COATINGS: LS'I‘:RSP:SPRAYING:WE}LDING:CLADDING & DIFFUSION METHODS
`
`gas annular blast at the lower end of the melt crucible as it
`emerges through a hole into a reduced pressure area containing
`the substrate to be coated (Dunstan ef al l985). The droplets
`undergo in—flight cooling at 10
`to 10
`degxvsecond followed by
`a very fast deposition cooling and slow subsequent cooling.By
`controlling these stages a very thin liquid film is maintained on
`the surface of the coated substrate. Dendrites form in flight and
`shatter on impact, giving many nucleation sites and a fine equi~
`axed structure.One of the authors (M.G.HJ has seen this Osprey
`process and strongly recommends it for development and applica-
`tion.
`
`6.7. COATING BY %LD1B£
`
`6.7.1. GENERAL:
`
`Surfacing by welding is one of the most widely practised bulk
`coating techniques.Cladding methods form the other group.Fig.
`6»l9 gives a comparative idea of the properties of major hard-
`facing alloys. The major welding processes currently used for
`surfacing are:
`
`(MIG),
`Gas, Powder, Manual Metal Arc (MMA), Metal— Inert Gas
`Tungsten—Inert Gas MTG)
`(which includes Plasma—Arc), Submerged
`Arc and Friction Welding.
`
`Low s————-———-—— Abrasion Resistance —-—~—e High
`
`‘
`
`14°/aM -1-2i%C
`
`Arc
`
`.
`
`_ tube carbide _
`
`acetyiane
`tube carbide
`
`Cr-steels
`
`l
`
`Austenitic
`stainless
`steels
`
`’
`
`High SP9‘-ad Steels
`steels
`
`Austenitic
`&
`.
`.
`afifrftsensmc
`
`Heat and
`C 0 rm 5 i on
`R B SiSt8.nCe
`
`’
`
`Co- 8: Ni-base alloys
`
`High .e....____——.._ Impact Resistance —————_——.as Low
`
`Fig.6-19: Position profiles of the major types of hardfacing
`
`106
`
`

`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`The tern1‘surfacing' is generally used where coatings are thicker
`than 1 mm on the substrate,
`to greater than 8 mm. Several
`layers
`can be applied by welding techniques,
`the thickness restricted
`only by the mechanical and physical limitations specified for the
`application of the product material
`i.e.
`the coated substrate.
`
`Bulk coating methods use coating material in the form of powder,
`sheet, strip,
`rod or paste, applied usually close to or above the
`melting point of the substrate—coating configuration. Amongst
`the bulk coating methods, welding, diffusion bonding and HIP
`methods result in an ig_situ diffusion bond unlike that obtained
`from overlay and other deposition methods. Bonding in overlay and
`other methods can occur by mechanical keying or over a period of
`time, by metallurgical bonding via thermal heat treatments, while
`bulk coating is wholly a metallurgical
`'bond‘
`formed during the
`liquid state with a good fraction of it containing the substrate
`material. Bulk coatings are applied to counter heavy wear and for
`use under conditions which impose mechanical and thermal shock.
`They have to be tough so as not
`to break under these conditions,
`apart from being metallurgically and chemically compatible to the
`substrate and resistant to the working environmenta The thickness
`is a compromise for the wear and tear demanded during service and
`the utility not
`too dependent on significant dimensional varia-
`tions as the coating wears. It is essential to achieve an opti~
`mised coating for the design service life, in View of the cost
`and bulk involved.
`
`6l?,2e WELD SURFACXEG
`
`A classification formulated by the British Steel Corporation may
`be considered representative of surfacing alloys normally used
`for coating by welding (Gregory 1980). The selection of
`the
`surfacing alloy involves three parameters at the outset, namely,
`the hardness,
`the purpose,
`and abrasion and/or oxidation resist-
`ance. Compatibility in characteristics with the substrate metal
`is the next
`important factor as only this ensures the integrity
`of fornxand performance of the finished component. In general,
`ferrous alloys are employed for abrasion resistance, nickel and
`cobalt alloys for oxidation and corrosion resistance, copper
`alloys for bearings, and, WC, chromium boride and similar com~
`pounds for wear resistance.
