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Electroforming of iron foil
Journal of Mechanical Working Technology, 1(1977/1978)231--243 231 O Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
E L E C T R O F O R M I N G O F IRON FOIL
S.H.F. LAI, J.A. MCGEOUGH and P. LAU Department of Engineering, University of Aberdeen, Marischal College, A berdeen AB9 1AS (Scotland) (Received November 1, 1976; in revised form April 18, 1977)
Industrial Summary An alternative to conventional processes for the preparation of iron foil, electroforming, is described. Its advantages over the former methods, including basic cost differences, are outlined. Experiments have shown that the foil can be so produced with an electrolyte which is mainly ferrous chloride solution. (Such an electrolyte can be obtained from industrial waste containing the metal salts.) The dependence of the current efficiency of electroforming and the quality of the foil so produced upon the main process variables, current density, electrolyte temperature and pH, are discussed. The main mechanical, electrical and magnetic properties of the foil are shown to depend on the process variables and on its thickness.
1. I n t r o d u c t i o n Metal strip, the thickness of which is a b o u t 0.1 mm, is usually known as "foil"; steel in that form has many applications. For example, because of its high strength-to-weight ratio, it is often used as a packaging material. Its mechanical properties, such as good ductility, make it attractive in the building industry as a roofing material and for constructional panels. Iron and its alloys also have electrical and magnetic properties which are of widespread practical interest, especially when the materials axe in the form of foil. A c o m m o n example is iron--nickel alloy, popular as a shield for the protection of sensitive electronic apparatus from stray magnetic fields. Another is silicon iron, laminations of which are used in the manufacture of transformer cores. In the former case, a high permeability is desirable; for the latter, low hysteresis and eddy current losses are the usual requirements, together with a reasonably high permeability. Moreover the lower the eddy current loss required, the higher usually must be the resistivity of the material. However, irrespective of the t y p e of steel required and its eventual application, the material is traditionally produced in sheet and foil form by rolling. Despite advances in that process [1], a persistent problem is the comparatively higher costs of making thinner gauges; for instance, the price per unit weight of steel foil is estimated to increase almost two-fold if a thickness of
232
0.05 mm, rather than 0.1 mm, is required [2]. An alternative method of manufacture of thin strip, "electroforming", is therefore under investigation. With this process, metal is deposited upon the cathode of an electrolytic cell, the treatment being so controlled that the deposit can be subsequently separated from the cathode to yield the " e l e c t r o f o r m e d " article. In the manufacture of foil by electroforming, individual sheets of metal can be produced with either flat or cylindrical cathodes; continuous formation of foil is achieved by means of a rotating cylindrical cathode. The main advantages of electroforming are: (i) thinner foils cost proportionally less than thicker material; (ii) the capital cost of electroforming plant is low; (iii) production of foil from processed industrial waste is possible. In this paper, a procedure for producing iron foil is first described, and then the efficiency of the process and the main properties of this material are discussed. Finally, typical properties of electroformed iron foil are compared with those of conventionally produced materials.
2. Procedure and apparatus The first step in this investigation was to establish the composition of an electrolyte and the operating conditions appropriate for the electroforming of iron foil. To that end, small-scale tests were performed with a Hull cell. This work showed that a suitable solution consists of 400 g/1 of ferrous chloride, 80 g/1 of calcium chloride together with 2 ml/l of a commercial nonpitting agent. (It might be noted that with recently developed industrial processes for electroforming iron foil, the main FeC12 electrolyte is obtained by dissolving scrap iron in hydrochloric acid [3,4] .) The addition of CaCl2 increases the current efficiency of the process, whilst the non-pitting agent improves the quality of the foil. A bright smooth electro-deposit of iron can be obtained for current densities in the range 10--100 A/dm 2 and at solution temperatures from 86 to 108°C. (No experiments have been performed at current densities and temperatures greater than 100 A/dm 2 and 108°C because of the limitations of the apparatus available.) With these operating conditions, iron foil of thickness 0.05 to 0.16 mm has been produced on the larger-scale apparatus shown in Fig. 1. Two separate rectangular sheets of foil, of side 240 by 45 mm, are obtained by electroforming on both sides of the cathode. No agitation of the electrolyte is used in this work, even although forced convection is often used to prevent pitting. Two recoguised methods, cathode movement and air agitation, have been tried and have been found to be unsatisfactory. At a typical current density, e.g. 30 A/dm 2, the time of electroforming is a b o u t 20 min for a foil of 0.1 mm thickness. The applied voltages are usually in the range 3--9 V [5,6]. Little comparative information is y e t available a b o u t industrial electroforming plants except that rotating cathodes are used (see Section 1) and that foil of thickness, 0.02 to 0.175 mm, and of various widths, ranging from approxi-
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mately 0.6--3 m, is produced [3]. Percentage constituents of the foil are typically 99.72 Fe, 0.14 C, 0.09 Si, 0.05 Cr, < 0.002 P, < 0.002 S. The presence of carbon has been discussed elsewhere [6] ; it is sufficient here to note that it does not exist in the form of iron carbide. 3. Efficiency of the process In this section the efficiency of the process is investigated in terms of the current efficiency and from study of the quality of the foil produced.
3.1 Current efficiency The time required to electroform foil of a required thickness can be estimated from Faraday's law, provided account is taken of the effects of the other process variables upon the current efficiency. As indicated in Fig. 2, IO0
FZ
I
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I
,
I
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CURRENTDENSITY(Aldn~) Fig. 2. Effect of current density upon current efficiency.
234 the current efficiency has been found to decrease with increasing current density, this change being apparently associated with an increase in hydrogen gas evolution at the cathode. The presence of ferric iron seems to have little influence on current efficiency. The efficiency decreased by only about three per cent, even when the ratio of ferric to ferrous iron reached its maximum level of 0.09 after 15 months of experiments [6]. Electrolyte temperature has little effect on current efficiency over the working range 86 to 108°C. The acidity of the electrolyte has also been found to have negligible influence on current efficiency over the range 0 < pH ~< 3. No experiments were carried out at higher pH, because deleterious ferric ions can exist in solution at a pH of about 3.3 (It might be significant for industrial operations, however, that ferric ions can be precipitated at a pH of 5, and that the precipitate can subsequently be filtered to maintain the electrolyte in its proper condition in the reservoir.)
