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UDC 669.715:621.791.72

EFFECT OF ANNEALING ON PRELIMINARY TREATMENT

OF ALUMINUM ALLOY 1421 BY LOW-ENERGY LASER PULSES

P. Yu. Kikin,1 A. I. Pchelintsev,1 and E. E. Rusin1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 25 – 27, July, 2013.

The effect of annealing on preliminary treatment of aluminum alloy 1421 by low-energy laser pulses is stu-

died. It is shown that after annealing the action of the preliminary laser treatment on the time of the start of fu-

sion under very intense laser treatment disappears.

Key words: low-energy laser treatment, annealing, aluminum alloy.

INTRODUCTION

It is shown in [1] that preliminary low-power pulsed la-

ser irradiation shortens the time of the start of the process of

fusion of aluminum alloys 1421, 1570 and AMG6 in subse-

quent laser heating with energies exceeding the fusion

threshold. It has been established that this effect manifests it-

self only until the moment of formation of microscopic crat-

ers on the surface of the alloy. After the formation of micro-

craters the time of the start of fusion increases upon growth

in the number of pulses of preliminary laser treatment. It has

been assumed that the effect of shortening of the time before

the start of fusion is connected with growth in the concentra-

tion of nonequilibrium vacancies in the surface layer of the

material in the preliminary treatment. The disappearance of

this effect upon formation of microcraters is caused by low-

ering of the concentration of nonequilibrium vacancies due

to their merging into pores, the appearance and growth of

which results in detachment of the oxide film on the surface

of the alloy. It should be expected that the decrease in the

concentration of vacancies due to annealing should lead to

suppression of the effect mentioned.

The aim of the present work was to study the effect of

annealing on the time of the start of fusion of aluminum alloy

1421 preliminarily subjected to low-power laser irradiation.

METHODS OF STUDY

We studied test pieces of alloy 1421 (Al – 5.5% Mg –

2.2% Li – 0.12% Zr – 0.2% Sc) with grains 10 – 20 �m in

size. The test pieces were irradiated by a YAG:Nd+3 pulse la-

ser. The irradiation was performed at an energy of up to 25 J

and a pulse length of about 8 msec.

The preliminary irradiation consisted of a series of five

low-energy laser pulses with energy density E1

� 1.0 �

102 J�cm2. The fusion threshold was not attained upon irradi-

ation with this energy density, and the surface of the test

piece did not change. The material fused under a high-inten-

sity pulsed action with energy density E2

� 6 � 102 J�cm2.

The time of the start of the melting process was determined

from the decline of the intensity of the signal reflected from

the surface of the alloy during the action of a laser pulse [2].

The tests were performed in the following mode. We sin-

gled out three regions on the surface of the test piece.

In the first region we determined the effect of the prelim-

inary laser irradiation on the start of the process of fusion of

the alloy. This region was subjected to irradiation with low-

energy laser pulses (five pulses) with energy density E1. We

detected the time � of the start of fusion in subsequent irradi-

ation of this region with a pulse with energy density E2. The

obtained time was compared with the time of the start of fu-

sion of the alloy under the action of a laser pulse with energy

density E2

without preliminary low-energy irradiation.

The second region was subjected to irradiation with

low-energy laser pulses with energy density E1. Then the test

piece was annealed at a temperature of 300°C for 2 h and

cooled with the furnace. After the annealing the second re-

gion was irradiated with high-energy laser pulses with en-

ergy density E2, and the time of the start of fusion was de-

tected.

In the third region of the test pieces, which had been sub-

jected to annealing, we again determined the effect of the

preliminary low-energy laser irradiation on the start of the

Metal Science and Heat Treatment, Vol. 55, Nos. 7 – 8, November, 2013 (Russian Original Nos. 7 – 8, July – August, 2013)

368

0026-0673/13/0708-0368 © 2013 Springer Science + Business Media New York

1Institute for Problems of Mechanical Engineering of the Russian

Academy of Sciences (IPM RAN), Nizhny Novgorod, Russia

(e-mail: [email protected]).

melting process under the action of a high-energy laser pulse.

For this purpose the surface of the third region of an an-

nealed test piece was first subjected to the action of low-

energy laser pulses and then irradiated with a laser pulse that

caused fusion of the alloy at energy density E2. The detected

time before the start of fusion of the test piece was compared

with the time of the start of fusion in this region only under

irradiation with energy density E2.

RESULTS AND DISCUSSION

The mean values of the times of the start of fusion of al-

loy 1421 in different regions of test pieces are presented in

Table 1. Analyzing the results we established the following.