`
`The process is known severally as hardfacing, weld-cladding,
`overlaying and by other proprietary names. Welding is particular~
`ly suited for local repair work of damaged bulk—coated compo-
`nents. It essentially involves melting the coating material with
`or without a flux and inert gas, on the substrate to be covered.
`
`107
`
`

`
`COATINGS: LST:RSP:SPRAYlNG:WELDING:CLADDING & DIFFUSION METHODS
`
`fiB7v3w EELD SURFACING PARAMETERS
`
`The foremost aspect is the dilu-
`tion which results on welding.
`When molten weld metal contacts
`the substrate the substrate sur-
`face also melts and the resul~
`
`\NmdBemj
`
`PWWWN Mam
`
`ting mixture is contained in the
`weld pool or
`the weld bead
`(Fig.6—2D). Dilution is defined
`as 100A A+B
`whereA is the
`amount 5; weld metal and B is
`the substrate metal
`in the weld
`pool. On solidification there
`will be a zone in between the_
`weld and the substrate which has
`a composition different from both the weld metal and the subs-
`trate. There will be an effective reduction in the original
`substrate thickness, but the total surfaced material will be
`thicker with the modified welded material bonded to the subs~
`trate. Dilution can vary from lO~40% and can be reduced to even
`5% depending on the welding method used.
`
`,.
`%’D”mmn==(A X‘Om“A'*B)
`F@l}20:DHmmn m a\NeM Bead
`
`The second parameter is the rate of deposition. The relative mass
`of coating dealt with in bulk surfacing far exceeds the normal
`rates ofildepositon. The latter are usually expressed in mg or g
`_ hr- while surfacing is defined in kilograms per hour.
`
`687943 EELD SURFACIEG METHODS
`
`Table 6:12 and Fig. 6-21 give some of the features of seven
`welding processes commonly used, Several variations on these
`basic themes are in practice. Oxy—acetylene methods are the
`slowest of all the methods causing also the minimum of dilution,
`as the flame temperature is much less than are temperatures. The
`feed material is a rod or powder and lands on the substrate in a
`small pool or as molten droplets respectively, which causes
`restrained fusion and hence there is very little or virtually no
`loss of the substrate into the weld zone.A precise control and
`on~site welding is possible but the methods are unsuitable for
`large scale operations.
`
`Very high temperatures are encountered in arc welding but TIG
`welding using a nonmconsumable W electrode is similar in scope to
`gas—welding in precision and operation, but somewhat higher in
`dilution and deposition rate. The plasma arc method improves on
`TIG— welding by employing two arc routes,
`the major arc current
`being the nonwtransferred arc passed between the W electrode and
`a water—cooled copper annulus around it, while the transfered arc
`passes between the electrode and the workpiece via the hot plasma
`issuing from the torch. Unlike TIG, plasma arc can cover large
`areas with precision and reguires minimum finish. But it cannot
`lay down a coating more than about 1,5 mm thick‘
`
`27?
`
`108
`
`

`
`ogmnwmgoenzm
`
`0.2
`
`<22
`
`~
`
`|NoomIuLu:_m+mE
`
`
`
`m1xu_+muugm
`
`oLmnu_L+oo_m
`
`-umo_o_;m:mmm
`
`
`
`_m+wEuou_o_;m
`
`mL_3m:o::_+coo
`
`
`mamam:0:cfl+co0
`
`cmLm>oo:x:_m
`
`m_nmm,:m:oms
`
`umm_cm:umE>..:¢
`tweenLO_m::m:
`
`
`
`_m:cmzm_nmm_cm;omz
`
`m_nm+Louc:oz
`
`m_am+Loa
`
`¢._+mmLm>umm>
`®«Dm%LOQICOZ
`
`N“
`
`
`
`L0_LQmLo++mumw
`
`
`
`+~m0umux:_+
`
`mvoL+uo_m
`
`“muoL+uo_mveg
`
`
`
`no;Lm_:n:+
`
`
`
`ecoomooL+om.m
`
`x:_+mc_c_m+
`
`omro.