3.2 Surface quality of the foil Both acidity and electrolyte temperature have a significant bearing on the surface quality of the foil. For pH < 0, the foil is highly stressed, brittle and sometimes cracked. For pH between 0 and 0.5, a bright and smooth foil is obtained. Further increase in pH to 3 makes little difference to the quality of foil. With little to be gained in either current efficiency or quality of foil from these higher pH values, the attainment of which also requires further additives to the electrolyte, the solution has therefore been operated at a pH value between 0 and 0.5. Similarly, the electrolyte has been maintained at temperatures between 85 and 108°C, because the material is of acceptable quality over that range. Figure 3(a) illustrates the flaky and blistered character of the iron foil, representative of deposits obtained at temperatures below 85°C, in contrast with Fig. 3(b) which shows the smooth foil achieved at temperatures greater than 85°C. A typical surface roughness of the latter material is 0.04 pm CLA for its cathodic face and 5.6 pm CLA for its anodic side. (The surface roughness of the cathode mandrel was 0.04 pm CLA.) 4. Measurement of mechanical, electrical and magnetic
properties
4.1 Mechanical properties Accurate measurement of the mechanical properties of the foil has presented special difficulties because of the thinness of the material. These problems, and the methods developed to overcome them, are discussed in detail by Lai and McGeough [6]. For convenience, though, the principal, and additional features are described here. Specimens for measurement were obtained by shearing each sample in a die, from which test pieces of the shape and dimensions given in Fig. 4 were produced. A Hounsfield 'tensometer', with a spring beam of 600 N and a constant loading rate of 1.58 m m / m i n , was then used to give a load--extension
235
Fig. 3. Typical surfaces of iron foil of thickness 0.4 mm. Electrolyte temveratures: (a) 45°C, (b) 100°C. (Other conditions: current density 30 A / d m 2, pH 1 .)
236 35
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Dimensions in mm
Fig. 4. Dimensions of tensile test specimens. (Gauge length for ductility measurements = 15 ram.) graph, from which tensile strength, Young's modulus and yield stress could be deduced. Ductility measurements, based on a gauge length of 15 mm, were made on the same specimens. Hardness was recorded by means of an 'Isoma' microhardness machine, with a load of 100 gf (0.981 N). The test procedures adopted have enabled tensile strength, Young's modulus, yield stress and hardness to be measured to respective accuracies of 3.5, 10, 8, and 3.3%. Internal stress is less frequently investigated as a mechanical property, although it can significantly affect the quality of electroformed components. This property describes the stressed state of the foil which arises with the structural distortions that occur during the electroforming of the metal. It can be evaluated by electro-depositing iron on both sides of a rigidly held metal strip, of known Young's modulus. Measurement of the subsequent changes in length, 4 , made on a dial gauge attached to the metal strip, together with estimates of the average thickness, t, of the iron electrolytically deposited, are substituted into the equation o = EdA/21t
(1)
where o is the internal stress, E the Young's modulus of the metal shim, d the thickness of the metal shim (0.05 mm). Typical values for A and t have been 0.152 m m and 0.102 mm, which yield a value of 38.5 MN/m 2 for the internal stress. The accuracy of these measurements is estimated at 2%. A full discussion on internal stress is available in Lai's thesis [5]. 4.2 Electrical and magnetic p r o p e r t i e s
The properties investigated were resistivity, relative permeability, saturation flux density, remanence, coercivity and hysteresis loss. Full details of the procedures used will be found elsewhere [7], but the significant information is given here. Values for the average resistivity of the foil were deduced from measurements of resistance, obtained from rectangular strips of foil, 70 mm by 7.5 mm, placed in series with a Wayne--Kerr Universal Bridge. This m e t h o d yielded results accurate to about 5%. The apparatus constructed for measurement of the magnetic properties of the iron foil was based on that recommended in B.S.601 [8] and is similar to that described by Schwartz and Mallory [9]. The basic circuit, which is shown in Fig. 5, consisted of a power supply, a
237
WATTMETER
A.C. FLUXMETER
OSCILLOSCOPE
AMMETER
Fig. 5. Circuit for magnetic measurements.
primary and secondary coil, and an AC fluxmeter for reading the flux linkage between the coils. This meter has an oscilloscope display with means for recording photographically the hysteresis loop of the circuit, from which the main magnetic properties can be deduced. The primary coil in this apparatus consisted of 2880 turns of copper wire; n o t more than two turns of copper wire were required for the secondary coil. The coils encompassed a test frame made from 20 foils, each 70 m m by 7.5 ram, arranged to form a square with the corners double lapping over an area 10 m m by 10 ram. Eddy current losses in the frame were rendered negligible by coating each strip with polyurethane varnish. (This procedure is similiar to that in B.S. 601 except that the latter suggests that the test frame should consist of strips of dimensions 30 m m by 280 mm. Because of the comparatively small a m o u n t of foil produced in the experiments, these dimensions had to be reduced to those stated above.) The measurements were considered to have an accuracy of approximately 10%. 5. Results and discussion
Mechanical properties of electroformed foil, such as tensile strength and hardness, can be altered by changing the process variables and also the thickness. For example, for a constant foil thickness of 0.060 m m and a current density of 10 A / d m 2, the tensile strength decreases with increasing electrolyte temperature, from 470 MN/m 2 at 86°(] to 400 MN/m 2 at 108°(]. Similarly, hardness decreases from about 250 to 140 VHN over the same temperature range. These properties also decrease with increasing thickness; from 470 MN/ m 2 and 250 VHN at a thickness of 0.060 mm, to 420 MN/m 2 and 160 VHN at 0.095 mm, the solution temperature being held constant at 86°C and the current density at 10 A/dm 2.
