Proceedings of the 12th International Conference on
Aluminium Alloys, September 5-9, 2010, Yokohama, Japan
©2010
2010 The Japan Institute of Light Metals
pp. 1938-1943

1938

Effect of Ultrasound Treatment on Age-Hardening Response
in an Al-Mg-Si Alloy
Shoichi Hirosawa 1, Junji Kamibeppu1, Mohamad Iqbal1,
Hideki Hosoda2 and Tatsuo Sato3
1

Department of Mechanical Engineering and Materials Science, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN
2
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, JAPAN3
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, JAPANAn attempt to improve age-hardening response in an Al-Mg-Si alloy has been made in thetemperature range of room temperature (RT) to 443K by exploiting ultrasound which penetrates
silicone oil in a bath with a frequency of ~1MHz. The application of ultrasound treatment has littleeffect on natural-aging behavior of the alloy sheet, but increases the age-hardenability by ~5Hv at373K and by ~20Hv at 443K. Such an increase in mechanical strength is suitable for the fabrication ofautomobile body panels, and therefore the mechanism is expected to be clarified not only from anacademic point of view but also from an industrial point of view. In this work, two possible effects ofultrasound; i.e. vibrational force by a liquid medium of silicone oil and shock wave by the collapse ofcavitation bubbles, have been quantified to clarify the mechanism by which the concentration ofvacancies, diffusion of atoms and/or nucleation and growth of nano-scale precipitates (nanoclusters)
might be influenced.
Keywords: Al-Mg-Si alloy ,ultrasound, age-hardening, microstructure, nanocluster

1. Introduction
Heat treatable Al-Mg-Si alloys are used for automobile body panels because of their good formability,
corrosion resistance, surface quality and precipitation strengthening after paint-bake treatment at~443K. It is well known that the alloys containing excess of Si over Al-Mg2Si quasi-binarycomposition; e.g. AA6022 alloy, exhibit a pronounced age-hardenability due to the formation of theβ” phase and/or its precursors; e.g. nanoclusters called Cluster(1) and Cluster(2) [1]. However, moreincreased age-hardening response is anticipated to meet industrial demands for high strength andgood dent resistance comparable to those of the conventional steel-made body panels.
Ultrasound is a clean and safe energy, and therefore a wide range of applications have been developedin the modern industries. The authors reported for the first time that the application of ultrasoundduring heat treatments of Al-4%Cu and Al-4%Cu-0.5%Mg alloys complexly changes their agehardenability [2]. It can be hypothesized that ultrasound assists in atomic vibration in the alloys,
promotes diffusion of atoms and therefore results in the accelerated precipitation from super saturated
solid solutions. However, it is also feasible to consider that the more excited atomic vibration
inversely inhibits the formation of nanoclusters, leading to suppressed age-hardening response.
In this work, the effect of ultrasound on age-hardening response of AA6022 alloy has been
investigated to confirm which of improvement or degradation in age-hardenability is achieved by thenewly developed ultrasound treatment. The mechanism of such an effect of ultrasound treatment wasalso quantitatively discussed from the viewpoints of vibrational force by a liquid medium of silicone
oil and shock wave by the collapse of cavitation bubbles.
2. Experimental

1939

The chemical composition of the alloy utilized in this work is listed in Table 1. The alloy ingot washomogenized, hot- and cold-rolled through the standard processes to the final thickness of 1.0mm.
The sheet specimens were solution heat-treated at 823K for 60s, and then water quenched. Agingtreatment was carried out at RT, 373 and 443K in a silicone oil bath equipped with an ultrasoundtransducer (Fig.1). The specimen with dimensions of 15×15×1 mm3 was fixed within silicone oil incontact with a thermocouple to keep a constant temperature, and vibrated by ultrasound penetrated
from silicone oil. The frequency and amplitude of the vibration at the upper side of the specimen were
960kHz and 4.4nm at 443K, as measured by a laser Doppler vibration meter.
Hardness test was conducted by a Vickers hardness tester with 98N load and dwell time of 30s tocompare age-hardening response of the specimens with and without ultrasound treatment. Increment
in electrical resistivity during aging ∆ρ,∆ρ = 17.24 / σ × 100,(1)

was also compared through electrical conductivity σ (in IACS%) measured by an eddy current tester.

Table 1 Chemical composition of the alloy utilized in this work [in mass%].
Mg

SiFe

Mn

Ti

Cu

Al

0.53

1.05

0.15

0.07

0.02

0.01

Bal.

