purpose of this work was to evaluate the role of this novel process on the cyclic properties of A356, AII2Si and 7075 aluminium alloys. Major contributors to the fatigue performance improvements were investigated in order to determine the optimum shock conditions. These were mainly compressive residual stress (RS) levels for which a large range of incident shock conditions was performed. We showed that stress levels were very sensitive to the laser fluence and the number of local impacts, and experimental RS measurements were found to be in good agreement with analytical modelling results. In comparison, a conventional shot peening (SP) treatment was found to lead to higher surface hardening and RS levels, but with a very detrimental roughening not observed after LSP. High cycle (107) fatigue tests carried out on laser- processed, shot-peened and untreated notched samples illustrated the efficiency of LSP as a new, promising method to improve the fatigue limits a D of structures, especially in comparison with enhancements displayed by SP ( + 22% vs. + 10%). According to crack detection electric measurements, fatigue performance improvements with LSP mainly occurred during the crack initiation stage.
Keywords: Laser shock processing; Shot peening; Aluminium alloys; High cycle fatigue behaviour
I. Introduction
The ability of high-energy pulsed lasers to generate shock waves and plastic deformation in metallic targets was demonstrated for the first time in 1963 [1] in the USA and extended in 1968 [2] with the development of confined ablation modes. Thereafter, the application to materials processing was initially investigated in the 1968-1981 period at Battelle Columbus Laboratories (OH, USA) with the application to fastener holes [3]. After the last publications of Battelle searchers, dated 1981, no further developments emerged because of the lack of laser sources to provide real industrial potential. In France, since 1986, and supported by the automotive industry (PSA) and other industrial partners, new studies have been started. Systematic investigations have centred on highlighting the main key points of the process at this time. These various studies have exam-
ined the confined interaction mode [4,5], breakdown factors limiting the amount of laser energy reaching the surface [6] and analytical modelling of the mechanical processes involved [7,8]. The original results reported here [10] follow from previous studies on the laser shock strengthening of steels [5,7,8] and nickel base superalloys [9]. Three commercial aluminium alloys were chosen on which to evaluate the effects of laser shock processing (LSP). The first two are AI-Si-type cast aluminium alloys (A112%Si (All2Si) and A1-7%Si (A356-T6)) often used in the automotive industry. The third is an aeronautical 7075-T7351 wrought alloy. This paper reports a series of experiments relating the influence of laser-induced shock waves on the surface modifications and high cycle fatigue properties of these aluminium alloys. The objective was to compare LSP effects with a conventional mechanical treatment (shot 0921-5093/96/$15.00 © 1996 - - Elsevier Science S.A. All rights reserved SSDI 0921-5093(95)10084-5
P. Peyre, R. Fabbro /Materials Science and Engineering A210 (1996) 102 113
103
(2 HEL amplitude) plastic waves Shock waves
1
I Peak Pressure
] -----~k elastic waves I • plastic waves
depth I T HEL Fig. 1. Laser shock processing (LSP) in the confinedablation mode. peening (SP)) which has been used for many years in the industry, except on mechanical parts when access to the treated surface is limited.
2. Process and experimental environment The generation of high-amplitude laser stress waves in metals requires irradiation with a short (tenths of nanoseconds), intense (greater than 1 GW cm-2), focused laser pulse. The laser-material interaction generates vaporization of the first atomic layers of the target, called an ablation phenomenon. Blowing off, the partially ionized gas (plasma) drives a shock wave by expansion from the irradiated surface. This shock wave propagates into the target, causes plastic deformation to a depth at which the peak stress no longer exceeds the Hugoniot elastic limit (HEL) of the metal (equivalent to the yield strength under shock conditions) and, in turn, induces residual stresses (RSs) throughout the affected depth. The experiment involves coating the surface to be treated with a sacrificial opaque material (black paint, metallic foil) followed by a dielectric material transparent to laser light (water, glass) (Fig. 1). As the laser beam passes through the water, the energy is initially absorbed by the opaque overlay, and only a thin layer (less than 10/~m) of this sacrificial material is ablated. Thus the material undergoes no thermal process or microstructural changes. Additionally, as the vapour continues to absorb the rest of the laser energy, it is readily heated and ionized into a plasma. Therefore the plasma expands and generates two shock wave systems,
one in the target and the other in the confining medium. As shown in Fig. 1, shock wave systems have two components: an elastic component below 1 HEL ((1 + 2/2/Z)ay) and an elastic-plastic component above 1 HEL. An attenuation in depth occurs when highvelocity elastic recoil waves come back from the surface and reduce the peak pressure amplitude of 2 HEL. As it is desirable that the laser beam is converted into a stress wave with the highest degree of efficiency, the plasma is trapped between the dielectric transparent material and the workpiece surface to retard its expansion and to achieve higher stress amplitude and duration. This configuration, which is termed the confined ablation mode, is the basis for the LSP of materials. It allows 5 GPa peak pressures vs. 1 GPa for the same incident laser power densities in direct ablation mode, and durations two to three times longer than the duration of the laser pulse. The resultant effect is the generation at the metallic surface of a higher compression wave, which causes enhanced deformations and higher compressive RS, provided that the metal has been covered with a sacrificial coating. Since this coating (usually black paint or metallic foil) protects the specimen from melting and vaporization, LSP is a pure mechanical treatment inducing plastic flow and negative RS. These compressive RSs are the key to the superior fatigue or stress corrosion performance of structures while reducing the applied surface stresses. A precise description of the confined ablation mode was given recently [4], with a three-step process for the pressure environment generation (heating, adiabatic cooling and final expansion of the plasma). Using this