238 These effects can be attributed to an increase in grain size with increasing electrolyte temperature and with increasing foil thickness. A complete explanation for the temperature effect has not y e t emerged, although possible mechanisms have been discussed by Lai and McGeough [6]. Micrographic evidence is available [6] which demonstrates that at the lowest temperature, 86°C, columnar grains, of typical size 3.9 t~m, are formed. With increasing temperature the grains increase in average size, to about 4.7 um at 108°C (for foil of approximate thickness 0.04 mm). The change in mechanical properties with increasing foil thickness is a consequence of the electroforming action. The thickness is directly proportional to the time of electroforming, and since the grain size of electro-deposited metals increases with time of electrolysis, a lighter gauge will have finer grains, and hence higher strength. (It should be realised, however, that electroformed foil in this condition does not have a uniform grain structure across its thickness.) Typical estimates of variation in grain size with thickness have been obtained from foil of thickness 0.04 mm. This material was electroformed at a current density of 25 A/dm 2 and at an electrolyte temperature of 100°C with a cathode of surface roughness 0.04 p m CLA. Over the region of foil formed in the vicinity of the cathode, equiaxial grains, about 1.34/am in diameter, were formed into bands, the widths of which ranged from a b o u t 4.0 pm--10.72 pm. Outside these bands were larger grains whose shapes varied from equiaxial to fibrous. Their dimensions were similarly varied, although typically they were 13.4 p m long and 6.7 p m wide. A small increase in strength can be gained by using a related feature of electroforming. If the process of electroforming is interrupted, for example by a m o m e n t a r y break in the passage of current, then on resumption the formation of crystals will again proceed from a fine-grained structure to a coarse-grained one, and n o t from the most recent stage of coarser grains reached before the interruption. Thus for foil of thickness of 0.050 mm produced at a current density of 50 A/dm ~ and at an electrolyte temperature of 96°C, continuous electroforming has yielded material with a tensile strength of approximately 480 MN/m 2, whereas intermittent electroforming for periods of 1 min, with 15 s intervals, has increased the tensile strength to a b o u t 620 MN/m 2. Even this improvement in the tensile strength is achieved only at the expense of a longer net time for electroforming. The time needed to obtain a specified thickness of foil can be reduced, of course, by increasing the current density. Tests on foil of thickness from 0.05--0.16 mm have shown that over the current-density range 10--100 A/dm 2, a slight increase is obtained in mechanical properties such as tensile strength and hardness. However, the decrease in current efficiency with increasing current density, indicated in Fig. 2, means that the time of electroforming to produce foil of specified thickness is correspondingly increased at higher current densities. The state of the cathode surface has a bearing on all mechanical properties,
239
although its influence on internal stress is particularly noticeable. For foil thicknesses up to 0.02 ram, the crystallographic nature of the substrate lattice plays a significant part [6]. Above 0.02 mm, the macroscopic condition of the cathode surface still affects the mechanical properties, although its effect lessens on further increase in foil thickness. The process variables then acquire main control. This gradual increase in the effects of the process variables at the expense of the influence of the cathode surface is illustrated in Fig. 6. As the foil thickness is increased from 0.02 to 0.07 mm, the sinuate variation of internal stress with current density is dampened. For a foil thickness of the order of 0.12 mm, internal stress is considered to be influenced mainly by the process variables. Nevertheless, its dependence upon the main process variables of current density and electrolyte temperature is still much more complex than that of tensile strength or hardness [6]. The hydrogen evolved at the cathode had little apparent effect on the mechanical properties; for example, no change in hardness occurred after heat-treatment at 650°C, which operation would have driven off any occluded hydrogen. (Other results n o t discussed herein have also indicated that internal stress is not affected by the generation of hydrogen.) The present work has therefore shown that the structure of the foil can be sufficiently controlled during electroforming to have a significant effect upon its mechanical properties. If the electrical and magnetic properties of electroformed iron could be similarly controlled, the difficulty of special preparation of conventionally produced foil, which is often necessary after rolling to endow the material with the specific properties for proposed applications, might be avoidable. Recent work to this end has indicated that these properties of the foil can also be influenced by the process variables [7]. Thus Table 1 shows that the
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240
resistivity of the material decreases with increasing electrolyte temperature, over the range 86 to 108°C. The effect is consistent with the changes in grain size caused by alteration of the electrolyte temperature, as discussed above. That the grain structure of metals affects their resistivity is, of course, well known; see, for example, reference 10. For foil of the same thickness, electroformed at a constant electrolyte temperature of 96°C, resistivity has been found to increase with current density, from a b o u t 55 X 10 -6 ohm cm at 10 A/dm 2 to 95 X 10 -6 ohm cm at 25 A/dm 2. Nevertheless, little change in grain size has been observed over this range of current densities. Instead, it may be relevant that this rise in resistivity is associated with an observed increase in the a m o u n t of impurities in the iron foil which occurs as the current density is increased. Increasing quantities of impurities are also known to increase the resistivity of metals [10]. Table 2 shows that the values of the magnetic properties decrease with increasing electrolyte temperature, the effect being particularly marked for hysteresis loss and relative permeability. This behaviour is again consistent with an observed increase in the average grain size of the foil with increasing electrolyte temperature. These results are in agreement with Bozorth's finding that an increase in grain size often causes a decrease in hysteresis loss [ 11]. Furthermore, Wolf has reported that the coercivity and remanence of electro-deposited metals can also be dependent upon electrolyte temperature [12] (and presumably, therefore, u p o n grain size). The former author has indicated that other