`
`hx—
`
`m.NA
`
`m._v
`
`.;m#c_»
`
`
`
`nmw:m_n_xm:
`
`
`
`m®L_3ummwmm
`
`@Wm®LU:_coo
`
`zxmx~_o+m+mL
`
`
`
`-u.m_;m+:o;+Mz
`
`.mmmmc_
`
`o.n_mmoa
`
`
`
`@C_L_3U@Lum>o_com
`
`x:_¢.u_mgm
`
`+_moamumeow>L®>
`com.mL000
`
`
`
`m>vL:+m>Lm>
`
`L0_mo_+Lw>
`
`mom+Lsmm_++__
`
`
`ow_m.mL_3umLoo
`
`m:Mu_m3umm;Lm>o
`
`mwwuoLa
`
`ogmnm
`
`m2WDDaAuIQAmB
`
`nm4m<P
`
`.mmwI+;m:_r*m+mz
`oL<:"m+m2:_m::m:
`+0:o_+m_._m>
`I+L@:_|E®+W@c3F
`
`o_~
`
`
`
`“oL<13ummwAmmo
`
`oLm:omL<
`
`m:~n_m2ummw
`:3ocx1om_<
`
`“mm
`
`
`
`ognmEmm_m
`
`w_k
`
`w:m_>+momI>xo
`ummmuogm
`
`m.nm+Loa:coz
`
`
`
`o_nm+Loa.mo»
`
`
`
`®_Dm+LOQ.mm>
`
`um+_mI:0
`
`-u_mz
`
`m_nmm_cmnumE
`
`_m::m2
`
`LO_mu:mz
`
`
`
`_a:cmE>__oc;
`
`um¢>»
`
`L@U30&
`
`uom
`
`su.mz
`
`
`
`comL<.uLmmc+
`
`cm+_o£.;uLo+
`
`
`IUDLwm+m:_ugo:ou
`
`o+:_um+com
`
`m:+o+c_>+_>mgm
`
`
`
`Lmv_o;.+mo;
`
`pm_=:mLO
`
`+um+coum+mMaoLu
`
`o+c,m+mLsumm+
`
`.mzVmam+Lmc_
`.mu_m_;mAw:
`
`_ooaIuwm3
`
`m+mL+mn:m
`
`su_mxmmoEm.+
`
`.mmo:m>um_ooa
`
`mvoL+ow_@3m_nwEamcour:oz
`m_>um+Lmvzom
`
`>9Lmaaocm
`mu_>oLa0+WULDOW
`
`_,c>._m:+g_>
`
`m:mo.o
`
`m.o
`
`mamsou»mEm_m
`umnzomoogm
`
`3,.
`
`“EEmmmcxu_;+
`
`:o_+_moumo
`
`|_m_Lm+mE
`
`uegom
`
`.ak:o,+:__a
`
`:o~+_moamQ
`
`
`
`ugxmxw+nL
`
`109
`
`

`
`a. Oxy—acetylene hardfacing
`
`b. Manual metal-arc (MMA)
`harclfacing
`15
`
`id. Tungsten inert-gas (TIG)
`hardtacing
`
`Fig.6-21: Weld Surfacing Techniques
`
`ti. Manual Metal Arc (MMA);
`Weld Surfacing Techniques I a: Oxy-acetylene;
`c. Metal
`lnert—gas (MIG; d. Tungsten inert-gas{TlG);
`a. submerged are
`a. 1.Parent metal, 2. Weld metal, 3. Filler rod. 4. Blowplpe nozzle
`(introducing oxy-
`acetylene gas mixture, 15. Welding direction
`b. 1,2 ,t5 ~ as in a; 5. Slag, 6. Consumable electrode (core wire with a flux cover),
`7. Gas shield, 8. weld pool
`c. 1,2,B,7,8,t5 as in a,b above; the consumable electrode is fed through a contact
`which also carries the protective shielding gas supply
`d.