104
P. Peyre, R. Fabbro / Mater&& Science and Engineering A210 (1996) 102-113
model, and considering the plasma to be a perfect gas, the scaling law of the pressure generation can be estimated by the following relationship
Pressure (GPa)
10
SRTgaussian I
P(GPa) =0.01 / ~ - ~
X/Z (g cm-2 s - l)x/Io(GW c m - 2)
DGI
(1)
o~ where I0 is the incident laser power density, P is the pressure, Z is the reduced shock impedance between the target and the confining medium and ~ is the efficiency of the interaction, where ~E contributes to the pressure increase and (1 - ~)E is devoted to the generation and ionization of the plasma (c~ = 0.1-0.2). All the experiments shown in this paper were performed with a water co.nfinement mode. In this kind of configuration, easily achievable in the industry, Eq. (1) gives P(GPa) = 1.02~/I0(GW cm - 2)
(2)
For a water confinement mode, the peak pressure is approximated as the square root of the incident laser power density. In Fig. 2, the laser and pressure pulses, monitored with a fast photodiode and an x-cut quartz gauge system respectively, are illustrated. It can be observed that the decay time is much slower for the stress pulse than for the laser pulse because of plasma confining effects. These higher stress wave durations are expected to increase the amount of shock-induced cold work, mainly for low-strength materials such as aluminium alloys. Fig. 3 shows the peak pressure measurements as a function of the power density for a water confinement mode and two different pulse shapes: a classical gaussian and a dissymmetrical short rise time (SRT) pulse [6]. It appears that pressures reach a limit over a power density of 4 GW cm-2 for the gaussian pulse. These saturations are assumed to reflect dielectric breakdown
GN• D []
[] •
•
BiD •
• •
•n
d~
I
I
I
I
I
•I
I
I
peak power density ( G W / c m 2 ) Fig. 3. Peak pressure levels as a function of the peak power density for the water confinement mode. Influence of the pulse shape (gaussian or SRT); piezoelectric quartz measurements [6].
phenomena in the confinement medium that limit the amount of energy reaching the metallic surface. As shown in Fig. 3, the breakdown thresholds can be increased (up to 5 GW cm 2) using SRT pulses. All our experiments were performed with the LALP pulsed Nd-glass prototype laser operating at 2 = 1.06 /zm and consisting of an oscillator followed by four amplifier stages. This system is capable of emitting up to 80 J in a pulse that is semi-gaussian in shape or has an SRT with a full width at half- maximum (FWHM) of 15-30 ns. Only 25 ns FWHM gaussian pulses were used since they allowed maximum peak pressures of 2.5 GPa, capable of treating any aluminium alloy. To provide a good energy reproducibility on the samples, the maximum repetition rate was limited to about one shot every 2 min. The laser energy was focused with a lens of 600 mm focal length to spots of 5-12 mm diameter on the targets. Distilled water (thickness, 2-5 mm) was used as confining medium. In this configuration, aluminium samples were submitted to 1-8 GW cm -2 irradiation resulting in 2.5 GPa maximum peak pressures. 3. Materials
Three industrial aluminium alloys were chosen to evaluate the effects of LSP. Both A1-Si alloys were chilled cast, solution treated, quenched and aged to the T6 condition. Age hardening occurred mainly by fine Table 1 Weight per cent compositions of the three aluminium alloys
I
-100
I~
pulse I
L
I
I
200
300
400
Si 0
100
time (ns) Fig. 2. Gaussian laser pulse and resulting pressure pulse submitted to the target.
All2Si A356 7075
Cu
Fe
12.2 1.48 0.16 7.05 0.05 0.2 0.08 1.48 0.17
Mg
Mn
Ni
Zn
1.3 0.33 2.6
0.03 0.03
1.25 0.03 0.03 5.95
Cr Ti
0.2
0.01 0.15 0.04
105
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102 113 Table 2 Monotonic and dynamic mechanical properties of the alloys Material
E (GPa)
HV (25 gf)
a v (compressive monotonic) (MPa)
crv (compressive dynamic) (MPa)
gv (tensile monotonic) (MPa)
A (tensile)'¼,
A356 T6 All2Si T6 7075--T7351
70 77 72
110 120 170
-218 - 238 -450
-260 - 310 - 550
225 170 415
8 0.7 11
precipitation of the coherent Mg2Si phase. The compositions are given in Table 1. Each cast alloy exhibited grain size coarsening to about 300/zm levels so that the RSs induced by LSP could not be measured by conventional X-ray diffraction. The A356 alloy was solution treated for 6 h at 480 °C, water quenched (20 °CA and aged for 6 h at 160 °C to the T6 condition. Tensile tests performed on 8 mm diameter samples revealed (with scanning electron microscopy (SEM) observations) ductile-brittle failure aspects. All2Si was obtained by solution treatment for 6 h at 480 °C, water quenching at 70 °C and aging for 6 h at 220 °C to the T6 condition. The microstructure of as-received All2Si-T6 consisted of the Mg2Si phase (like A356) with A12Cu partially coherent precipitates. In alloying A1 with 12% Si the material was found to lose most of its plasticity behaviour (to less than 1% elongation values). The starting 7075 material was 4 cm thick sheet oriented with the direction parallel to the hot rolling direction. It was delivered in the T7351 thermal state which involved quenching, tensile stress relieving and two-stage aging (8 h at 135 °C and 1 h at 190 °CA. Optimization of the shock conditions and characterization of the mechanical effects induced by LSP on the three alloys were performed using 15 mm thick plane specimens. It has been known for some time that the resultant effects of LSP are primarily mechanical, since thermal effects are confined to the surface layers of the sacrificial coating. Therefore LSP is usually considered as a pure high strain rate (106 s ~) uniaxial compression, creating surface layer stretching during interaction. The mechanical properties of the alloys were determined by monotonic tensile and compressive tests (strain rate, 10- 3 s 1) and by dynamic compressive tests (103 s - i ) using the Split-Hopkinson pressure bar (SHPB) system. However, as SHPB monitors information only over 2% plastic strain, dynamic yield strengths were extrapolated from the first 2%-5% of plastic strain. The resulting mechanical properties are summarized in Table 2. Compressive measurements on cylindrical samples (radius, 5 mm; height, 15 mm) show that strain rate sensitivities between 10 3 s J and 10 3 s 1 lead to + 20%- + 30°/,, increases in the elastic limits o-y of each material.