TABLE 1
Dependence o f resistivity o n electrolyte temperature for e l e e t r o f o r m e d i r o n foil o f a p p r o x i m a t e t h i c k n e s s 0 . 0 7 5 m m (current density = 10 A / d i n 2) Resistivity ( o h m c m ) x 10 -6 E l e c t r o l y t e t e m p e r a t u r e (°C)
60 86
55 96
30 108
TABLE 2 E f f e c t o f e l e c t r o l y t e t e m p e r a t u r e o n m a g n e t i c p r o p e r t i e s o f i r o n foil, o f a p p r o x i m a t e thickness 0 . 0 7 5 ram, a n d e l e c t r o f o r r n e d at a current density o f 20 A ] d m ~ ( N o t e : e d d y current loss at t h e f r e q u e n c y o f m e a s u r e m e n t ( 5 0 Hz) for t h e foil was negligible) Electrolyte temperature (°C)
Relative permeability
Saturation flux d e n s i t y (Wb m -2 )
Remanence (Wb m -~)
Coercivity ( A m -1 )
86 96 108
360 255 155
3.0 2.8 2.2
2.4 1.8 1.4
19.7 17.3 17.7
Hysteresis loss (J m -3 cycle -~ ) 715 560 370
241 magnetic properties may be similarly affected. Nevertheless, he emphasises t h a t the dependence of the magnetic properties on grain size is complicated by the variations in crystal orientation which arise with the m e t h o d of preparation of the material and by the inclusion of impurities. The complex mechanisms at work in electroforming and the practical problems of producing iron foil free of impurities may therefore hinder tight control over magnetic properties through the dependence of grain size upon electrolyte temperature. Nonetheless, the present work does indicate a definite effect of electrolyte temperature, and hence of grain size, upon these properties. Little change in the magnetic properties was noted over the current density range 10--30 A/dm. This observation is consistent with the micrographic evidence for negligible change in grain size with current density. Tables 2, 3 and 4 are useful for comparing the properties of a typical electroformed iron foil with those of other conventionally produced materials. From Table 3 the iron foil is seen to have a higher tensile strength, yield stress and ductility, but lower hardness and Young's modulus, than an annealed silicon steel. It also has a slightly greater tensile strength than a rolled tin plate, Because of the rising cost of aluminium, its replacement as a foil by iron has been considered. Table 3 shows t h a t the thinnest aluminium foil has a higher tensile strength and lower ductility than the iron foil, and t h a t on increasing the thickness of the aluminium, its mechanical properties become lower than those of the iron. Comparison of Table 1 with Table 4 shows that the resistivity of the iron foil is higher than that of commercially pure iron and t h a t an increase in electrolyte temperature can change the resistivity from a value smaller than, TABLE 3 Mechanical properties of electroformed iron foil and of conventionally produced metal
foil Material
Electroformed iron foil (temperature 108°C and current density 25 A/dm: ) Annealed 3.5% Si steel Rolled tin plate* Aluminium foil*
Thick- Tensile
H a r d - Yield
ness
strength
ness
(ram)
(MN/m2)
0.072
Young's modulus
(VHN) (MN/m2)
Ductility (percentage elongation)
371
147
364
5.1
140
0.045
352
163
336
2.76
191
0.013 0.017 0.02 0.05
325 595 315 160
190 ----
-----
-2.3 2.0 0.79
*Data from the British Steel Corporation.
stress
(GN/m ~)
I
m
*Data f r o m various sources.
10
14
--
--
-
48
--
-
60
0.075
Electroformed iron foil ( t e m p e r a t u r e 108°C and current density 10 A / d i n 2 ) Silicon i r o n * Pure iron* Commercially p u r e iron* L o w c a r b o n steel*
Resistivity ( o h m c m X 10 -6)
Thickness (ram)
Material
1.98
500--1500 104 150
155
Relative permeability
1.0
1.95 2.16 2.16
2.2
Saturation flux d e n s i t y (Wb m -s)
4 x 103
0.8 1.3 1.3
1.4
Remanence (Wb m -2)
24 0.8 7
17.7
Coercivity ( A m -1)
Electrical a n d m a g n e t i c p r o p e r t i e s o f e l e c t r o f o r m e d iron foil a n d o f c o n v e n t i o n a l l y p r o d u c e d f e r r o u s metals.
TABLE 4
35O
370
Hysteresis loss (J m - 3 cycle- 1
b~
bO
243 to an amount greater than, that of silicon iron. Finally, typical magnetic properties of other ferrous materials are presented for comparison in Table 4. The relative permeability of the electroformed iron is greater than that of its closest conventionally produced relations, pure iron, low carbon steel and commercially pure iron, whereas the saturation flux densities are comparable. The remanence of the two groups are similar, but the coercivity of the electroformed material is most comparable with that of silicon iron. Tables 2 and 4 indicate that the hysteresis loss of electroformed iron can resemble that of silicon iron or, depending upon the electrolyte temperature. (Although the tables provide a useful guide to the properties of the electroformed foil, it should be realised that a full comparison is difficult, because of the probable variations in specimen size and forms of testing used in compiling the results given in Tables 3 and 4). Acknowledgements
This project is supported by SRC Grant No. B/RG/8037.3. The investigation reported herein was partly reported in a paper presented at the IEE Conference on Electrical Methods of Machining, Forming and Coating, 1975 (Conference Publication 133) and the permission of the IEE to reproduce that work is gratefully acknowledged. The authors appreciate the interest and support of Professor T.M. Charlton, by courtesy of whom an Aberdeen University Engineering Department Research Studentship~is held by one of them (S.H.F.L.). Mr. W.A. Crichton is thanked for providing the information on grain size and Mr. N. Milne for some useful comments. References 1 2 3 4 5 6 7 8 9 10 11 12
E.C. Larke, The Rolling of Strip, Sheet and Plate,Chapman and Hall, London, 1963. H.M. Finniston, Philos. Trans. R. Soc. London, A, 275 (1973) 313--327. See, for example, Phys. Bull.,26 (1975) 229. H. Silman, in J.D. Thornton (Ed.), Inst. Chem. Eng. Symp. Series,Electrochemical Engineering, 1 (1973) 1195--1211. S.H.F. Lai, M.Sc. Thesis, Aberdeen University, 1974. S.H.F. Lai and J.A. McGeough, J. Mech. Eng. Sci., 18 (1976) 19--24. P. Lau, M.Sc. Thesis, Aberdeen University (to be submitted). BritishStandards Institution No. 601, Part 5, 1973. M. Schwartz and G.D. Mallory, J. Electrochem. Soc., 123 (1976) 606--614. K.J. Pascoe, Properties of Materials for ElectricalEngineers, John Wiley and Sons, London, 1972, Ch. 10. R.M. Bozorth, Ferromagnetism, Van Nostrand, London, 1957, Ch. 4, p. 87. I.W. Wolf, J. Appl. Phys., 33 (1966) 1152--1159.
