Module 9 Non conventional Machining. Version 2 ME, IIT Kharagpur

Transcription

1 Module 9 Non conventonal Machnng

2 Lesson 38 Electro Chemcal Machnng

3 Instructonal bjectves () () () (v) (v) (v) (v) (v) (x) (x) (x) Identfy electro-chemcal machnng (ECM) as a partcular type of non-tradton processes Descrbe the basc workng prncple of ECM process Draw schematcally the bascs of ECM Draw the tool potental drop Descrbe materal removal mechansm n ECM Identfy the process parameters n ECM Develop models for materal removal rate n ECM Analyse the dynamcs of ECM process Identfy dfferent modules of ECM equpment Lst four applcaton of ECM Draw schematcs of four such ECM applcatons 1. Introducton Electrochemcal Machnng (ECM) s a non-tradtonal machnng (NTM) process belongng to Electrochemcal category. ECM s opposte of electrochemcal or galvanc coatng or deposton process. Thus ECM can be thought of a controlled anodc dssoluton at atomc level of the work pece that s electrcally conductve by a shaped tool due to flow of hgh current at relatvely low potental dfference through an electrolyte whch s qute often water based neutral salt soluton. Fg. 1 schematcally shows the basc prncple of ECM. In ECM, the W workpece T s connected to the postve termnal W of a low T voltage hgh current DC generator or power source. The tool s shaped and shape of the tool s transferred R to the workpece. The tool s connected to the negatve termnal. R Machnng takes place due to anodc electrolyte dssoluton at atomc level of the work materal K due to electrochemcal L reacton. A gap between the tool K and the L workpece s requred to be mantaned to proceed wth steady state machnng. Intal stage of ECM Steady state of ECM Fg. 1 Schematc prncple of Electro Chemcal Machnng (ECM) 2. Process Durng ECM, there wll be reactons occurrng at the electrodes.e. at the anode or workpece and at the cathode or the tool along wth wthn the electrolyte. Let us take an example of machnng of low carbon steel whch s prmarly a ferrous alloy manly contanng ron. For electrochemcal machnng of steel, generally a neutral salt soluton of sodum chlorde (NaCl) s taken as the electrolyte. The electrolyte and water undergoes onc dssocaton as shown below as potental dfference s appled NaCl Na + + Cl - H 2 H + + (H) -

4 As the potental dfference s appled between the work pece (anode) and the tool (cathode), the postve ons move towards the tool and negatve ons move towards the workpece. Thus the hydrogen ons wll take away electrons from the cathode (tool) and from hydrogen gas as: 2H + + 2e - H 2 at cathode Smlarly, the ron atoms wll come out of the anode (work pece) as: Fe Fe e - Wthn the electrolyte ron ons would combne wth chlorde ons to form ron chlorde and smlarly sodum ons would combne wth hydroxyl ons to form sodum hydroxde Na + + H - NaH In practce FeCl 2 and Fe(H) 2 would form and get precptated n the form of sludge. In ths manner t can be noted that the work pece gets gradually machned and gets precptated as the sludge. Moreover there s not coatng on the tool, only hydrogen gas evolves at the tool or cathode. Fg. 2 depcts the electro-chemcal reactons schematcally. As the materal removal takes place due to atomc level dssocaton, the machned surface s of excellent surface fnsh and stress free. e W R K Fe ++ Fe ++ H Na + FeCl 2 H Cl - H + H Fe ++ H + Fe(H) 2 Na + H H 2 T L e H Cl - Fg. 2 Schematc representaton of electro-chemcal reactons The voltage s requred to be appled for the electrochemcal reacton to proceed at a steady state. That voltage or potental dfference s around 2 to 30 V. The appled potental dfference, however, also overcomes the followng resstances or potental drops. They are: The electrode potental The actvaton over potental hmc potental drop Concentraton over potental hmc resstance of electrolyte Fg. 3 shows the total potental drop n ECM cell.