4. Mechanical effects induced by LSP
4.1. General trends As fatigue cracks mostly originate at the surface of materials, the fatigue behaviour of mechanical parts depends strongly on their mechanical and geometrical surface states. For instance, a compressive surface layer prevents crack opening and growth and therefore has a beneficial effect on the fatigue life, whereas a high surface roughness generates local stress concentration factors detrimental to crack initiation. Therefore a large part of our work was devoted to the experimental description of surface changes due to laser shock waves, mainly in terms of the compressive RSs induced within the materials. Our interest was stimulated by the need to investigate a large range of power densities in order to optimize the shock conditions and to generate maximum RS levels at the surface of the target materials. The experimental results were correlated with analytical predictions [7] of the optimum shock conditions and RS fields induced. In the normal configuration (thermoabsorptive overlay + confining medium), the generation of RSs by LSP may be summarized as follows: during laser material interaction, the pressure pulse generated by the blow off of the plasma crushes the treated area and creates pure uniaxial compression along the direction of shock wave propagation and tensile stretching in the plane parallel to the surface. After reaction of the surrounding zones, a compressive stress field is generated within the affected volume (Fig. 4(b)), while the underlying layers are in a tensile state.
4.2. Analytical modelling An elastic plastic model was developed recently [7] to predict the amount of plastic strain and RS induced by any fast impact considered as a pure uniaxial deformation. This model is based on the calculation of the elastic-perfectly plastic response of a material to longitudinal and planar shock wave systems. The following assumptions were used for the calculations: (1) the shock-induced deformation is uniaxial and planar; (2) the pressure pulse is uniform in space; (3) materials obey a Von Mises plasticity criterion.
106
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
Treating the elastic and plastic loadings and unloadings separately, the model comes to the following definite conclusions: (1) plastic deformation increases linearly between 1 HEL and 2 HEL (HEL < P < 2 HEL); (2) above 2 HEL, plastic deformation reaches a maximum limit; (3) above 2.5 HEL, surface release waves focus and amplify from the edges of the impacts thus modifying the RS field, regardless of the impact shape (circular or square). As a consequence, according to the model, the optimal peak pressure corresponds to 2-2.5 HEL (Fig. 4(a)). Thereafter, calculation of the RS field was performed using the case of a parallelepiped or circle inclusion in a semi-infinite body. The analytical formulation of surface RS developed in Ref. [7] was recently extended [8] to the case of a prestressed material (with an initial stress field of %). The main analytical calculations from Refs. [7] and [8] are summarized in Table 3.
® HEL
0 el~¢ deformation
reverse straining with surface release waves
N plastic deformation bounding
3~+2g E p~
(~
5.1. Experimental techniques On the 7075 alloy, RS measurements were performed with X-ray diffraction using a 20 = f(sin2¢) law which allows easy determination of the multiaxial deformations and stresses with a 1.5 mm spot size, 2 ~b angles and 11 ¢ angles. The incremental hole drilling destructive method [11] was applied on cast A356 and AI12Si alloys because of grain size coarsening up to 300 /~m which is detrimental to X-ray measurements (X-ray diffraction patterns were not obtained). As reported in Ref. [11], hole drilling to D diameter (1-3 mm) allows RS determination over D/2 depth. Strain variations during layer removal are thus measured at the surface of the part with three strain gauges in a rosette. Finite element software allowed the calculation of the principal RSs from the strain measurements.
5.2. One-impact LSP
2 HEL
-2 HEL
5. RSs induced by LSP: experimental results
1. during the interaction : tensile stretching of the impacted area
P
2. After the switch-off of the laser pulse : reaction of the surrounding matter induces a compressive stress field
~q Fig. 4. (a) Plastic deformation induced by LSP as a function of peak pressure [7]. (b) Compressive residual stresses induced by LSP: general principles.