The time before the start of fusion of the alloy with prelimi-

nary low-energy treatment under the action of high-energy

laser irradiation was shorter than without such treatment (see

the first region before annealing in Table 1).

In the second region of the test piece, which had been

subjected to a preliminary treatment, the action of the high-

intensity laser pulse after annealing did not cause shortening

of the time before the start of fusion. The time before the

start of fusion of the material in the third region turned out to

be equal to the time of the start of fusion of the alloy in the

first region without preliminary treatment (see the second re-

gion in Table 1, after annealing). Thus, the annealing removed

the effect of the preliminary laser treatment on the time of the

start of fusion of the alloy under the action of a high-intensity

laser pulse. The third region of the test piece was also an-

nealed. Preliminary treatment of this region again shortened

the time before the start of fusion of the alloy under the ac-

tion of subsequent high-intensity laser irradiation as com-

pared to the action of only a high-intensity laser pulse on this

region (see the third region in Table 1, after annealing).

The results obtains are explainable by the effect of the

concentration of nonequilibrium vacancies on the start of fu-

sion of the alloy. The concentration of nonequilibrium vacan-

cies increases under the action of preliminary low-energy la-

ser pulses on the surface of the alloy. The growth in the con-

centration may be a result of (a) an increase in the concentra-

tion of thermal fluctuation vacancies [3] and (b) an increase

in the concentration of vacancies due to growth of the oxide

film [4]. In the short time of the action of a laser pulse

(10 – 3 sec) and in about the same time of cooling the major

part of vacancies do not have enough time for going into

sinks, and the material preserves “frozen” nonequilibrium

vacancies.

Growth in the concentration of nonequilibrium vacancies

increases the absorptivity of the material connected with ad-

ditional scattering of conduction electrons in the skin layer of

the alloy [5]. Annealing and slow cooling bring the concen-

tration of vacancies to equilibrium due to their sinking (for

example onto grain boundaries), which is responsible for dis-

appearance of the mentioned influence of the preliminary la-

ser treatment on the time of the start of fusion. Preliminary

low-power treatment of the alloy subjected to annealing has

also caused accumulation of vacancies in the near-surface

layer, growth in the absorptivity, and decrease in the time of

the start of fusion.

CONCLUSIONS

Annealing of aluminum alloy 1421 irradiated prelimina-

rily by a series of low-energy laser pulses causes disappear-

ance of the influence of preliminary treatment on the time of

the start of fusion of the alloy under high-intensity laser irra-

diation.

The authors are sincerely grateful to V. N. Perevezentsev

for the helpful discussion of the results of this work.

REFERENCES

1. P. Yu. Kikin, V. N. Perevezentsev, E. E. Rusin, and E. N. Razov,

“Effect of preliminary high-power laser irradiation on the pro-

cess of melting of aluminum alloys,” Zh. Teor. Fiz., 82(2),

46 – 49 (2012).

2. P. Yu. Kikin, V. N. Perevezentsev, A. I. Pchelintsev, and E. E. Ru-

sin, “Treatment of ultrafine-grained aluminum alloys by pulsed

laser irradiation,” Prob. Mashonostr. Nadezhn. Mash., No. 5,

87 – 91 (2007).

3. Ya. I. Frenkel, An Introduction into the Theory of Metals [in Rus-

sian], Izd. Tekhn.-Teor. Literatury, Moscow (1950), 383 p.

4. N. Henney, The Solid State Chemistry [Russian translation], Mir,

Moscow (1971), 223 p.

5. F. Kh. Mirzoev, V. Ya. Panchenko, and L. A. Shelepin, “Laser

control of processes in a solid,” Usp. Fiz. Nauk, 166(1), 3 – 32

(1996).

Effect of Annealing on Preliminary Treatment of Aluminum Alloy 1421 By Low-Energy Laser Pulses 369

TABLE 1. Time of the Start of Fusion of Alloy 1421 under Pulsed Laser Irradiation with Energy Density E2 after Treatment by Different Modes

Treatment mode

Region I Region II Region III

Irradiation with energy

density E2

(prior

to annealing)

Preliminary treatment and sub-

sequent irradiation with energy

density E2

(prior to annealing)

Preliminary treatment before an-

nealing, annealing, and subsequent

irradiation with energy density E2

Annealing and subsequent irra-

diation with energy density E2

(without preliminary treatment)

Annealing, preliminary treat-

ment, subsequent treatment

with energy density E2

Time before start of fusion �, msec

1.07 0.82 1.06 1.07 0.78