Vacuum/volume 44/number B/pages 867 to 861 I1993 0042-207X/93$6.00+.00 Printed in Great Britain 0 1993 Pergamon Press Ltd

Electron beam alloying of aluminum alloys Peter Petrov and Dimiter Dimitroff, Institute of Electronics, 72 Trakia Blvd, 1784 Sofia, Bulgaria

received 1 January 1992 and in revised form 25 May 1992

This paper reports on studies of structural and mechanical changes in layers of eutectic aluminum alloys after doping with Fe, Cr, Ni, and Co followed by electron beam treatment. A theoretical approach is developed for determination of the processing conditions.

I. Introduction

During modification of aluminum alloys of the Al-Si system with a beam of energetic electrons, various structures are formed which are characterized by the presence of highly dispersed and metastable phases’-3. The degree of both structural and mech- anical changes may be substantially influenced by introducing additional dopant elements”‘.

This paper is concerned with a study of the structural and mechanical changes which occur in eutectic aluminum alloys as

a result of doping with Fe, Cr, Co, and Ni powders followed by a subsequent electron beam (EB) treatment.

2. Experimental and theoretical procedures

The study was carried out on an alloy of the following chemical composition: 10.57% Si, 5.7% Cu, <0.03% Ni+Fe and 83.7% Al. An electron beam extracted from an EB-gun was deflected following a sinoidal function with a frequency F (kHz) and amplitude 2A (cm), perpendicular to the direction of sample movement.

The doping was done using powders based on Ni, Fe and Co which were previously deposited by means of a standard plasma technique. The thickness of the coatings was up to 1 mm. The chemical content of the dopant powders is given in Table 1.

The starting samples with dimensions of 50 x 50 x 18 mm were prepared employing bulk thermal processing under the following sequence : (i) homogenization at 505°C for 4 h and at 515°C for another 4 h ; (ii) water quenching; and (iii) aging at 200°C for 15 h.

The process parameters were theoretically determined prior to treatment on the basis of the following approach. It has been shown in refs 7 and 8 that the equation of heating of a semi- infinite body from a moving point heat source is

Table 1. Chemical composition of dopant powders

Elements (%)

Dopant powder Ni Fe Co Cr Si B C

NikCr 11.76 - 16.3 3.27 3.2 0.23 Fe--Cr 2.34 - 21.7 3.63 2.1 0.88 CcvCr 3.33 1.20 22.2 - - -

w, y, 4 = & exp F W+W/W, (1)

where Tis the temperature (K), Q is the heat source power (W) ; Q = UZ, CT is the accelerating voltage (V), Z is the beam current (A), a is the coefficient of temperature conducitivity (m* s-l), 1 is the coefficient of thermal conductivity (W m- ’ K- ‘), V is the velocity of sample movement (m s-l), x, y, z are the spatial coordinates (m) ; R* = x2+y*+z2, and x is the direction of sample movement. Let the width of treated zone which is heated to the melting temperature Tm (and above) be 2A. The value of Y at which a maximum temperature is reached, i.e. Y = A, is determined under the following condition

-‘=o ax . From equations (1) and (2) with z = 0 and A2 = Yz = R*-X2 one can obtain the following parametric equations :

where I++ = VR/2a and 8 = 4daTmlQ V. Taking different values of 111 we find the values of VA/2a which correspond to the melting temperature. This relationship is represented graphically in Fig- ure 1 in the form of a dimensionless diagram and can be used for assessment of the dimensions of the treated zone. This may be done in the following way :

(i) using the given energy parameters of the process (U, i, V)

one can calculate the value of 8 ; and (ii) with the aid of Figure 1 one can obtain the values of

I+G = VA/2a and A.

For example, at a beam power Q = 3.7 kW and velocity of sample movement V = 3 cm s- ’ we obtain f3 = 2. From the curve in Figure 1 we find the value of $ = 0.4 and accordingly A = 0.26 cm. The corresponding energy parameters were used in our exper- iments. The electron beam treatment was performed using a Leybold EWS 300/l 5-60 machine.