`i,2,3,7,8,t5 — as in a,b, above; 9. Non»-consumable electrode with a supply of
`shielding gas in a co-axial tube. 13. see ‘:2’ below.
`e. 1,2,5,8,15 - as in a, b above;
`10. Excess weld metal, 11. Granular Flux, 12. Flux
`Tube feed, 13. Gas shield, 14. Arc submerged in the excess weld pool
`
`tube
`
`110
`
`

`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`MMA and MIG methods are more versatile and sturdy, capable of
`very high deposition rates. Vertical or overhead runs are possi-
`ble and on~site operations. The flux~cored arc welding method can
`give thicker deposits and higher deposition rates using flexible
`feed wire packed with its flux and additives. A fully mechanised
`but strictly workshop method is submerged—arc welding. Granular
`flux is deposited ahead of a continuous wire and the arc submer—
`ges, under the flux resulting in a weld surface requiring mimimum
`finish. Friction welding or surfacing is performed by pressing a
`rotating metal rod with its axis at right angles to a laterally
`moving substrate.
`
`A multiple strip weld surfacing schematic is shown in Fig. 6522.
`
`4. strip sheet electrodes
`3. are
`2. weld overlay
`1. Parent metal
`passed through feed rollers 5. are current source
`
`Flg.6~22. Schematic of multiple strip weld surfacing
`
`5.7.5. WELD COATED FIEISH
`
`Weld coated surfaces are usually wavy and slightly convoluted
`ending with a surface roughness of up to 2 mm difference between
`the maximum and minimum thickness. The wave nature is due to the
`electrode of a limited width having to pass to and fro on the
`work surface. Surface tension of the weld pool settling on the
`substrate which subsequently solidifies as the electrode passes
`on, causes an unevenness in the weld—coated surface.This sur~
`face roughness requires to be smoothed out by machining, grind-
`ing, polishing, or light abrading, For components like bulldozer
`blades or rock Crushers this may not be necessary, but it is
`essential for gear wheels or valve seats.
`
`Shrinkage effects during cooling can distort the coated component
`as stresses are created.lBending for instance, can be countered
`by offering the work pre—bent
`in the opposite direction along the
`same axis. Where thick layers or heavily restrained coatings are
`involved cracking is a possible or
`inevitable hazard. This can be
`compensated by applying an intermediate coat of soft~meta1
`(ductile) and by intermittent applications with pre-heating of
`components especially when hard and brittle alloys are deposited.
`
`286
`
`111
`
`

`
`COATINGS : LST:RSP:SPRAYING :WELDING:CLADDING 8: DIFFUSION METHODS
`
`6 . 3 . CLAD SURFACIEEG
`
`Cladding is a broad term used which includes methods such as
`explosive impact and magnetic impact bonding, or hot isostatic
`pressing (HIP) or cladding {BIC}, or mechanical bonding such as
`extrusion. This would introduce an overlapping between cladding
`and diffusion bonding when the above methods have to categorized.
`
`Cladding methods can be classified on the basis of the speed with
`which substrate to coating material bonding can be achieved.
`Table 6:13 shows the three groups with the bonding they achieve
`and other details (Bucklow l983). In general, the substrate is
`referred to as the backing plate and the coating material is
`called the flyer plate. Ferrous materials form the most handled
`clad components, with Ni-based alloys coming second. Other clad-
`ding alloys like Co alloys on steel are less used, chiefly in
`view of cost and availability. More detailed information may be
`had from the references appended (Bucklow 1983; Edmonds 1978;
`Bahrani
`l978; Bahrani
`l967}.