E L E C T R O F O R M I N G O F IRON FOIL
S.H.F. LAI, J.A. MCGEOUGH and P. LAU Department of Engineering, University of Aberdeen, Marischal College, A berdeen AB9 1AS (Scotland) (Received November 1, 1976; in revised form April 18, 1977)
Industrial Summary An alternative to conventional processes for the preparation of iron foil, electroforming, is described. Its advantages over the former methods, including basic cost differences, are outlined. Experiments have shown that the foil can be so produced with an electrolyte which is mainly ferrous chloride solution. (Such an electrolyte can be obtained from industrial waste containing the metal salts.) The dependence of the current efficiency of electroforming and the quality of the foil so produced upon the main process variables, current density, electrolyte temperature and pH, are discussed. The main mechanical, electrical and magnetic properties of the foil are shown to depend on the process variables and on its thickness.
1. I n t r o d u c t i o n Metal strip, the thickness of which is a b o u t 0.1 mm, is usually known as "foil"; steel in that form has many applications. For example, because of its high strength-to-weight ratio, it is often used as a packaging material. Its mechanical properties, such as good ductility, make it attractive in the building industry as a roofing material and for constructional panels. Iron and its alloys also have electrical and magnetic properties which are of widespread practical interest, especially when the materials axe in the form of foil. A c o m m o n example is iron--nickel alloy, popular as a shield for the protection of sensitive electronic apparatus from stray magnetic fields. Another is silicon iron, laminations of which are used in the manufacture of transformer cores. In the former case, a high permeability is desirable; for the latter, low hysteresis and eddy current losses are the usual requirements, together with a reasonably high permeability. Moreover the lower the eddy current loss required, the higher usually must be the resistivity of the material. However, irrespective of the t y p e of steel required and its eventual application, the material is traditionally produced in sheet and foil form by rolling. Despite advances in that process [1], a persistent problem is the comparatively higher costs of making thinner gauges; for instance, the price per unit weight of steel foil is estimated to increase almost two-fold if a thickness of
232
0.05 mm, rather than 0.1 mm, is required [2]. An alternative method of manufacture of thin strip, "electroforming", is therefore under investigation. With this process, metal is deposited upon the cathode of an electrolytic cell, the treatment being so controlled that the deposit can be subsequently separated from the cathode to yield the " e l e c t r o f o r m e d " article. In the manufacture of foil by electroforming, individual sheets of metal can be produced with either flat or cylindrical cathodes; continuous formation of foil is achieved by means of a rotating cylindrical cathode. The main advantages of electroforming are: (i) thinner foils cost proportionally less than thicker material; (ii) the capital cost of electroforming plant is low; (iii) production of foil from processed industrial waste is possible. In this paper, a procedure for producing iron foil is first described, and then the efficiency of the process and the main properties of this material are discussed. Finally, typical properties of electroformed iron foil are compared with those of conventionally produced materials.
2. Procedure and apparatus The first step in this investigation was to establish the composition of an electrolyte and the operating conditions appropriate for the electroforming of iron foil. To that end, small-scale tests were performed with a Hull cell. This work showed that a suitable solution consists of 400 g/1 of ferrous chloride, 80 g/1 of calcium chloride together with 2 ml/l of a commercial nonpitting agent. (It might be noted that with recently developed industrial processes for electroforming iron foil, the main FeC12 electrolyte is obtained by dissolving scrap iron in hydrochloric acid [3,4] .) The addition of CaCl2 increases the current efficiency of the process, whilst the non-pitting agent improves the quality of the foil. A bright smooth electro-deposit of iron can be obtained for current densities in the range 10--100 A/dm 2 and at solution temperatures from 86 to 108°C. (No experiments have been performed at current densities and temperatures greater than 100 A/dm 2 and 108°C because of the limitations of the apparatus available.) With these operating conditions, iron foil of thickness 0.05 to 0.16 mm has been produced on the larger-scale apparatus shown in Fig. 1. Two separate rectangular sheets of foil, of side 240 by 45 mm, are obtained by electroforming on both sides of the cathode. No agitation of the electrolyte is used in this work, even although forced convection is often used to prevent pitting. Two recoguised methods, cathode movement and air agitation, have been tried and have been found to be unsatisfactory. At a typical current density, e.g. 30 A/dm 2, the time of electroforming is a b o u t 20 min for a foil of 0.1 mm thickness. The applied voltages are usually in the range 3--9 V [5,6]. Little comparative information is y e t available a b o u t industrial electroforming plants except that rotating cathodes are used (see Section 1) and that foil of thickness, 0.02 to 0.175 mm, and of various widths, ranging from approxi-
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spheres
~/] ]-T-~4F- Electrolyte
I_ 90ram H J~ II s°'uti°n ] -t~ I'~ ~L-Rubber lined ]---_ H I I I I electrolyte tank • ~-~J~l I (260dm'Copadty) ~ R e d a n g u l a r
~stainless ,,'cathode
( ~Filter pump
steel
(2/.,0x 45mm2)
Fig. 1. Experimental apparatus for electroforming of iron foil.
mately 0.6--3 m, is produced [3]. Percentage constituents of the foil are typically 99.72 Fe, 0.14 C, 0.09 Si, 0.05 Cr, < 0.002 P, < 0.002 S. The presence of carbon has been discussed elsewhere [6] ; it is sufficient here to note that it does not exist in the form of iron carbide. 3. Efficiency of the process In this section the efficiency of the process is investigated in terms of the current efficiency and from study of the quality of the foil produced.
3.1 Current efficiency The time required to electroform foil of a required thickness can be estimated from Faraday's law, provided account is taken of the effects of the other process variables upon the current efficiency. As indicated in Fig. 2, IO0
FZ
I
~
I
,
I
i
I
i
I
i
I
~
I
~
I
t
I
i
I
CURRENTDENSITY(Aldn~) Fig. 2. Effect of current density upon current efficiency.
234 the current efficiency has been found to decrease with increasing current density, this change being apparently associated with an increase in hydrogen gas evolution at the cathode. The presence of ferric iron seems to have little influence on current efficiency. The efficiency decreased by only about three per cent, even when the ratio of ferric to ferrous iron reached its maximum level of 0.09 after 15 months of experiments [6]. Electrolyte temperature has little effect on current efficiency over the working range 86 to 108°C. The acidity of the electrolyte has also been found to have negligible influence on current efficiency over the range 0 < pH ~< 3. No experiments were carried out at higher pH, because deleterious ferric ions can exist in solution at a pH of about 3.3 (It might be significant for industrial operations, however, that ferric ions can be precipitated at a pH of 5, and that the precipitate can subsequently be filtered to maintain the electrolyte in its proper condition in the reservoir.)