5 Anodc overvoltage Voltage Anode potental Actvaton over potental ohmc potental concentraton potental concentraton potental hmc drop actvaton overpotental cathodc potental Voltage cathodc overpotental anode cathode Fg. 3 Total potental drop n ECM cell 3. Equpment The electrochemcal machnng system has the followng modules: Power supply Electrolyte fltraton and delvery system Tool feed system Workng tank Fg. 4 schematcally shows an electrochemcal drllng unt. Flow control valve Pressure relef valve Flow meter Pressure gauge Constant feed to Tool the tool -ve PS Low voltage +ve hgh current power supply Pump M Spent electrolyte centrfuge Flters M P sludge Fg. 4 Schematc dagram of an electrochemcal drllng unt

6 4. Modellng of materal removal rate Materal removal rate (MRR) s an mportant characterstc to evaluate effcency of a non-tradtonal machnng process. In ECM, materal removal takes place due to atomc dssoluton of work materal. Electrochemcal dssoluton s governed by Faraday s laws. The frst law states that the amount of electrochemcal dssoluton or deposton s proportonal to amount of charge passed through the electrochemcal cell, whch may be expressed as: m Q, where m mass of materal dssolved or deposted Q amount of charge passed The second law states that the amount of materal deposted or dssolved further depends on Electrochemcal Equvalence (ECE) of the materal that s agan the rato of the atomc wegh t and valency. Thus A m α ECE α Thus m α QA ν where F Faraday s constant coulombs ItA m Fν ν m IA MRR tρ Fρν where I current ρ densty of the materal The engneerng materals are qute often alloys rather than element consstng of dfferent elements n a gven proporton. Let us assume there are n elements n an alloy. The atomc weghts are gven as A 1, A 2,.., A n wth valency durng electrochemcal dssoluton as ν 1, ν 2,, ν n. The weght percentages of dfferent elements are α 1, α 2,.., α n (n decmal fracton) Now for passng a current of I for a tme t, the mass of materal dssolved for any element s gven by m Γa ρα where Γ a s the total volume of alloy dssolved. Each element present n the alloy takes a certan amount of charge to dssolve. QA m Fν Fmν Q A FΓa ραν Q A The total charge passed

7 Now Q Q T T Γ MRR t It Q αν It FΓ a ρ A a 1 I Fρ. αν A 5. Dynamcs of Electrochemcal Machnng ECM can be undertaken wthout any feed to the tool or wth a feed to the tool so that a steady machnng gap s mantaned. Let us frst analyse the dynamcs wth N FEED to the tool. Fg. 5 schematcally shows the machnng (ECM) wth no feed to the tool and an nstantaneous gap between the tool and workpece of h. electrolyte job tool dh h Fg. 5 Schematc representaton of the ECM process wth no feed to the tool Now over a small tme perod dt a current of I s passed through the electrolyte and that leads to a electrochemcal dssoluton of the materal of amount dh over an area of S V V I R rh s Vs rh then dh dt 1. F A ρν x x Vs 1. rh s

8 1 A x V.. F ρν x rh for a gven potental dfference and alloy dh A xv 1 c. dt F ρν xr h h where c constant A xv Fρν xr V c αν Fρr A dh c dt h hdh cdt At t 0, h h o and at t t 1 and h h 1 h 1 hdh c dt ho t h1 ho 2ct That s the tool workpece gap under zero feed condton grows gradually followng a parabolc curve as shown n Fg. 6 h h o t Fg. 6 Varaton of tool-workpece gap under zero feed condton As dh dt c h Thus dssoluton would gradually decrease wth ncrease n gap as the potental drop across the electrolyte would ncrease

9 Now generally n ECM a feed (f) s gven to the tool dh c f dt h Now f the feed rate s hgh as compared to rate of dssoluton, then after sometme the gap would dmnsh and may even lead to short crcutng. Under steady state condton the gap s unform.e. the approach of the tool s compensated by dssoluton of the work materal. Thus wth respect to the tool, the workpece s not movng dh c Thus 0 f dt h c f h or h* steady state gap c/f Now under practcal ECM condton t s not possble to set exactly the value of h* as the ntal gap. Thus t s requred to be analysed f the ntal gap value would have any effect on progress of the process dh c Now f dt h h hf Now h ' h * c 2 ft f t And t' h * c dh' f / c dh 1 dh.. 2 dt' f / c dt f dt dh c Thus f dt h dh' c cf f f f dt' h'h * h' c dh' 1 h' f f dt' h' dh' 1 h' dt' h' h' dt' dh' 1 h' Now ntegratng between t 0 to t t when h changes from h o to h 1 t' h1' h' dt' dh' 1 h' 0 t ' ho' h1' ho' ( h' ) ( 1 h' ) d 1 h1' + d 1 ' ho' ( h' ) ' ' ho 1 t' ho h1 + ln ' h1 1 now for dfferent value of h o, h 1 seems to approach 1 as shown n Fg. 7