RS fields induced by single impacts were investigated first. In general terms, the amplitude of the surface stress increases with the magnitude of the pressure pulse, which is related to the incident power density ~b. When ~b exceeds a given value, RS increases with depth but decreases at the surface of the materials (mainly at the centre of the impacts) because of surface waves previously examined in Ref. [9]. For instance, for the A356-T6 alloy, surface stresses increase up to - 1 4 5 MPa for 1.3-1.5 GPa pressure levels (1.5-2 GW c m 2), but further power density enhancements (up to 3 GW cm -2) tend to reduce the stress level to - 1 0 0 MPa, whereas in-depth levels continue to increase (Fig. 5(a)). Consequently, it is expected that all materials have an optimal shock condition. Fig. 5(b) shows the in-depth RSs induced by optimized shock conditions in A356, AI12Si and 7075 alloys with one circular spot. A comparison between experiments and calculations is presented in Table 4 in terms of the optimum shock conditions and the surface-induced RS. As predicted by the model (see Table 3), the maximum surface stress field induced ar~sSurris dependent on the yield strength ay of the unshocked material. The experimental data agree closely with the analytical predictions, except for the 7075 alloy where the magnitudes of the surface stresses differ by about 30%. For both AI12Si and A356 cast alloys, the results are in agreement with many previous studies [7-9]: surface stresses reach a maximum level of - 0.6ay, while 7075 only exhibits a stress magnitude of - 0.35o-v. For 7075, it has been suggested that shock-induced plastic deformation may not be saturated by only one impact, even under severe shock conditions (4 GPa impact only achieves an RS magnitude of - 175 MPa).
P. Peyre, R. Fabbro / Materials Seience and Engineering A210 (1996) 102-113
107
Table 3 Analytical calculations of the mechanical effects induced by a fast laser shock impact on an elastic perfectly plastic material [7,8] Calculated value
Analytical formula
Plastic strain condition PH
HEL = (1 + + ) ( 6 y - - 0 " o ) =
Plastic deformation
cp = - 3k + 21l ~ -
Optimal pressure
P = 2 2.5HEL
Plastified depth Superficial residual stresses (circular impact)
Comments
1.75 (O'y-- O'o)
2HEL(P )
Starts at HEL, saturates at 2HEL and depends linearly on P
1
C~,Cp, r ( P - HEL'] L = Cel-- Cpl \ 2HEL /
For a pure uniaxial deformation increases with cr~ < 0
Drives to c p saturation
triangular pulse
a""':¢ = 6°--I/t~P '1~ +-' 1 v + aoJ7 [1 - 4 ~
The plastified depths, which are almost linearly dependent on the pulse duration, are better than 1 mm at the optimized shock conditions. Moreover, very small surface stress gradients are found after LSP. This latter point is beneficial because it is known to be the key to little or no cyclic stress relaxation.
5.3. LSP with repeated impacts In the future, application of LSP to extended areas will necessitate the repetition or at least overlapping of impacts in order to generate large homogeneous compressive zones. Therefore our investigations have centred on repeated impacts. On cast alloys, as predicted by the model described above, no surface stress increases can be achieved with two or three impacts at the same point, but in-depth RS and plastified depths are strongly enhanced. For 7075, a different behaviour is observed: cumulative impacts have a very beneficial effect on the stress levels in the superficial layers as shown in Figs. 6(a) and 6(b). For instance, a 4 G W c m - 2 treatment generates - 170 MPa at first impact, - 2 4 0 MPa with a second impact and - 340 MPa with a third (Fig. 6(a)). Moreover, X-ray diffraction peak broadening observed in 7075 ( + 7% to + 15%) reveals that this particular material, submitted to numerous impacts, seems to undergo local cyclic hardening leading to much higher RS levels (which depend mostly on o-v). Bauschingerlike effects have also been proposed as a mechanism
(1 + v)
Depends linearly on the pressure duration r Increases with eP Decreases with L Increases with a o < a Increases with the size of the impact
which would tend to reduce the plastic flow level during the first impact and allow much more deformation and RS during the following impacts.
6. Comparison of LSP and SP surface effects A356 and 7075 were selected to explore the influence of LSP and SP on the surface modifications and fatigue properties. The issues relevant to RS, hardening and surface roughening are reviewed and discussed. SP involves the impact of high-speed small beads with about 100%-200% covering rates. Contrary to LSP, SP mechanical effects generally result from the influence of surface hammering by b e a d - m e t a l contact, which creates tangential stretching of the surface layers and Hertz-type uniaxial loading with a maximum shear stress in the subsurface. Thus shot-peened surfaces are submitted to more multiaxial, intense loadings than laser- shocked materials. A quantitative comparison of the loading conditions is presented in Table 5. Perhaps the most distinctive change in the impact conditions involves the peak pressure durations which are 10-20 times longer in the case of SP. As shown by recent investigations on steels [7,8], shot-peened specimens generally display much higher RS levels than laser- shocked materials, mainly in the first 100-200/~m of depth. Our experimental results on A356 cast alloy [12] show that surface stresses of - 210 MPa are achievable with SP against only - 150 MPa
108
P. Peyre, R. Fabbro /Materials Science and Engineering A210 (1996) 102-113
with LSP. A t first glance, these results c o u l d indicate t h a t the plastic d e f o r m a t i o n i n d u c e d will be lower with L S P t h a n with SP, a l t h o u g h c a l c u l a t i o n s give o p p o s i t e q u a n titative d e s c r i p t i o n s (calculated values o f ε p are n e a r 0 . 2 % - 0 . 3 % for SP a n d 0 . 5 % - 1 % for LSP). H o w ever, as s h o w n in Fig. 7(a), in 7075, L S P c a n d e v e l o p an a l m o s t equal R S level in the first 1 0 0 / ~ m by m u l t i p l e impacts. M o r e o v e r , in all the m a t e r i a l s investigated so
0
I
-20 -4 0
] I I '
-60
I
[] JL
I
1,5 GW/cm2 -- 1,3 GPa 0,9 GW/cm2 = 1 GPa 3 GW/cm2 = 1,9 GPa I
I I i I •
-50
t
-
-12
Calculation Experiment
AI12Si-T6
7075-T7351
Poptimum
O'surface Poptimum °'surface Poptimum
°'surface
(GPa)
(MPa) (GPa)
(MPa) (GPa)
(MPa)
1.2 1.3-1.4
- 150 1.3 - 145 1.6-1.8
- 120 2 - 125 2-2.2
-280 - 160
S u r f a c e r e s i d u a l stresses (MPa) 0 I F I I I
-
-,o -80
A356-T6
far, the stress field has been shown to extend two to five times d e e p e r t h a n is typical for SP ( 0 . 2 - 0 . 3 mm).
