Rare Metal Materials and Engineering
Volume 42, Suppl. 2, November 2013
Cite this article as: Rare Metal Materials and Engineering, 2013, 42(S2): 217-221
Surface Defects Produced by Laser Shock Peening with
Aluminium Foil Ablative Layer
Cao Ziwen, Gong Shuili, Zou Shikun, Che Zhigang
National Key Laboratory of Science and Technology on Power Beam Processes, Beijing Aeronautical Manufacturing Technology Research
Institute, Beijing 100024, China
Abstract: The ablative layer acts as a sacrificial layer to prevent a slight burning of the surface in laser shock peening. Aluminium
foil was applied to be ablative layer in almost all of industrial applications, because of its excellent toughness and easy
maneuverability. In this experiment, Nd: glass laser system with pulse duration of 30 ns and spot size of 4 mm×4 mm was applied
for peening TC17 titanium alloy covered by aluminium foil. Surface defects on peened areas were studied by surface morphology
detecting, EDX chemical composition analysis and SEM observation. Some micro impression defects were found on peened areas,
and carbon element was detected in and around micro marking defects. Results show that aluminium foil thickness is decreased as
increasing impact times, thus a tiny broken surface of aluminium foil is found just at overlapped area by four square spots, and no
heat affected layer is observed on substrate surface. In case of relatively large area of aluminium foil damage or laser pulse directly
irradiated on base material surface, the thin recast layer with thickness of 1~3 μm is generated, and a number of micro-cracks are
found in recast layer and some pores locate at interface between melting layer and original material.
Key words: laser shock peening; surface defect; aluminium foil ablative layer
For the laser shock peening (LSP) configuration, material
surfaces are usually covered with ablative layers. This ablative
layer can prevent from a slight burning of plasma thermal effects, in order to provide a pure mechanical effect on peened
material[1]. In past several decades, the materials of ablative
layer conformed with development of LSP technique and upgrade of laser system. As energy-transition medium, ablative
layer should endure intense laser irradiation and high-speed
plasma explosion, and keep integrated during LSP process.
Black paint, adhesive paper and aluminium foil are used to be
ablative layer for different surface conditions[2-4]. Black paint
is usually used to almost all surface conditions, but paint does
not have sufficient tensile strength to keep from local shearing
when the shock locally compresses the surface. It also fractures and debonds when the shock ends and the surface rebonds. Adhesive paper is recommended to be apply to single
impact situation. So far, aluminium foils with adhesive backing have proved to be a better ablation material, and it shows
excellent toughness and easy maneuverability. Thus, it is used
as ablative layer to meet overlapping LSP requirement[5]. Because of high power density (>109 GW/cm2) of laser pulse
during LSP, almost all laser energy is absorbed to convert to
internal energy and mechanical energy of plasma[6]. Generally,
the applicable thickness of aluminium foil is approximately
70~100 μm[7,8]. Air bubble located at interface between aluminium foil and substrate material is not allowed when aluminium foil is deposited onto the surface.
However, some surface defects has been found on laser
peened surfaces by aluminium foil as ablative layer, such as
micro impression, tiny area covered by melted aluminium foil
and laser direct irradiated surface. These surface defects could
eliminate the benefit imparted by LSP and were studied in this
paper.
1
LSP Experiment
LSP experiment was carried out with a Q-switch Nd: glass
pulse laser system with pulse duration of 30 ns and frequency
of 0.1 Hz. The flow chart of LSP with aluminium foil ablative
layer is illustrated in Fig.1. The base material used in this experiment was TC17 titanium alloy. First step was cleaning the
material surface intended for peening with absorbent cotton
dipped in acetone solution. After acetone cleaning, material
surface was cleaned again by use of soakage waterless ethanol
absorbent cotton, and the cleaned area should be more 10 mm
above LSP zone borderline. The aluminium foil was pasted
onto material surface carefully by manual labour, and this step
Received date: April 25, 2013
Foundation item: NSFC (50824001)
Corresponding author: Cao Ziwen, Master, Engineer, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, P. R. China, Tel:
0086-10-85701493, E-mail: [email protected]
Cao Ziwen et al. / Rare Metal Materials and Engineering, 2013, 42(S2): 217-221
was most important preparation before LSP treatment. Aluminum foil pasting region should be 5 mm more than laser
shock area, and the pasted aluminum foil should be flat, with
no air bubble, no wrinkle or no scratch. Flowing de-ionized
water with approximately 1 mm thickness was implemented
over aluminium foil for restraining plasma overexpansion. The
laser beam output from laser system was a conventional round
laser beam with diameter of 20 mm, this laser beam travelled
through a beam-shaping element and a focusing lens, then it
irradiated onto a target surface with a square laser spot of 4
mm×4 mm. Laser pulse energy set in this experiment was 50 J.
Two adjacent laser spots overlapped with overlapping ratio of
8%. After LSP, the aluminium foil was uncovered and material
surface was cleaned using acetone solution.