10 Smulaton for h o ' 0, 0.5, 1, 2, 3, 4, 5 h 1 ' h h t' Fg. 7 Varaton n steady state gap wth tme for dfferent ntal gap Thus rrespectve of ntal gap h fh h ' 1 1 h * c c h f c A xv 1 or f. h Fρν r h A x f Fρν A x f Fρν x x x V. rh I. s A x. Fρν s x MRR n mm / s Thus t seems from the above equaton that ECM s self regulatng as MRR s equal to feed rate. 6. Applcatons ECM technque removes materal by atomc level dssoluton of the same by electrochemcal acton. Thus the materal removal rate or machnng s not dependent on the mechancal or physcal propertes of the work materal. It only depends on the atomc weght and valency of the work materal and the condton that t should be electrcally conductve. Thus ECM can machne any electrcally conductve work materal rrespectve of ther hardness, strength or even thermal propertes. Moreover

11 as ECM leads to atomc level dssoluton, the surface fnsh s excellent wth almost stress free machned surface and wthout any thermal damage. ECM s used for De snkng Proflng and contourng Trepannng Grndng Drllng Mcro-machnng Tool Work De snkng 3D proflng Fg. 8 Dfferent applcatons of Electro Chemcal Machnng tool drllng work (drllng)

12 tool work trepannng Fg. 9 Drllng and Trepannng by ECM 7. Process Parameters Power Supply Type drect current Voltage 2 to 35 V Current 50 to 40,000 A Current densty 0.1 A/mm 2 to 5 A/mm 2 Electrolyte Materal NaCl and NaN 3 Temperature 20 o C 50 o C Flow rate 20 lpm per 100 A current Pressure 0.5 to 20 bar Dluton 100 g/l to 500 g/l Workng gap 0.1 mm to 2 mm vercut 0.2 mm to 3 mm Feed rate 0.5 mm/mn to 15 mm/mn Electrode materal Copper, brass, bronze Surface roughness, R a 0.2 to 1.5 μm

13 Quz Test 1. For ECM of steel whch s used as the electrolyte (a) kerosene (b) NaCl (c) Deonsed water (d) HN 3 2. MRR n ECM depends on (a) Hardness of work materal (b) atomc weght of work materal (c) thermal conductvty of work materal (d) ductlty of work materal 3. ECM cannot be undertaken for (a) steel (b) Nckel based superalloy (c) Al 2 3 (d) Ttanum alloy 4. Commercal ECM s carred out at a combnaton of (a) low voltage hgh current (b) low current low voltage (c) hgh current hgh voltage (d) low current low voltage Problems 1. In electrochemcal machnng of pure ron a materal removal rate of 600 mm 3 /mn s requred. Estmate current requrement. 2. Composton of a Nckel superalloy s as follows: N 70.0%, Cr 20.0%, Fe 5.0% and rest Ttanum Calculate rate of dssoluton f the area of the tool s 1500 mm 2 and a current of 2000 A s beng passed through the cell. Assume dssoluton to take place at lowest valency of the elements. A N ρ N 8.9 ν N 2 A Cr ρ Cr 7.19 ν Cr 2 A Fe ρ Fe 7.86 ν Fe 2 A T 47.9 ρ T 4.51 ν T 3 3. In ECM operaton of pure ron an equlbrum gap of 2 mm s to be kept. Determne supply voltage, f the total overvoltage s 2.5 V. The resstvty of the electrolyte s 50 Ω-mm and the set feed rate s 0.25 mm/mn.

14 Answers Answers to Quz Test 1 (b) 2 (b) 3 (c) 4 (a) Soluton to Prob. 1 m AI MRR m. t Fν m AI MRR. Γ ρt Fρν MRR 600 mm 3 /mn 600/60 mm 3 /s 10 mm 3 /s 10x10-3 cc/s 56xI 10x x7.8x2 As A Fe 56 ν Fe 2 F coulomb ρ 7.8 gm/cc x10x10 x7.8x2 I 56 I A Answer Soluton of Problem 2 Now, Now 1 ρ alloy α ρ 1 αn α Cr αfe α T ρn ρcr ρfe ρt gm / cc m I MRR ρt αν Fρ A x x8.07x cc/sec 2.14 cc/mn 2140 mm 3 /mn 0.2x x2 0.05x

15 MRR 2140 Rate of dssoluton 1.43 mm / mn answer Area 1500 Soluton to Prob. 3 c h * f VAFe where c FρFerν Fe ( V 2.5) x55.85 C x7.8x10 x50x2 ( V 2.5) c ( V 2.5) h* 2 f x 60 V V 8.73 Volt. Answer