3.2 Surface quality of the foil Both acidity and electrolyte temperature have a significant bearing on the surface quality of the foil. For pH < 0, the foil is highly stressed, brittle and sometimes cracked. For pH between 0 and 0.5, a bright and smooth foil is obtained. Further increase in pH to 3 makes little difference to the quality of foil. With little to be gained in either current efficiency or quality of foil from these higher pH values, the attainment of which also requires further additives to the electrolyte, the solution has therefore been operated at a pH value between 0 and 0.5. Similarly, the electrolyte has been maintained at temperatures between 85 and 108°C, because the material is of acceptable quality over that range. Figure 3(a) illustrates the flaky and blistered character of the iron foil, representative of deposits obtained at temperatures below 85°C, in contrast with Fig. 3(b) which shows the smooth foil achieved at temperatures greater than 85°C. A typical surface roughness of the latter material is 0.04 pm CLA for its cathodic face and 5.6 pm CLA for its anodic side. (The surface roughness of the cathode mandrel was 0.04 pm CLA.) 4. Measurement of mechanical, electrical and magnetic
properties
4.1 Mechanical properties Accurate measurement of the mechanical properties of the foil has presented special difficulties because of the thinness of the material. These problems, and the methods developed to overcome them, are discussed in detail by Lai and McGeough [6]. For convenience, though, the principal, and additional features are described here. Specimens for measurement were obtained by shearing each sample in a die, from which test pieces of the shape and dimensions given in Fig. 4 were produced. A Hounsfield 'tensometer', with a spring beam of 600 N and a constant loading rate of 1.58 m m / m i n , was then used to give a load--extension
235
Fig. 3. Typical surfaces of iron foil of thickness 0.4 mm. Electrolyte temveratures: (a) 45°C, (b) 100°C. (Other conditions: current density 30 A / d m 2, pH 1 .)
236 35
!to
30
T
"f
~
35
T j .
"
~h.
.
1
.
RU
Dimensions in mm
Fig. 4. Dimensions of tensile test specimens. (Gauge length for ductility measurements = 15 ram.) graph, from which tensile strength, Young's modulus and yield stress could be deduced. Ductility measurements, based on a gauge length of 15 mm, were made on the same specimens. Hardness was recorded by means of an 'Isoma' microhardness machine, with a load of 100 gf (0.981 N). The test procedures adopted have enabled tensile strength, Young's modulus, yield stress and hardness to be measured to respective accuracies of 3.5, 10, 8, and 3.3%. Internal stress is less frequently investigated as a mechanical property, although it can significantly affect the quality of electroformed components. This property describes the stressed state of the foil which arises with the structural distortions that occur during the electroforming of the metal. It can be evaluated by electro-depositing iron on both sides of a rigidly held metal strip, of known Young's modulus. Measurement of the subsequent changes in length, 4 , made on a dial gauge attached to the metal strip, together with estimates of the average thickness, t, of the iron electrolytically deposited, are substituted into the equation o = EdA/21t
(1)
where o is the internal stress, E the Young's modulus of the metal shim, d the thickness of the metal shim (0.05 mm). Typical values for A and t have been 0.152 m m and 0.102 mm, which yield a value of 38.5 MN/m 2 for the internal stress. The accuracy of these measurements is estimated at 2%. A full discussion on internal stress is available in Lai's thesis [5]. 4.2 Electrical and magnetic p r o p e r t i e s
The properties investigated were resistivity, relative permeability, saturation flux density, remanence, coercivity and hysteresis loss. Full details of the procedures used will be found elsewhere [7], but the significant information is given here. Values for the average resistivity of the foil were deduced from measurements of resistance, obtained from rectangular strips of foil, 70 mm by 7.5 mm, placed in series with a Wayne--Kerr Universal Bridge. This m e t h o d yielded results accurate to about 5%. The apparatus constructed for measurement of the magnetic properties of the iron foil was based on that recommended in B.S.601 [8] and is similar to that described by Schwartz and Mallory [9]. The basic circuit, which is shown in Fig. 5, consisted of a power supply, a
237
WATTMETER
A.C. FLUXMETER
OSCILLOSCOPE
AMMETER
Fig. 5. Circuit for magnetic measurements.
primary and secondary coil, and an AC fluxmeter for reading the flux linkage between the coils. This meter has an oscilloscope display with means for recording photographically the hysteresis loop of the circuit, from which the main magnetic properties can be deduced. The primary coil in this apparatus consisted of 2880 turns of copper wire; n o t more than two turns of copper wire were required for the secondary coil. The coils encompassed a test frame made from 20 foils, each 70 m m by 7.5 ram, arranged to form a square with the corners double lapping over an area 10 m m by 10 ram. Eddy current losses in the frame were rendered negligible by coating each strip with polyurethane varnish. (This procedure is similiar to that in B.S. 601 except that the latter suggests that the test frame should consist of strips of dimensions 30 m m by 280 mm. Because of the comparatively small a m o u n t of foil produced in the experiments, these dimensions had to be reduced to those stated above.) The measurements were considered to have an accuracy of approximately 10%. 5. Results and discussion
Mechanical properties of electroformed foil, such as tensile strength and hardness, can be altered by changing the process variables and also the thickness. For example, for a constant foil thickness of 0.060 m m and a current density of 10 A / d m 2, the tensile strength decreases with increasing electrolyte temperature, from 470 MN/m 2 at 86°(] to 400 MN/m 2 at 108°(]. Similarly, hardness decreases from about 250 to 140 VHN over the same temperature range. These properties also decrease with increasing thickness; from 470 MN/ m 2 and 250 VHN at a thickness of 0.060 mm, to 420 MN/m 2 and 160 VHN at 0.095 mm, the solution temperature being held constant at 86°C and the current density at 10 A/dm 2.