stresses ( M P a )
residual
Table 4 Optimum laser shock conditions on aluminium alloys in a water confinment mode. Comparison of experimental and calculated values.
o [] •
[] []
-100
%D
-150
I
1 impact 2 impacts 3 impacts
[]
[]
D []
-200
D []
,
-16 0
0.2
.250
.
0.4
0.6
depth
(mm)
0.8
1
-300 -350
• I
-400 0
1
7075-T7351
I
I
[
2
3
4
power
i
i
5 6 (GW/cm2)
density
-
I
7
8
Residual stresses (MPa) 0
I
i
t
-40 -
I
•
R e s i d u a l stresses ( M P a )
-
E'o'5-T'351i 50 0,
-80
-120 -160
t-
I
/
~
.200 8 0
m
I
-
I
~ 2 , 5
]
&
7075-4,2
GW/cm2
-100 -
~
-15G
GW/cm2
A356- 1,5 GW/cm2
I
I
I
I
0.2
0.4
0.6
0.8
depth
I
, , , , , , , , , I ' ' ' ' ' ' ' ' ' i ' ' ' ' ' ' ' ' '
-20G 1
-250
1 impact 3 impacts
-
1
(mm) -300
Fig. 5. Optimization of the residual stress fields induced by LSP in aluminium alloys in the water confinement mode. Experimental measurements (incremental hole drilling method). (a) A356-T6 at various shock conditions. (b) Maximum surface stress fields induced With single impacts.
'
0
~
J
,
J
,
,
,
I
r
:
J
,
,
,
,
,
I
0.5
I
1 depth
I
I
s
,
t
I
III
.5
(mm)
Fig. 6. Residual stresses induced by repeated impacts on 7075-T7351 alloy: (a) surface effects; (b) in-depth effects.
109
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102 113
Table 5 Comparative loading conditions induced by laser shock processing and shotpeening with industrial configurations (LSP in a water confinment mode) Peak Diameter Pressure Mechanical Strain pressure of impacts duration impulse rate (GPa) (ram) (as) (GPa/is) (s 1) LSP 0 6 Shot peening 3 10
1 15 0.2-1
0.05 0.5 1
0 0.3 1-10
106 10 4
The second comparative factor investigated was the surface hardening. Fig. 7(b) reveals that SP induces double the surface hardness increase ( + 20%) generated by LSP ( + 10%), even when LSP is performed under severe shock conditions (with a glass confinement mode for instance). This may be attributed to differences in the pressure duration, which result in higher dislocation generation and motion, and to the number of slip planes activated by multiaxiai surface loading such as SP. In the same way, the X-ray diffraction peak broadening induced by LSP remains inferior to SP, even for multiple laser impacts, suggesting that SP is capable of achieving greater lattice distortion and work hardening in the alloys. However, as some fundamental questions concerning these mechanisms still remain unanswered, transmission electron microscopy (TEM) may be helpful in future investigations. Surface geometry modifications were also investigated. They are observed as a general metal depression for LSP and an important roughening for SP. Table 6 shows the comparative roughening effects induced in A356 and 7075 alloys. In both cases, while the lasershocked surface remains partially unchanged, SP generates a detrimental roughened surface with large increases in the mean and peak roughnesses R~ and Rt. By comparison, laser-peened surfaces generally exhibit a 5--15 /~m homogeneous depression with little roughness modification. As shown above, the advantages of LSP include the greater affected depths and the preservation of the surface state, whereas SP is found to generate higher RS and hardness enhancements. This further demonstrates the differences in the two processes in terms of the mechanical and geometrical changes generated on the metallic surfaces. Regarding fatigue limit modifications, the interesting work of Wang et al. [13] on shot-peened 7075 samples showed that the important contributors reviewed above can be quantitatively separated as follows: + 50% and + 20% enhancements of the fatigue limit aD due to RS and hardening beneficial effects respectively and - 3 5 % decrease in 0"D due to roughening effects.
7. Application
to fatigue
behaviour
The cyclic behaviour of mechanical parts has previously been shown [13] to depend strongly on the metallurgical, mechanical and geometrical surface states of the materials involved. Since many materials usually display pronounced improvements in fatigue limits with SP or cold rolling, LSP was envisaged as a substitute to conventional treatments for materials displaying a reduced fatigue limit increase. As reported in Section 6, the beneficial effects of LSP may originate from the large affected depth and surface state quality, which are (HV-25g)
Hardness 190
....
