Tamer Elshenawy,Salah Soliman,Ahmed Hawwas

Technical Research Center,Cairo,Egypt

Influence of electric current intensity on the performance of electroformed copper liner for shaped charge application

Tamer Elshenawy*,Salah Soliman,Ahmed Hawwas

Technical Research Center,Cairo,Egypt

Electrolytic Copper used in the shaped charge liner manufacturing can be produced from acid solution using electro-deposition technique.The intensity of the applied electric current controls the quality of the produced copper grade.The electric current intensity within the electrolytic acidic solution cell with the minimum oxygen and sulfur elements in the produced copper was optimized and found to be 30-40 A/Ft2.The elemental composition of the obtained electrolytic copper was determined using highend stationary vacuum spectrometer,while the oxygen was determined precisely using ELTRA ONH-2000 apparatus.Besides,SEM was used to investigate the shape of the copper texture inside the deposited layers and to determine the average grain size.New relations have been obtained between the applied current intensity and both the oxygen and sulfur contents and the average grain size of the produced copper.Experimental result showed that when the applied current density increases to a certain limit,the oxygen and sulfur content in the electrolytic copper decreases.Performance of the produced copper liner was investigated by the static firing of a small caliber shaped charge containing an electro-formed copper liners,where the penetration depth of the optimized electrolytic liner was enhanced by 22.7%compared to that of baseline non-optimized liner.

1.Introduction

The electrochemical process,which involves the deposition of ametallic anode on a different metal cathode by passing a specified current-through a solution called an electrolyte is called electroplating.The electrolyte allows the transfer of metal ion from anode to the desired cathode depending on the solution concentration and temperature,type and surface area of both anode and cathode and the applied current intensity per unit area of the cathode[1].Many advance in electroplating technology,which enhances the application of electroplating industry in different industry was discussed in details in Ref.[2].Different factors,which affect the electroplating process have been discussed by many researchers.Anik et al.found that the deposition was greatly affected by PH values of the electrolyte solutions that was found to be 9 for effective throwing power[3].The variation of the PH values between the cathodic films and the bulk electrolyte will enhances the production of the undesired result like spongy or powdered deposits[4].In the electroplating process,the amount of material,which will be deposited on the cathode will be varied according to the applied current density.Different metal thickness and the formation of undesired result as pores cracks and other irregularity will result according to the non-optimised current density[4,5].Operating temperature is an effective tool for controlling the deposition rateand the qualityof plate.Higheroperating temperate can be used for decorative plating as it required high deposition rate[6].It was found that the higher the temperature of the copper solution,the higher the throwing power of this solution[7].Furthermore,agitation rate and the concentration of bath will affect the deposition rate and the structure of copper metal deposited[2,6].

Recently,researchershave shownincreased interest in the effect of copper material impurities on the ductility of the copper used as shaped charge liners.In 2003,Schwartz et al.[8].described the dependence of cooper ductility on the total type and number of impurity atoms.The copper used was 4N(99.99%)purity and this liner was manufactured by cold forging technique to extrude it to a hollow cone shape.After the cold forging process,the produced liners are annealed at 315°C for one hour or 400°C for 10 min to stabilize the microstructure of sulphur doping.It has been shownthat the presence of impurities such as oxygen and sulphur decreases the ductility of the copper due to the segregation of the impurities at the grain boundaries[8].Therefore,the breakup time significantly decreases and hence the predicted penetration depth will be affected based on the total number of sulphur impurities present in the copper liner[8].

Fujiwara and Abiko[9]performed experiments on the ultrahigh purity copper in order to investigate the effect of impurity presence and operating temperatures on the copper ductility.In this study,the ultra-high purity copper was produced by electronic beam refining and vacuum melting technique.The tensile test was performed on the ultra-high purity copper 6N,8N and compared with commercial purity copper rod 3N(99.9%)under high vacuum of 7×10-4Pa at a strain-rate of 4.2×10-5s-1.The average grain size for the three copper specimens was 30,50 and 100μm for 3N,6N and 8N,respectively[9].This implies that the copper impurities have a significant effect on its mechanical properties and performance as a shaped charge liner.

2.Electroformed copper liner manufacturing

By application of different current densities for designed electroplating lab scale cell,a very fine grain liner is produced by the anode electron deposition(sometimes referred as electro-plating)of pure copper on a polished stainless steel(sometimes lead cathodes are used).The anode material is 4N pure copper,while the electrolytic solution CuSO4.7H2O(300 g/l)was used.A concentrated sulphuric acid(180-270 g/L)was added to increase the solution conductivity and increase the throwing power of the electrolytic cell.The substrate,on which the copper ions will be deposited,is a stainless steel(or lead)conical shape mould treated mechanically and chemically to allow the separation of the formed copper liner from it.A symmetrical liner wall thickness at different distances from its axis is obtained using a DC step motor,which produces fine and more equi-axial pure copper texture.Detailed procedures for the production method of the electrolytic copper liner was explained in Ref.[10].Fig.1 shows the electroplating cell used for manufacturing the copper liner.