238 These effects can be attributed to an increase in grain size with increasing electrolyte temperature and with increasing foil thickness. A complete explanation for the temperature effect has not y e t emerged, although possible mechanisms have been discussed by Lai and McGeough [6]. Micrographic evidence is available [6] which demonstrates that at the lowest temperature, 86°C, columnar grains, of typical size 3.9 t~m, are formed. With increasing temperature the grains increase in average size, to about 4.7 um at 108°C (for foil of approximate thickness 0.04 mm). The change in mechanical properties with increasing foil thickness is a consequence of the electroforming action. The thickness is directly proportional to the time of electroforming, and since the grain size of electro-deposited metals increases with time of electrolysis, a lighter gauge will have finer grains, and hence higher strength. (It should be realised, however, that electroformed foil in this condition does not have a uniform grain structure across its thickness.) Typical estimates of variation in grain size with thickness have been obtained from foil of thickness 0.04 mm. This material was electroformed at a current density of 25 A/dm 2 and at an electrolyte temperature of 100°C with a cathode of surface roughness 0.04 p m CLA. Over the region of foil formed in the vicinity of the cathode, equiaxial grains, about 1.34/am in diameter, were formed into bands, the widths of which ranged from a b o u t 4.0 pm--10.72 pm. Outside these bands were larger grains whose shapes varied from equiaxial to fibrous. Their dimensions were similarly varied, although typically they were 13.4 p m long and 6.7 p m wide. A small increase in strength can be gained by using a related feature of electroforming. If the process of electroforming is interrupted, for example by a m o m e n t a r y break in the passage of current, then on resumption the formation of crystals will again proceed from a fine-grained structure to a coarse-grained one, and n o t from the most recent stage of coarser grains reached before the interruption. Thus for foil of thickness of 0.050 mm produced at a current density of 50 A/dm ~ and at an electrolyte temperature of 96°C, continuous electroforming has yielded material with a tensile strength of approximately 480 MN/m 2, whereas intermittent electroforming for periods of 1 min, with 15 s intervals, has increased the tensile strength to a b o u t 620 MN/m 2. Even this improvement in the tensile strength is achieved only at the expense of a longer net time for electroforming. The time needed to obtain a specified thickness of foil can be reduced, of course, by increasing the current density. Tests on foil of thickness from 0.05--0.16 mm have shown that over the current-density range 10--100 A/dm 2, a slight increase is obtained in mechanical properties such as tensile strength and hardness. However, the decrease in current efficiency with increasing current density, indicated in Fig. 2, means that the time of electroforming to produce foil of specified thickness is correspondingly increased at higher current densities. The state of the cathode surface has a bearing on all mechanical properties,
239
although its influence on internal stress is particularly noticeable. For foil thicknesses up to 0.02 ram, the crystallographic nature of the substrate lattice plays a significant part [6]. Above 0.02 mm, the macroscopic condition of the cathode surface still affects the mechanical properties, although its effect lessens on further increase in foil thickness. The process variables then acquire main control. This gradual increase in the effects of the process variables at the expense of the influence of the cathode surface is illustrated in Fig. 6. As the foil thickness is increased from 0.02 to 0.07 mm, the sinuate variation of internal stress with current density is dampened. For a foil thickness of the order of 0.12 mm, internal stress is considered to be influenced mainly by the process variables. Nevertheless, its dependence upon the main process variables of current density and electrolyte temperature is still much more complex than that of tensile strength or hardness [6]. The hydrogen evolved at the cathode had little apparent effect on the mechanical properties; for example, no change in hardness occurred after heat-treatment at 650°C, which operation would have driven off any occluded hydrogen. (Other results n o t discussed herein have also indicated that internal stress is not affected by the generation of hydrogen.) The present work has therefore shown that the structure of the foil can be sufficiently controlled during electroforming to have a significant effect upon its mechanical properties. If the electrical and magnetic properties of electroformed iron could be similarly controlled, the difficulty of special preparation of conventionally produced foil, which is often necessary after rolling to endow the material with the specific properties for proposed applications, might be avoidable. Recent work to this end has indicated that these properties of the foil can also be influenced by the process variables [7]. Thus Table 1 shows that the
2~C
o~
Z V)
Z 0:: LU I--
_z ,
~
,
,
[
,
,
1~ 0
CURRENT DENSITY (A/din 2)
Fig. 6. Effect of foil t h i c k n e s s and current density upon internal stress.
240
resistivity of the material decreases with increasing electrolyte temperature, over the range 86 to 108°C. The effect is consistent with the changes in grain size caused by alteration of the electrolyte temperature, as discussed above. That the grain structure of metals affects their resistivity is, of course, well known; see, for example, reference 10. For foil of the same thickness, electroformed at a constant electrolyte temperature of 96°C, resistivity has been found to increase with current density, from a b o u t 55 X 10 -6 ohm cm at 10 A/dm 2 to 95 X 10 -6 ohm cm at 25 A/dm 2. Nevertheless, little change in grain size has been observed over this range of current densities. Instead, it may be relevant that this rise in resistivity is associated with an observed increase in the a m o u n t of impurities in the iron foil which occurs as the current density is increased. Increasing quantities of impurities are also known to increase the resistivity of metals [10]. Table 2 shows that the values of the magnetic properties decrease with increasing electrolyte temperature, the effect being particularly marked for hysteresis loss and relative permeability. This behaviour is again consistent with an observed increase in the average grain size of the foil with increasing electrolyte temperature. These results are in agreement with Bozorth's finding that an increase in grain size often causes a decrease in hysteresis loss [ 11]. Furthermore, Wolf has reported that the coercivity and remanence of electro-deposited metals can also be dependent upon electrolyte temperature [12] (and presumably, therefore, u p o n grain size). The former author has indicated that other