....
180 -:~',
I ....
I ....
I'
'--7
' ' I'''~l
[ _~._~ / 7075
Shot-Peening
~ 27A
17C
-~
1613-
LSP
- ~
150.4GW/cm2
~
~
-
d
(glass)
140-~t-Peening
F15-20
!
A
1 3(~ ~ ~ 1 . ~ "
l-A-35 ~
12d
.~
1 10 -
LSP ~ L l 4 ~ ~ l [ ~ - ~ - ~ GW/cm2 (glass) ~ " ~ 100 ,, L ,~,F,,~ N,,,~I, , b ,, I, ~ 0
100
Residual 50
0
,
stresses ,
...........
-35
, 0
200
,
!
,
300
400
500
depth
(~tm)
700
(MPa) i
,
!
,
~ ,
!
. . . . . .
i
~ I, 0.2
I
600
.
,
,
, 0.4 depth
I ~ ~ 0.6
.
.
.
.
.
.
0.8
(mm)
Fig. 7. Mechanical effectsinduced by LSP and SP: (a) comparison of the residual stress fields induced in 7075 (LSP: 3 impacts, 4 GW c m 21; (b) Vickers hardness measurements with a 25 gf load (A356T6, 7075-T7351).
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
110
Table 6 Comparative roughening effects of laser shock processing and shot peening on A356 and 7075 alloys Material and Processing
R a (pm)
R t (#m)
A356 as milled A356 + SP F 3 8 - 5 0 N (0.3 m m beads) A356 + LSP 2 G W cm 2 (2 impacts) 7075 as milled 7075 + SP F20-23A (125%, 0.6 m m beads) 7075 + LSP 4 G W cm - 2 (3 impacts)
0.7 5.8 1.1 0.6 5.7 1.3
6.2 33 7.5 5.2 42 11
expected to influence favourably the initiation and cracking stages. High cycle bending tests were performed on the three alloys in order to demonstrate the efficiency of LSP and to distinguish between the fatigue behaviour of unshocked, shot-peened and laser-processed aluminium alloys. 7.1. L S P of fatigue samples
Notched rather than plane samples were selected in order to reduce the number of impacts per sample and to localize crack initiation at the notch root with stress concentration factors (Kt = 1.6-1.7). With such conditions, fatigue cracks are initiated in the laser-shock-processed zones thus restricting the failure cycle dispersion. The specimen geometries are presented in Fig. 8. The LSP conditions were close to those previously optimized through RS measurements (Section 5). Each notch was covered with a sacrificial aluminium scotch r
foil and immersed in 5 mm water confinement medium. A computer-controlled X - Y table was used to move the water tank containing the specimen in the required direction during processing. Different aspects of the treatment are discussed. For instance, on 7075, since it has been demonstrated that repeated impacts may have a beneficial effect on the surface stresses (see above), particular attention was focused on the overlapping rate (up to 67%). As a consequence, 7075 notches were submitted to three local deformations against only two on cast alloys (Table 7). For A356, square- and ellipse-shaped impacts were investigated so that the role of the impact shape in achieving fatigue life enhancement could be studied at constant incident power density. 7.2. Fatigue tests
High cycle (up to 107) fatigue experiments were performed using a three-point bending test in a 100 kN servo-hydraulic material testing system machine (stress ratio R = 0.1; test frequency, 40-50 Hz) in an ambient air atmosphere. Despite the small number of fatigue data points (about 10-14 points for each configuration with four levels of the maximum stress magnitude), statistical regressions were performed to yield the regression representations of W6hler curves a = f(N) and to determine the fatigue limits aD of the unshocked, laser-processed and shot-peened specimens. Regression laws were calculated with Esope software derived from the Bastenaire-type formula U=
A exp[ - (a - CrD/B) c]
(3)
o - - o-D
"t"-I
I I
I
I
d
i"
H
"1 ~ e
I i ~ ~'lll L
I
I
7.3. Experimental results on cast alloys
i~i~iii!i AS12UNG : LSP with 50 % overlapping
of 3 squared shape impacts
A356
where A, B and C are parameters adjusted with the software, aD is the fatigue limit at 107 cycles and N is the number of cycles to failure for a maximum applied stress o-.
AS12UNG
7075
L (mm)
86
86
90
H (mm)
24
24
22
, h (mm)
21
22
19,5
e (mm)
12
12
11
r (mm)
6
6
5
Fig. 8. Notched fatigue specimens.