After electron beam processing the samples were subjected to

857

P Petrov and D Dimhoff; Electron beam alloying of aluminum alloys

2 50 r

200-

/ 50 -

8

I oo-

0 - 50

I I I I 0 0 50 I 00 I 50 2 00 2 50 300

VA/2a

Figure 1. Theoretical nomogram for &termination of the operating conditions.

aging at 200’C for 2 h. Prior to analyses by optical and electron microscopy, the specimens were cut into appropriate pieces and a standard procedure was performed which included developing

in Keller’s solution and electrolytic etching. Sample micro- hardness was assessed using a LEITZ measuring instrument with a loading force of 1 N. X-ray structural analysis was carried out employing DRON-UM-I facilities with Co-Ka emission.

Strength measurements were performed on samples whose dimensions and treatment are shown in Figure 2. These measure- ments were performed immediately after electron beam pro- cessing as well as after a furnace heat treatment at 250°C up to 200 h.

Corrosion examinations were carried out by means of a poten- tiostat in a solution of 5% NaCl. The anodic potential was determined by measuring the anodic current density. Sample areas unaffected by chemical corrosion were covered with a layer of varnish

Figure 2. Schematic representation of samples to be examined for tensile strength : ;I treated zonea.

3. Results and discussion

The basic dopant clement present in the AlpSi alloy is copper whose maximum solubility in an g-hard solution is up to 5.5% under equilibrium crystallization. The dopant elements intro- duced from the deposited powders form the following com- pounds’, I” :

(i) Fe forms FcSiAl, and occurs in many other compounds. Upon liquid phase hardening, oversaturated solutions arccreatcd containing up to 8.4% Fe :

(ii) Cr forms CrAI, with Al but is commonly present in the composition of (CrFe),Si,Al, 3 or (CrFe) ,Si,Al,. At high cooling rate (> IO’ K s ‘) the hard solution becomes oversaturated with

Cr up to 10% ; (iii) Ni forms NiAI, with Al which in turn dissolves Fe and

Cu. On liquid phase hardening its solubihty may reach 15%. Ni also combines with Al to form NiAl, and occurs in inter- metallides such as Cu,NiAl, and (CuFeNi)?Al i : and

(iv) the solubility of Co in Al upon liquid phase hardening may reach 6.5%. Co forms CoZAI, and CoCu2Al, compounds and is partially soluble in FeCuzAl, and (CuFc)Al,,.

Electron beam heat treatment results in the formation of altered regions featuring a variety of dimensions and configura- tions (Figure 3).

The structure observed as a result of electron beam treatment is quite different from the starting one. The crystallization is of

Figure 3. Cross-section of the treated zones: U = 60 kV, I = 62 mA, V = 3 cm s ‘. 2A = 0.52 cm and ,“ = 5 kHz.

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P Petrov and D Dimitroff: Electron beam alloying of aluminum alloys

a dendritic nature. Electron beam irradiation without additional doping results in the formation of an a-hard solution and a highly dispersed intermetallic compound as shown in Figure 4(b). The average size of both Si inclusions and eutectical cells is reduced by a factor of 2 to 3 due to the high cooling rate, even on a finer scale after additional doping [Figures 4(c)-(e)]. A possible reason for this is the heterogeneous crystallization caused by the phase fluctuations (clusters). As a result, a carcass of high-density inter- metallic compounds is formed. Grains of secondary phases sep- arated from the oversaturated solutions can be seen in the Al- phase. On doping with an Fe-based powder the lattice constant of the cc-hard solution is found to decrease from 4.053 to 4.050 A. Subsequent aging at 250°C results in the extraction of Cu and formation of a (I-phase. Changing the amount of dopant elements in the Al crystal lattice affects its inherent tensile stress as con- firmed by the total width of the diffraction profile. For instance, if there is no doping, the total width of the starting material is 5.53 x 10 ’ rad; after aging up to 200 h this is observed to decrease to 4.90 x lo- ’ rad. In the case of doping, however, the difference between the above values is greater, i.e. from 5.75 x lo- A to 4.2 x I Om 3 rad, which is evidence for additional dopant oversaturation of the solution.

Figure 5 displays the variation of microhardness with duration of aging (up to 200 h) at 200°C. A reduction in microhardness is seen with an increasing time of aging. Maximum microhardness occurs in zones which were doped with an Fe-Cr powder.