2
a
b
20 μm
1 mm
c
d
10 μm
e
10 μm
10 μm
Fig.2 Aluminium foil surface and its thickness after LSP
Aluminium Foil State after Laser Irradiation
After high intensive laser pulses directed onto aluminium
foil through water confining layer, it is shown in Fig.2a obviously burned areas on aluminium foil irradiated by four overlapped square spots. Overlapped regions are easily distinguished from burned area, and width of this overlapped regions are greater than laser spot overlapped width. Fig.2b
shows the aluminium foil surface morphology after LSP
treatment. The employed aluminium foil surface is melted by
laser and plasma rapidly, and this melted liquid can not timely
vaporize off, because of the high plasma pressure. So this
melted liquid is boiling in superheat state. When laser pulse
switches off, the boiling liquid mixed with mass air bubbles and
superheated droplet, generates phase explosion. Combined influences of phase explosion and plasma expansion cause cavitations and splash substances on aluminum foil surface.
According to previous study by Ren Naifei[9], aluminium
foil instantaneously evaporates and ionizes to produce plasma,
and the thickness of evaporated layer is near about 10 μm.
Aluminium foil thickness decreases with increasing impact
times. Fig.2c, 2d, 2e show cross-section thickness of aluminium foil corresponding regions labeled in Fig.2a. The original
thickness of foil is near about 120 μm, and this value decreases to 80, 55 and 25 μm via one impact (Fig.2c), two impacts (Fig.2d) and four impacts (Fig.2e) respectively.
3
Micro Impression Defects on Laser Shocked
Surfaces
The micro impression defects were found by naked-eye observation on surfaces finished by LSP after aluminium foil
ablative layer had been uncovered. Typical micro impression
defects were detected by use of white light interference
method, and the results are shown in Fig.3. A number of tiny
pits and notches appear on laser shocked surface. The mean
size of pit defects and the width of notch defects is order of
100 μm, and the length of notches is from several micrometers
to order of milimeter. The depth of these defects is 1~2 μm,
and it is obviously found that the material surface around defects is elevated by defect formation. Thus, these micro impressions are likely to deteriorate finished surface roughness
of LSP, and fatigue cracks tend to initiate at defect area because of stress concentration effect[10]. This type of defect is
hard to be found on LSP treated surface with adhesive paper
or black paint protective layer. According to surface observation of used aluminium foil, the similar defect patterns also are
Notch defect
Pit defect
1.79
Height/μm
1.00
0.21
–0.57
–1.36
–2.15
0
162
324
486
647
Surface/μm
Fig.1 Flow chart of LSP with Al foil ablative layer
Fig.3 Pits and notches defects on peened surface
809
Cao Ziwen et al. / Rare Metal Materials and Engineering, 2013, 42(S2): 217-221
found at the corresponding positions on aluminium foil. A part
of defects on foil may be generated by tear force while foil is
uncovered from tape rolling. Other defects on foil may be
produced by process of depositing protective layer to material
surface. These foil defects have higher hardness than original
foil, and are pressed into material surface under high-speed
shockwave loading, and then micro impressions are marked
onto material surface. To prevent impression defects from laser peening, a hybrid ablation layer that comprises a separate
under layer is applied to a material to be peened. The underlayer is in contact with the surface to be peened and is applied
in a manner so that it does not have bubbles and voids.
4
Ablative Defects with Slight Broken Aluminium
Foil
Although aluminium foil acts sacrificial layer to protect substrate material surface, burned black points still were found on
peened surfaces because of local aluminium foil damage, especially at overlapped regions. Reasons attributed to break aluminium foils may be instable laser intensity, excessive impacts
or abnormal situation of aluminium foil. A broken foil appears
at the region where four square spots overlap, as shown in
Fig.4a, and two scratches are found on the aluminium foil surface near broken area as well. Combination of four impacts and
scratch defects on foil seems to cause foil damage. Fig.4b, 4c
illustrate SEM images of burned black point on base material
surface. The burned black point is uneven and different sizes of
melted bumps appear. EDX analysis results (Table.1) show the
main chemical composition of burned area at six different locations which is near about 1 μm size. The main composition of
melted bright bumps (spectrum 1, 2) is aluminium element.
During process of LSP with overlapped square-spot, the preceding laser pulses destroyed integrity of aluminium foil, and
the subsequent laser pulses made broken aluminium fragment
melt onto base material surface. The dark regions (spectrum 3)
are confirmed to distribute carbon element which originated
from glue on back surface of aluminium foil. The relative even
a
region (spectrum 4) seem to be base material surface, and its
chemical composites are close to original material before LSP
except for a little carbon element. According to the microstructures of burned black point from cross-section of the sample, as
shown in Fig.4d, the original TC17 microstructures extend to
the surface of specimen, and it proves that burning layer resulted from laser irradiation or plasma ejection is very thin. It is
likely that the substrate material of TC17 titanium alloy is accidentally protected by the melting aluminium layer. The residual
stresses were measured by X-ray diffraction method at burned
black point (approximately diameter 0.5 mm) with X-ray diameter of 1 mm. the residual stress of burned black point is 118
MPa (compression). This value is much lower than normal
treated surface with 500 MPa (compression).