Advantages of this technique are the produced small grain size and rotational symmetric liner structure around its axis.Fig.2 shows the cathode mould and the produced electroformed optimized copper liner before outer surface machining and polishing.

3.Current density effect on the concentration of oxygen and sulfur for the prepared copper liner

New relation have been obtained between the current density and the oxygen percentage on the resulted copper as shown in Fig.3.As the current density increased till the 80 APSF value the amount oxygen produced decreased.This reveals the advantage of using the 80 APSF as the operation current density for copper liner production using electroplating process.Also ductility enhancement when using 80 APSF compared with low current density process is predicted.

By plotting the current density against sulphur concentration shown in Fig.4,linear relation between sulphur content and the current density has been obtained showing that as the current density increases,the sulphur concentration decreases.The obtained relation showed the dependence of the sulphur content on the current density as described by a liner approximation.

Figs.3 and 4 show that the higher the current density,the better the performance of the produced liner.However,the higher current density greater than 50 APSF produces a spongy like shape as shown in Fig.5 for different current densities applied to electroplating process,which means the electric current intensities above 50 APSF were abandon for their unaccepted disadvantages.

4.Grain size measurements for prepared copper liner

Different steps have been carried out for measuring the grain size of the produced copper liner,firstly,the copper material is first cut into a convenient size then hot mounted(about 150°C)using a mounting press in a thermo softening plastice.g.acrylic resin,after which mounted specimens are ground with rotating discs of abrasive paper of 1200 grit.Etching is necessary to reveal the microstructure of the copper through selective chemical attack.Saturated aqueous solution of ferric chloride,containing a few drops of hydrochloric acid is used as a copper convenient etchant material.Care must be taken to avoid over etching of the copper material caused by localized chemical attack if the etching time is too long.This over etching may generate pits,which tend to grow,and obscure the main features to be observed,and in most cases they do not represent features of the microstructure.

The deposited copper layers were first deposited on the outer surface of the cathode,after which the layers start to accumulate parallel to the initial surface as shown in Fig.6.The liner structure was produced by subsequent cycles of electroforming deposition to complete the liner thickness(1.2 mm).The growth rate was occurred at the room temperature(25°C),in which copper deposited material prefer to grow in the AB direction.

The fine and equi-axial microstructure of the electro-deposited copper is shown in Fig.6,wherethe average grain size of the copper material produced by this technique measured by SEM was found to be of in the range of 0.5-2μm(Fig.7)depending on the applied electric current intensity within the electrochemical cell as shown in Fig.8.Thisfigureproved theinverse relation between the current intensity and the produced grain size.For penetrative shaped charge with longer breakup times,small average grain sizes are preferred asshown in Refs.[8,10,11]forcoppermaterials.Nevertheless,current intensities higher than 50 APSF are not preferred for their disadvantages such as porosity and spongy texture,thus optimum current intensity range that is recommended is 30:40 APSF.

5.Performance test using a prepared shaped charge copper liner

Two shaped charge liners have been tested for their performance due to the optimized electrolytic copper liner.The first was the baseline non-optimized liner of random non-uniform current intensity(i.e.60:70 APSF);while the other was the optimized one(precisely 30APSF applied current intensity).Both of them was fitted within small caliber shaped charge of caliber 30 mm loaded with 30 gm of C4 plastic explosive of detonation velocity of 7100 m/s.both shaped charges were fired against laminated RHA of thickness 2.54 cm each.Results show that the optimized liner has an enhanced penetration depth of 12.88 cm rather than 10.50 cm for the baseline traditional liner(i.e.22.7%better than that of the baseline liner)as shown in Fig.9.

6.Conclusions

Linear relationships have been obtained between the applied current intensity and the resultant sulphur and oxygen contents within the electroformed copper liners.Besides,different copper average grain sizes have been produced for various current intensities.Experimental tests for the penetration of the prepared copper linershave been carried out against RHA armour targets and showed 22.7%increase in the penetration depth for the optimized copper liner when compared to the baseline non-optimized one.

Acknowledgement

The authors would like to thank Dr Tamer Ezz and Dr Mahmoud Anwar for their assistance to do the SEM measurements.

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13 February 2017

in revised form 12 April 2017 Accepted 24 May 2017 Available online 27 May 2017

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Corresponding author.

E-mail address:tamershenawy@yahoo.com(T.Elshenawy).

Peer review under responsibility of China Ordnance Society.