Fig.1 Schematic illustration and outlook of the ultrasound bath utilized in this work.

3. Results
3.1 Age-hardening response
Isothermal aging curves of hardness at RT, 373 and 443K are shown in Fig.2 for the specimens
with and without ultrasound treatment. The application of ultrasound treatment has little effect onnatural-aging (Fig.2(a)), but increases the age-hardenability at 373K by ~5Hv (Fig.2(b)). At 443K,
furthermore, age-hardening response is significantly improved by ~20Hv, suggesting that ultrasoundtreatment is useful for the fabrication of aluminum-made body panels with higher strength and better
dent resistance.
3.2 Electrical resistivity change

Vickers hardness,Hv

Vickers hardness, Hv

Vickers hardness,Hv

194080(a)706050With ultrasoundultrasonicWithout ultrasoundultrasonic

40300.01
90A.Q

(b)

800.1

1

Aging Time, t /ks

10100

706050With ultrasoundultrasonicWithout ultrasoundultrasonic

40300.01
140 A.Q
1200.1(c)

1Aging Time, t /ks

10100

100
80
60
40

With ultrasoundultrasonicWithout ultrasoundultrasonic

20
00.01
A.Q

0.1

1

Aging time, t /ks

10100

Fig.2 Age-hardening curves of the specimens with and without ultrasound treatment
at (a)RT, (b)373K and (c)443K.

Increment in electrical resistivity, ⊿ ρ/nΩm3

RT21373K

00.01
A.Q

-1-2-30.1

1With ultrasoundultrasonic(443K)
Withoutultrasoundultrasonic(443K)
Withultrasoundultrasonic(373K)
Without ultrasoundultrasonic(373K)
With ultrasoundultrasonic(RT)Without ultrasoundultrasonic(RT)

10100

443K

Aging time, t /ks

Fig.3 Increment in electrical resistivity of the specimens with and without
ultrasound treatment at RT, 373 and 443K.

1941

Fig.3 shows the increment in electrical resistivity ∆ρ of the specimens with and without
ultrasound treatment at RT, 373 and 443K. The specimen with ultrasound treatment exhibits sluggish
decrease in ∆ρ at 443K compared to the specimen without ultrasound treatment (see at aging times of>1.2ks). In general, it is believed that ∆ρ arises from the superimposition of the increases in both size
and number density of precipitates, and the decrease in residual solute concentrations in the matrix [3].
Therefore, the sluggish decrease in ∆ρ is likely to indicate that refined precipitates distribute more
densely in the specimen with ultrasound treatment. This interpretation well agrees with the fact that
greater increases in hardness are observed with ultrasound treatment in Fig.2(b),(c). Note thatmicrostructural observation by transmission electron microscopy (TEM) is quite difficult to capture
any differences in morphology and distribution of such refined precipitates.
4. Discussion
Based on results in Figs.2 and 3, it can be concluded that ultrasound treatment has positive effect on
age-hardening response of the Al-Mg-Si alloy at 373 and 443K. In this chapter, two possible effects
of ultrasound; i.e. vibrational force by a liquid medium of silicone oil and shock wave by the collapseof cavitation bubbles, have been quantified to clarify the mechanism of such an effect of ultrasoundtreatment.
4.1 Vibrational force by a liquid medium of silicone oil
As the frequency f0 and amplitude A0 of vibration at the upper side of the specimen were measured
to be 960kHz and 4.4nm at 443K, the maximum velocity vmax of silicone oil can be estimated throughvmax=(2πf0) A0 = 2.65×10-2 [m/s].

(2)

In general, the sound pressure P0 and velocity v0 at the upper side of the specimen are related to thesound pressure Pi and velocity vi at the lower side of the specimen;− jpa ca sin k a t  cos k a t
 pi   p0 j  = 
 v  ,sin k a t
cos k a t−
 0 
 vi   ρ ca a(3)where ka=2πf0/Ca, Ca, ρa and t are wave number (1185[m-1]), speed of sound (5092[m/s]) in thespecimen at 443K, the density (2700[kg/m3]) and thickness (1 × 10-3[m]) of the specimen,respectively. Pi is therefore expressed aspi = jv max (ρ s c s cos k a t + jρ a c a sin k a t ) ,(4)because ultrasound wave penetrated from the specimen is no longer reflected in silicone oil. Here, Csand ρs are speed of sound in silicone oil (663[m/s]) and the density of silicone oil (841[kg/m3]) at443K. It was found from such estimation that the specimen is subjected to stress |Pi|p i = v max(ρ c cos (k t ) + ρ c sin (k t )) = 338 [kPa],s2s22aa2a22a(5)and elastically deformed by strain of 4.8×10-6 because Young’s modulus of the Al-Mg-Si alloy isaround 70GPa at 443K. Note that such a stress is repeatedly applied to the specimen with a frequencyof 960kHz.
4.2 Shock wave by the collapse of cavitation bubbles
Ultrasound achieves its chemical and/or mechanical effects by generating bubbles within a liquidmedium, a process called cavitation. The vibrational energy by ultrasound causes the molecules in theliquid to be alternately compressed and stretched to produce bubbles. These bubbles are subjected tovibrational stresses and eventually collapse. Although there are no visible cavitation bubbles in the1942