`
`Amongst the cladding methods listed, rolling and extrusion proce-
`sses are perhaps the most widely applied. Explosive bonding was
`accidentally discovered in 195?; hot isostatic pressing (HIP) and
`electromagnetic impact bonding (EMIB) are comparatively new;
`diffusion bonding process spans the early 20th century ferrous
`technology to date on Ni— base and other high temperature alloys
`for special applications. Some of the outstanding features of the
`above processes will be given in the following sections.
`
`538.1. ROLLIEG & EXTRUSI%
`
`This is the most economical of all cladding methods and the
`restriction in rolling and extrusion to sizes of components is
`only shop—capacity dependent. Both rolling and extrusion methods
`are practised for ferrous alloys with a wide variety of composi-
`tions and applications and for a number of Ni—based alloy/ferrous
`alloy combinations, Al~alloys
`and others have also been devet
`loped to meet
`increasing requirements for corrosion and oxidation
`resistance for ambient and high temperature applications in a
`variety of sulphur and chloride containing environments, and for
`high temperature uses ranging over energy, aviation and nuclear
`industries. Type 310 steel coextruded with 5OCr~5ONi alloy has
`been reported to have very good service under hot corrosion
`conditions in boiler systems (Meadowcroft 1987). Both rolled and
`extruded composite materials are only demand—limited, with the
`normal metallurgical limitations. Cladding materials are usually
`strip form, but powders can be employed quite effectively (Sugano
`et al l968L
`
`112
`
`

`
`TABLE 6:13 (L.H.S.)
`
`CLADDINS PROCESSES
`
`Group A
`UlTra—rapid cladding
`
`Process
`
`(6) Explosive
`Cladding
`
`(ll) Elecfromagnaflc
`lmpacf—Bondlng
`(a) Hot
`(b) Cold
`
`Tlme—facTor
`
`Mlcro~seconds
`
`Micro-seconds
`
`Pressure
`developed
`
`Subsfrafe
`shape
`
`Subsfrafe
`slze
`
`nof given
`
`350 MP8
`
`Simple; sheefs
`
`Simple; Tubes or rings only
`
`15 m2; max 55 m2
`
`<:30O nmz
`
`General
`requlremenf
`
`Thickness 3>lO mm;
`and af leasl 2X
`cladding sheaf
`lhlckness; suf-
`flcienfly ducflla
`
`Sufflclenf sfrengfh To wlTh-
`sTand magnelic discharge
`lmpacf
`
`Cladding
`maferlal
`
`Bonolng
`
`Sheef; reasonably
`ducTlle
`
`Shes? or pre~slnTered powder
`spray
`
`Solid slale, mech-
`anical keylng by
`shear flow,
`impacl
`& scouring acfion,
`afomlc conlacf &
`bonding
`
`Componenls hea-
`fed To recry-
`slaliisafion
`Temperalure;
`high speed
`hof pressure
`diffusion bonding
`
`in explosi-
`As
`ve cladding ~
`alomic keying
`as a resulf of
`a shock wave
`
`Bulky equlpmenf, expensive;
`small shapes resfrlcflon;
`C0mpOnenTS musf have good
`elecfrical conducfivlfy
`
`Safer fhan exploslve cladding
`very fasf
`
`Llmilaflons
`
`Advantages
`
`Comparafively
`lameama~
`subsfrale/clad
`sheets as They are
`easy To handle; Ex-
`ploslve hazard;
`Some maferial de—
`formafion needs
`correction
`
`Very fasf; very
`sfrong 8 confl-
`nuous bonds;
`unllmlled combi-
`nations; very
`lilfle surface
`preparafion
`required
`
`113
`
`

`
`TABLE 5—13 €R.H«S¢)
`
`CLADDENS PRGCESSES
`
`Group B
`Medium-flme claddlng
`
`Rolllng
`
`(ll) Exfruslon
`
`Group C
`Slow cladding
`
`(i) Diffusion-
`bonding
`
`(ll) Ho+ Isosfaflc
`Presslng (HlP)
`
`1-'2 minufes
`
`1-2 minufes
`
`10 mlnufes To >lO hours
`
`variable;
`10-100 N/mmz
`
`IOU-200 MP8
`
`Tube; rods
`
`Simple or complex
`
`Shop-capaclly
`
`Shop-capaclly
`
`Furnace—slze—
`dependenf
`
`Reasonably small
`
`Creep—reslsfan+
`
`Creep—resls+anf
`
`Shes?