5
Direct Ablation without Aluminium Protection
If aluminium foil has a large-area damage or laser pulse
shock onto base material surface which no aluminium foil
protects, the base material surface will be ablated by high intense laser pulse. Fig.5 shows the ablative area induced by one
laser pulse with 50 J from laser spot center to outside of laser
spot without aluminium foil protection. Secondary electron
image and backscatter diffraction image are acquired from
same field. The surface roughness gets worse after burning.
The rugged topography on the ablative surface seems to be
produced by phase explosion and plasma instantaneous expansion. It is clearly found that numerous micro-cracks
emerge on the ablative surface, as shown in backscatter diffraction image of Fig.6. Nearby laser spot center, micro-cracks
are relatively rare and non-directional, but micro-cracks become densely and directional from laser spot center to border
of ablative area. These micro-cracks could be caused by thermal stresses which induced by super rapid cooling. In addi-
b
Scratch defect
Ablative
region
100 μm
c
10 μm
d
10 μm
Spectrum
Mass Fraction/%
Ti
Al
C
1
12
81
-
O
7
2
15
43
19
22
3
17
4
79
-
4
78
9
4
Fig.5 SEM image of Broken aluminium foil and ablative surface
Fig.4 Broken aluminium foil and ablative surface
and EDX results of ablative area
Cao Ziwen et al. / Rare Metal Materials and Engineering, 2013, 42(S2): 217-221
100 μm
Fig.6 Topography and surface micro-cracks after direct laser irradiation
tional, density of micro-cracks increased with multiple impacts at general ablative surface.
On intense laser pulse ablates material surface, a mass of
free electrons in material surface absorb laser energy, and free
electrons are heated and transferred heat to ambient crystal
lattices to produce recast layer. Fig.7 shows the cross-section
images of the recast layers after direct laser irradiation. Although ablative surfaces are uneven, the interfaces of recast
layers and substrate materials are straight, and the interfaces
are easily recognized by different microstructures. The recast
layers are very thin and its thicknesses are 1~4 μm. Thus, in
some cases, the recast surfaces can be polished carefully to get
finished surfaces. Because of super rapid heating and cooling
conditions, the transition layer of between recast layer and
substrate material is hard to generate. Most of micro-cracks
only generated in recast layers and stopped to propagate at interface of recast layers and substrate materials. With increasing impact times, micro-cracks in the recast layer become
wide, as shown in Fig.7c. A very interesting and meaningful
phenomenon is detected, that these micro-cracks which generated by multiple impacts continue to propagate along the interface. For this phenomenon, the direct ablative micro-cracks
could propagate into substrate materials to lead to fatigue
fracture under fatigue loading, and maybe the recast layer will
spall from base materials as micro-crack propagating. Additionally, some pores are found in ablative layer, and it is not
beneficial to fatigue life of component.
b
a
c
1 μm
Fig.7 Ablative layers and micro-defects after direct laser irradiation
6
Conclusions
As excellent ablative layer, aluminium foil protects substrate materials from heat effect and endures multiple impacts of LSP without ablative layer damage. The pit defects
and notch defects can be caused by unacceptable quality of
aluminium foil during shock wave loading. The aluminium
foil breaks at excessive repeated impacts, and the slight
broken aluminium foil can also block partial laser to irradiate substrate materials. In case of large-area damage of
aluminium foil or direct laser irradiation without aluminium foil, the material surfaces are ablated totally to form
recast layers (thickness of 1~4 μm), and numerous micro-cracks emerge on the ablative surfaces. Fortunately,
these micro-cracks can’t propagate into original material
regions, multiple impacts drive these micro-cracks to
propagate along the interfaces of ablative layers and original materials.
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Cao Ziwen et al. / Rare Metal Materials and Engineering, 2013, 42(S2): 217-221
铝箔防护条件下激光冲击强化产生的表面缺陷研究
曹子文，巩水利，邹世坤，车志刚
(北京航空制造工程研究所 高能束流加工技术国防科技重点实验室，北京 100024)
摘
要：利用脉宽 30 ns 的钕玻璃激光器强化铝箔烧蚀层覆盖的 TC17 钛合金，激光光斑尺寸为 4 mm×4 mm。对强化表面的缺陷进行
表面形貌分析、能量色散 X 射线成份分析（EDX）和扫描电镜分析。在强化表面发现了许多微小的压痕缺陷，在微小压痕内及其附近
探测到了碳元素分布。随着激光冲击次数增加，铝箔烧蚀层的厚度逐渐降低，因此在 4 个方形光斑中心搭接区的铝箔发生微小的破损，
但未发现基体材料表面发生烧蚀情况。当铝箔表面较大面积发生破损或者激光直接照射在基体材料表面，将产生厚度为 1~3 μm 的再铸
层，并且在再铸层中发现数量众多的微裂纹和气孔。
关键词：激光冲击强化; 表面缺陷; 铝箔吸收层
作者简介：曹子文，男，1980 年生，硕士，工程师，北京航空制造工程研究所，北京 100024，电话：010-85701493，E-mail: [email protected]