Fig. 9 shows the fatigue data and their regression representations for as-machined and laser-shock- processed A356 and All2Si specimens. The fatigue data were plotted using maximum stress vs. fatigue life. As presented in Fig. 9, W6hler curves show much better performance for laser-treated samples than for as-machined specimens. For a fatigue life equal to 10 million cycles (at which the fatigue limit determination was made), the improvements in o'D are approximately + 36% for A356-T6 (110-150 MPa) and +22% for AII2Si-T6 (103-126 MPa). A large dispersion is found in the number of cycles to failure N for a given applied stress a. This can be attributed to casting defects due to dilation gradients between the Si primary phase and aluminium matrix
111
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113 Table 7 Summary of experimental LSP conditions on fatigue samples in a water confinement mode Material
0 (GW cm
,~356
2.3
~Xll2Si 7075
2.5 3.8
Impacts
Overlapping rate (%)
5 squares (6 mm) or 3 ellipses 3 squares (10 mm) 4 circles (11 mm)
50
2)
during the quenching stage. Indeed, it is clear from the examination of the fracture surfaces that fatigue cracks mostly initiate on porosities or surface Si inclusions. Therefore, for the particular case of cast aluminium alloys, LSP can be considered as a means to reduce the notch sensitivity of surface defects. Similar, earlier investigations of Forget [9] on Ni base superalloys also demonstrated that a laser-induced RS field could prevent fatigue cracks from initiating on surface inclusions as long as the local applied stresses remained inferior to the macroscopic plastic flow limit. The effect of LSP on the dispersion of the results was investigated statistically, but no real dependence was found since fatigue cracks initiate over a very large range of cycles. However, increasing the fatigue limits using LSP is very efficient for cast aluminium alloys. This is somewhat surprising since these materials are known to be very sensitive to work hardening effects that are not generated by LSP. Concerning the impact shape effects on A356, no real influence was found, thus indicating that the greater fatigue lives are attributable to the peak power density and pressure achieved at the workpiece surface. We have demonstrated that LSP can provide large increases in the fatigue limits of both A356 and All2Si cast aluminium alloys. These improvements are ascribed to a combination of compressive RSs and surface integrity effects. 7.4. L S P and S P effects on 7075
7075 notched specimens were laser treated or shot peened. As shown in Fig. 10(a), fatigue limit increases of about + 22'/o for laser-impacted specimens and only + 10% for shot-peened specimens are achieved. The surface roughness is considered to be the main detrimental contributor to the fatigue behaviour of shotpeened samples, compared with LSP which maintains a safe surface geometry and an almost equal RS level. Nevertheless, for shot-peened specimens, core initiations are detected at a stress amplitude of 220 MPa and at depths between 0.35 and 0.4 mm from SEM observations (below the shot-peened layer). Using a local fatigue strength concept taking into account the RS field [14], we confirm that the local fatigue strength is lower
Number of cyclic deformations
50 67
than the applied stress (calculated with a Creager approach) only beyond a depth of 0.35 mm. For higher applied stresses (greater than 220 MPa), initiation always occurs at the surface. According to these considerations, it can be concluded that with a deeper SP treatment or a different notch geometry (with a higher K,), LSP may display higher fatigue improvements. 7.5. Detection o f crack initiation on 7075 using an a.c. potential drop method
An a.c. potential drop method was developed to detect crack initiation and to investigate at what stage fatigue life increase occurred. Experimentally, an electric crack detector (ANS) connected to both sides of the notch was used. Current pulses (2 A, 0.02 s) were delivered to the samples during fatigue tests at a frequency of 30 Hz, and crack initiation (limit of detection, 50-100 /zm) was considered as the number of cycles corresponding to a change in potential down the notch root. Fig. 10(b) shows that, for a stress of 260 MPa, fatigue life improvements induced by LSP can be separated into a sevenfold increase in the crack initiation stage and only a threefold increase in the propagation stage. In contrast, SP provides a homogeneous two to threefold increase in both initiation and propagation durations. In addition, the ratios between initiation and propagation are changed from 85%-15% for shotpeened samples to 90°/,7 10% for laser-processed samples. This different behaviour can be ascribed to surface embrittlement and surface roughening due to SP, which will tend to reduce the beneficial effects of compressive RSs, whereas the electric detection results are indicative of delayed initiation stages in the case of laser-processed materials.
8. Combination of SP and L S P
LSP is generally envisaged for high-cost, fully localized mechanical parts, whereas SP can be used on larger parts with a smaller cost. In this context, preliminary studies were performed to investigate the combination of the two treatments. Fig. 11 shows that laser
112
P. Peyre, R. Fabbro / Materials Science and Engineering A210 (1996) 102-113
shocking a pre-shot-peened zone combines and enhances the advantages of the two treatments with both in-depth and superficial RS increases. Therefore SP beneficial effects are preserved (hardening) or increased (RS field) by LSP treatment. Further fatigue studies should provide interesting information on the mechanical improvements generated by S P + L S P treatment which seems to be a very promising process.
m a x (MPa) 320 ....................................................................................... ~.............................
........................... i............................i............................ i ..........~---..-D .i............................i............................ i............................ ~ 280 ".. . 3i8 G W / c m 2 i i
260..............
oi
2 o o ............................~ n t r ; ~ d ~
-.:::: .~91
;ii;~
1 8 0 ...................................................... t~ ...................... -2-O-~ ..................... G
max
160 10000
(MPa)
105
. . . . . . . . . . . . .
160
........O N - • I ' I - I - - - - ~ -
140
...........• . . . ~ . . . . . .
120
.....u.n: ho ck e d - - - ~ ~ . - l ~ : ~ l 0 M P a :
[]
[ 107
10~
N
2 2 0 ................................................................................................................ ~._.. I A356"T6 I 2 0 0 ............13 ............ !.....................................i....................................i ::LSP 2,3 GW/cm2 ': 180
[ 11~
D-----D
..........
.l~(~-----=----;50--~p a
• ...................L ~ . . . . . ! )
N cycles
..~.............