Figure 6 shows the variation of tensile strength R as a function of dopant elements at RT [Figure 6(a)] and at elevated tem- peratures [Figure 6(b)]. It is evident that the highest tensile strength occurs in the case of doping with Fe-Cr powders.

The results obtained from the microhardness and stretch measurements are a consequence of both the high dispersiveness and the grain size of the structure in question. This structure influences the dislocation movement via the increased boundary area between the new phases formed and the Al-hard solution as

240-

60 --

40 I I I I I I 0 40 80 120 160 200 240

Tune (h)

Figure 5. Changes in the microhardness of the AILSi alloy during aging up to 200 h at 250°C : (1) starting material ; (2) after EB-treatment ; (3) after Ni-Cr doping followed by EB-treatment; (4) after Fe-Cr doping followed by EB-treatment; and (5) after CoCr doping followed by EB-treatment.

(b)

II 3

-

4 D 5

z60 4

1

1

@z 40- 2 3 4 5

20

Figure 6. Variation of the tensile strength R as function of dopant: [(a) RT; (b) elevated temperature] : (1) starting materials ; (2) after EB- treatment; (3) after Ni&Cr doping followed by EB-treatment : (4) after Fe---Cr doping followed by EB-treatment ; and (5) after Co-Cr doping followed by EB-treatment.

well as through blocking the Gine Preston’s zones by the dopant elements dissolved.

It is apparent from the results of corrosion examination that the Ni-Cr doped layer has the highest corrosion resistance whereas the FeCr doped layer exhibits the lowest corrosion counteraction. However, it must be pointed out that in all cases the corrosion resistance after doping is lower than that of the starting material. The reason for this is the increased potential difference between the Al-hard solution and the new phases formed, as well as between the new phases themselves. The absorption of Ni and Cr in the a-hard solution reduced the above- mentioned difference. The increased boundary area also favors the enhanced corrosion processes. Figure 7 shows the variation of anodic current ICC with beam power. Maximum corrosion resistance is observed at a beam power of 3 kW for undoped samples and at 2.4-3 kW in the case of doping with Ni-Cr. The increase in ICC at higher beam power is likely to be due to the lower crystallization rate and the appearance ofcoarser structural

01 I 50 zoo 250 300 350 400

Power (kW)

Figure 7. Variation of anodic current as a function of the dopant element: (1) starting materials; and (2) after Ni-Cr doping followed by EB-treatment.

P Pefrov and D Dimitroff: Electron beam alloying of aluminum alloys

inclusions. At lower beam power the increase in lo! is possibly associated with the formation of inclusion of non-dissolved dopant elements.

4. Conclusions

Electron beam surface doping of AlSi eutectic alloys with Fe, Co, Ni and Cr changes the structural morphology and the degree of doping of the Al-hard solution, which results in an increased hardness.

An increase in the tensile strength is observed at RT as well as at elevated temperatures, whereas the corrosion resistance is found to diminish. Variation of the corrosion resistance with beam power is also observed.

References

’ S Schiller, Me& Prog, 4, 39 (1986). 2 S Schiller and S Panzer. A Reu Mater Sci, 18, 121 (1989). ‘A Munitz, Met Trans B, 16, 149 (1985). ‘J Ayers, Thin Solid Films, 81, 323 (1981). jP Bonora, M Bassoli, G Cerison, P De Anna, G Battaglin, G Della Mea and P Mazzoldi, Thin Solid Films, 81,339 (1981). bP Petrov and D Dimitroff, 3rd International Beam Technologies Conference, Karlsruhe, p 178. DVS, Dusseldorf (1991). ‘H Carslaw and J. Jager, Conduction of the Heat in Solids. Oxford University Press, Oxford (1959). “N Rykalin, A Ualov. A Zuev and A Kokora, Laser and Electron Beam Treatment of Makials. Mashinostroene, Moscow (1959). 9 L Mondolfo, Structure and Properties of Aluminium Alloys. Metallurgia, Moscow (1979). “‘R Cahn and P Haasen, Physical Metallurgy. North-Holland and Physics Publishing, Amsterdam (1983).

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