utilized ultrasound bath (Fig.1), the maximum pressure of shock wave Pmax by the collapse ofcavitation bubbles could be estimated throughP max P (γ − 1 ) = Pg  ePgγ (γ − 1 ),(6)under the assumption that a bubble is formed with the critical radius re determined by cativationthreshold; i.e. 0.046µm, and the bubble grows up to the radius r of 0.1µm before the collapse [4]. Peand Pg are effective and internal pressures of the bubble (in this work Pe was regarded as |Pi|, whereas
Pg was calculated by Pg = (P1 +2σ/re -Pv) (re/r)3K where P1 is hydrostatic pressure of 165Pa, σ issurface tension of 20.3mN, Pv is vapor pressure of 133Pa and K=1 because of the isothermal change)and γ is the ratio of specific heats (if the bubble is assumed to be filled with O2, γ=Cp/Cv=1.40),
respectively. It was found from such estimation that the specimen is subjected to a comparable stressof Pmax =111kPa to that by the above mentioned vibrational force, but probably with a much lower
frequency.
4.3 Mechanism of accerelated precipitation by ultrasound treatment
It is obvious that age-hardening response of the Al-Mg-Si alloy is attributed to precipitation
phenomena involving both of diffusion of Mg and Si atoms by the aid of quenched-in excess
vacancies, and nucleation and growth of nano-scale precipitates. The application of ultrasoundtreatment was found to improve the age-hardenability of the alloy (Figs.2 and 3), and therefore it isplausible to consider that ultrasound influences either or both of them. The quantitative estimation of
the effects of ultrasound treatment suggested that both of vibrational force by silicone oil and shock
wave by the collapse of cavitation bubbles exert small stresses on the specimen in the order of 102kPa.
These stresses are macroscopically within those generating only elastic deformation, but atomistic
behaviors occurring in the alloy; e.g. formation of vacancies, diffusion of atoms, nucleation andgrowth of nanoclusters, might be influenced. Further investigation on the effects of ultrasound will beneeded to clarify the mechanism of ultrasound treatment.
5. Conclusions
The effect of ultrasound treatment on the age-hardenablity of AA6022 alloy has been investigated.
Although there was no significant difference in age-hardening response at RT, the increase inhardness during aging at 373 and 443K was found to be improved by the application of ultrasoundtreatment. This suggests that the newly developed ultrasound treatment is useful for the fabrication ofaluminum-made body panels with higher strength and better dent resistance. The quantitative
estimation of the effects of ultrasound treatment suggested that both of vibrational force by silicone
oil and shock wave by the collapse of cavitation bubbles exert small stresses on the specimen in theorder of 102kPa. Although these stresses are macroscopically within those generating only elastic
deformation, atomistic behaviors occurring in the alloy; e.g. formation of vacancies, diffusion ofatoms, nucleation and growth of nanoclusters, might be influenced. Further investigation on theeffects of ultrasound treatment is in progress.
Acknowledgements
The authors gratefully acknowledge Prof. K.Nakamura (Tokyo Institute of Technology), Executive
Director T.Miyamoto and Dr. N.Okada (Honda Electronics Co.,Ltd) for their valuable advice on
ultrasound treatment. The generous financial support by Japan Aluminum Association is also
acknowledged.
References

1943[1] A.Serizawa, S.Hirosawa and T.Sato: Metall. Mater. Trans. 39A (2008) 243-251.
[2] H.Hosoda: Patent of Japan 2006-274312 (2006).
[3] K.Osamura, N.Otsuka and Y.Murakami: Phil. Mag. 45 (1982) 583-599.
[4] K.Takagi: Cho-onpa Binran (Ultrasound handbook) (Maruzen, Tokyo, 1999) pp.305-309.