Laser-peened
-
-
3105 -
100 .................................... !.......... • .................... --~-~---P--- ................. : 2 80 I 10 5
10 6
10 7
10 8
• []
Cracking + failure Initiation
2105 -
N 1105
Shot-peened
-Untreated 0 --
Fig. 10. (a) O ' m a x - N c u r v e s for unshocked, shot-peened and lasershocked 7075 alloy. (b) Crack detection tests on 7075 alloy with a.c. potential drop method. Comparison of initiation and cracking stages at O'max = 260 MPa.
G max (MPa)
[ 140
" ~ N
0 ....................,
100
:
~
LSp- 2,5 GW/~cm2
10 5
i
". . . . . . . . . . . i....... [] ........ O - i ........................... i
~
I
2
6
A
M
p
~ - - ~ - - - - - ~ I - I ) 3 - - - - ~ i - P
60 10000
It
:
2
::
2
I 10 6
10 7
a
a
10 8
N Fig. 9. High cycle fatigue tests on A356-T6 and ASI2UNG-T6 cast aluminium alloys, amax-N curves for unshocked and laser- shockprocessed specimens.
9. C o n c l u s i o n s
LSP is a very attractive, promising, new surface treatment for aluminium alloys yielding large fatigue improvements in A356, All2Si and 7075 alloys. In our study, mechanical surface modifications were investigated in terms of the RS fields induced, surface hardening and surface roughness modifications. In all three alloys investigated, the RS fields extend to a greater depth than is typical o f other techniques, with affected depths better than 1 mm. Hardening is limited to + 10% o f the initial value, which is half the increase achievable by conventional SP treatment. The main difference between the two treatments has been
P. Peyre, R. Fabbro /Materials" Science and Engineering A210 (1996) 102 113
113
Appendix A. Nomenclature
::::::: :::::::i:i:i..... ...............................iiiiii!
i
II.
1
.........
i .400
, II I
0
0.2
~_I_
shot-peen,ng
li
a - - +LS-I,50Pa li , , +LS-2,1GPa li ,
0.4 0.6 depth (ram)
'
,
0.8
=i
1
Fig, 1I. Residual stresses induced by a combination of shot peening and laser shock treatment on 7075 alloy.
demonstrated to originate from the surface roughening: LSP generates a general depression of the material with no roughness changes; SP strongly affects the surface and creates potential initiation sites. Therefore much better fatigue performance is observed with laser-shocked 7075 than with conventional shotpeened specimens. This is attributed, through fatigue crack detection, to larger increases in the crack initiation stage for LSP than for SP specimens. It has also been shown that a combination of LSP and SP treatments is capable of achieving an optimum surface layer. Whether a high level of fatigue behaviour improvement of LSP + SP materials is possible has not yetbeen demonstrated, but this combination is expected to provide better fatigue resistance than any single process alone. Despite the interesting potential of LSP, no real industrial applications could be found, mainly because of the low repetition rates of the laser sources presently available. However, the newly developed excimer gas laser [15] can deliver output characteristics similar to those required in the industry (with a repetition rate of better than 10 Hz). In the near future, with the use of such laser sources which will enable the problems inherent in the process to be overcome, LSP may replace conventional methods for many applications such as fatigue or stress corrosion behaviour.
a L P r0 ep 2, /~ v a~,rr av a0 r
square-shaped impact edge (mm) plastically affected depth (mm) shock pressure circle-shaped impact radius (mm) plastic deformation rate induced by LSP Lam6 elastic constants (GPa) Poisson's ratio (0.3) surface residual stress (MPa) static yield strength (MPa) initial residual stress (for unshocked material) (MPa) pressure pulse duration (ns)
References [1] R.M. White, J. Appl. Phys., 34 (1963) 2123. [2] N.C. Anderholm, Bull. Am. Phys. Soc., 13 (1968) 388. [3] A.H. Clauer, B.P. Fairand and J. Holbrook, in L. Murr (ed.), Shock Waves and High Strain Phenomena in Metals-~ Concepts and Applications, Plenum, New York, 1981, pp. 38, 675. [4] R. Fabbro, J. Fournier, P. Ballard, D. Devaux and J. Virmont, J. Appl. Phys., 68 (1990) 775. [5] J. Fournier, Thesis, Ecole Polytechnique, France, 1989. [6] D. Devaux, R. Fabbro, L. Tollier and E. Bartnicki, J. Appl. Phys., 74 (1993) 2268. [7] P. Ballard, Thesis, Ecole Polytechnique, France, 1991. [8] C. Dubouchet, Thesis., University of Orsay, France, i993. [9] P. Forget, Thesis, Ecole des Mines de Paris, France, 1993. [10] P. Peyre, Thesis, University of Technology of Compi6gne, France, 1993. [11] J. Lu, A. Niku Lari and J.F. Flavenol, J. Mech. Work. Tech., 11 (1985) 167. [12] P. Peyre, P. Merrien, H.P, Lieurade and R. Fabbro, Materiaux et Techniques, 6 7 (1993) 7. [13] R. Wang, X. Li and H. Wu, Proceedings of the 3rd International ConJerence on Shot-Peening (ICSP3), Elsevier, GarmischPartenkirchen, 1987, p. 563. [14] R. Lin and T. Ericsson, in Residual Stresses, DGM-Hauk-Hougardy-Macherauch, Francfort, February 1993, p. 505. [15] B. Godard, M. Stehle, P. Murer, J. Bonnet and D. Pigache, Proceedings of ConJerence on Lasers and Electro- Optics, Baltimore, USA, June, 1993.