TABLE 1
Dependence o f resistivity o n electrolyte temperature for e l e e t r o f o r m e d i r o n foil o f a p p r o x i m a t e t h i c k n e s s 0 . 0 7 5 m m (current density = 10 A / d i n 2) Resistivity ( o h m c m ) x 10 -6 E l e c t r o l y t e t e m p e r a t u r e (°C)
60 86
55 96
30 108
TABLE 2 E f f e c t o f e l e c t r o l y t e t e m p e r a t u r e o n m a g n e t i c p r o p e r t i e s o f i r o n foil, o f a p p r o x i m a t e thickness 0 . 0 7 5 ram, a n d e l e c t r o f o r r n e d at a current density o f 20 A ] d m ~ ( N o t e : e d d y current loss at t h e f r e q u e n c y o f m e a s u r e m e n t ( 5 0 Hz) for t h e foil was negligible) Electrolyte temperature (°C)
Relative permeability
Saturation flux d e n s i t y (Wb m -2 )
Remanence (Wb m -~)
Coercivity ( A m -1 )
86 96 108
360 255 155
3.0 2.8 2.2
2.4 1.8 1.4
19.7 17.3 17.7
Hysteresis loss (J m -3 cycle -~ ) 715 560 370
241 magnetic properties may be similarly affected. Nevertheless, he emphasises t h a t the dependence of the magnetic properties on grain size is complicated by the variations in crystal orientation which arise with the m e t h o d of preparation of the material and by the inclusion of impurities. The complex mechanisms at work in electroforming and the practical problems of producing iron foil free of impurities may therefore hinder tight control over magnetic properties through the dependence of grain size upon electrolyte temperature. Nonetheless, the present work does indicate a definite effect of electrolyte temperature, and hence of grain size, upon these properties. Little change in the magnetic properties was noted over the current density range 10--30 A/dm. This observation is consistent with the micrographic evidence for negligible change in grain size with current density. Tables 2, 3 and 4 are useful for comparing the properties of a typical electroformed iron foil with those of other conventionally produced materials. From Table 3 the iron foil is seen to have a higher tensile strength, yield stress and ductility, but lower hardness and Young's modulus, than an annealed silicon steel. It also has a slightly greater tensile strength than a rolled tin plate, Because of the rising cost of aluminium, its replacement as a foil by iron has been considered. Table 3 shows t h a t the thinnest aluminium foil has a higher tensile strength and lower ductility than the iron foil, and t h a t on increasing the thickness of the aluminium, its mechanical properties become lower than those of the iron. Comparison of Table 1 with Table 4 shows that the resistivity of the iron foil is higher than that of commercially pure iron and t h a t an increase in electrolyte temperature can change the resistivity from a value smaller than, TABLE 3 Mechanical properties of electroformed iron foil and of conventionally produced metal
foil Material
Electroformed iron foil (temperature 108°C and current density 25 A/dm: ) Annealed 3.5% Si steel Rolled tin plate* Aluminium foil*
Thick- Tensile
H a r d - Yield
ness
strength
ness
(ram)
(MN/m2)
0.072
Young's modulus
(VHN) (MN/m2)
Ductility (percentage elongation)
371
147
364
5.1
140
0.045
352
163
336
2.76
191
0.013 0.017 0.02 0.05
325 595 315 160
190 ----
-----
-2.3 2.0 0.79
*Data from the British Steel Corporation.
stress
(GN/m ~)
I
m
*Data f r o m various sources.
10
14
--
--
-
48
--
-
60
0.075
Electroformed iron foil ( t e m p e r a t u r e 108°C and current density 10 A / d i n 2 ) Silicon i r o n * Pure iron* Commercially p u r e iron* L o w c a r b o n steel*
Resistivity ( o h m c m X 10 -6)
Thickness (ram)
Material
1.98
500--1500 104 150
155
Relative permeability
1.0
1.95 2.16 2.16
2.2
Saturation flux d e n s i t y (Wb m -s)
4 x 103
0.8 1.3 1.3
1.4
Remanence (Wb m -2)
24 0.8 7
17.7
Coercivity ( A m -1)
Electrical a n d m a g n e t i c p r o p e r t i e s o f e l e c t r o f o r m e d iron foil a n d o f c o n v e n t i o n a l l y p r o d u c e d f e r r o u s metals.
TABLE 4
35O
370
Hysteresis loss (J m - 3 cycle- 1
b~
bO
243 to an amount greater than, that of silicon iron. Finally, typical magnetic properties of other ferrous materials are presented for comparison in Table 4. The relative permeability of the electroformed iron is greater than that of its closest conventionally produced relations, pure iron, low carbon steel and commercially pure iron, whereas the saturation flux densities are comparable. The remanence of the two groups are similar, but the coercivity of the electroformed material is most comparable with that of silicon iron. Tables 2 and 4 indicate that the hysteresis loss of electroformed iron can resemble that of silicon iron or, depending upon the electrolyte temperature. (Although the tables provide a useful guide to the properties of the electroformed foil, it should be realised that a full comparison is difficult, because of the probable variations in specimen size and forms of testing used in compiling the results given in Tables 3 and 4). Acknowledgements
This project is supported by SRC Grant No. B/RG/8037.3. The investigation reported herein was partly reported in a paper presented at the IEE Conference on Electrical Methods of Machining, Forming and Coating, 1975 (Conference Publication 133) and the permission of the IEE to reproduce that work is gratefully acknowledged. The authors appreciate the interest and support of Professor T.M. Charlton, by courtesy of whom an Aberdeen University Engineering Department Research Studentship~is held by one of them (S.H.F.L.). Mr. W.A. Crichton is thanked for providing the information on grain size and Mr. N. Milne for some useful comments. References 1 2 3 4 5 6 7 8 9 10 11 12
E.C. Larke, The Rolling of Strip, Sheet and Plate,Chapman and Hall, London, 1963. H.M. Finniston, Philos. Trans. R. Soc. London, A, 275 (1973) 313--327. See, for example, Phys. Bull.,26 (1975) 229. H. Silman, in J.D. Thornton (Ed.), Inst. Chem. Eng. Symp. Series,Electrochemical Engineering, 1 (1973) 1195--1211. S.H.F. Lai, M.Sc. Thesis, Aberdeen University, 1974. S.H.F. Lai and J.A. McGeough, J. Mech. Eng. Sci., 18 (1976) 19--24. P. Lau, M.Sc. Thesis, Aberdeen University (to be submitted). BritishStandards Institution No. 601, Part 5, 1973. M. Schwartz and G.D. Mallory, J. Electrochem. Soc., 123 (1976) 606--614. K.J. Pascoe, Properties of Materials for ElectricalEngineers, John Wiley and Sons, London, 1972, Ch. 10. R.M. Bozorth, Ferromagnetism, Van Nostrand, London, 1957, Ch. 4, p. 87. I.W. Wolf, J. Appl. Phys., 33 (1966) 1152--1159.