`
`Tube or duclile
`
`sheef
`
`Sheef or powder
`
`Shes? or powder
`
`Mechanloal
`
`Mechanical
`
`Me?alEurgical
`
`Mefallurgical
`
`Shape resTrlcTlon;
`Several passes
`required lo ensure only ducfila coa-
`sound bonding.
`Ting maferials can
`Clad maferials
`be handled
`mus? be malleable
`
`Expensive equipmenf and Time consuming;
`uneconomlcal for large componenfs; hlgh
`Technology componenls are usually con-
`sidered for These fwo cladding mefhods
`
`Economical
`
`Leasl expensive;
`several pairs can
`be rolled in a pack;
`also allows com-
`posife rolling
`
`Well suilod for very complex shapes and
`high lechnology componenfs requlring close
`mefallurglcal confrol. HlP can be used for
`mefal—ceramlc bonding
`
`114
`
`

`
`1. Quality check of the two component tube materials
`2. Preparation of a ‘sleeve’ length of each component
`3. Special cleaning of
`the tube intervcontact surfaces
`necessary for a faultless metallurgical bond.
`Accurate surface machining and a thorough removal
`of contamination are vital steps.
`lnter—tube Gap controlled within tight
`
`limits
`
`.
`
`5\.*s~.~I-.'«I-.'~.\.x\\'s-t'~{\\\\~.*~tt
`
`Sleeves
`
`“
`
`“
`
`‘
`
`L
`
`lnter—tube
`
`The sleeves are end—weided to prevent contami-
`nation oiuring further processing
`
`6. The composite billet enters a furnace to bring
`it to extrusion temperature.
`7. Powdered glass is applied on the billet. At the
`extrusion temperature the molten glass acts as
`a iubricant.
`
`inner
`mandrel
`
`An inner mandrel concentric with an outer die
`are aligned as the composite billet bearing the
`lubricant enters the extrusion press (3000 tons)
`
`9. The bitlet undergoes a reduction as well as an
`etongation in the extrusion press.
`
`10. The extruded. clad tube is then cooled, and trimmed
`Further reductions can be carried out by reducing
`either cold or cooler temperatures.
`
`Fig.6—23: Co—Extrusion Process Steps
`
`115
`
`

`
`COATINGS: LST:RSP':SPRAY:NG:WELDING:CLADDING 8: DIFFUSION METHODS
`
`some of the special features in rolling and or extrusion are:
`
`(i) A thin primary layer of electrodeposit usually ensures a
`better bond between the substrate creep resisting steel and th
`overlay clad material.
`’
`
`(ii) Cladding can be given with the substrate as a sandwich.
`
`(iii) With a parting agent, usually Cr2O3, several composite
`pairs or trios can be packed together in a single thin container
`or "can" of steel and then rolled together» The can is sometimes
`evacuated or back—filled with N2.
`
`(iv) The exposed edges oi the clad/substrate configuration are
`sealed by welding to prevent relaxation stresses from wrenching
`apart the rolled or extruded product.
`
`Covextrusion of clad tube is shown in Fig;6~23,
`
`5.8.2. EXPLOSIVE CLDDING
`
`This technique is also called explosive-welding.An accidental
`discovery in 1957 when a sheet metal got stuck to its die while
`an explosive experiment was carried out led to this method.An
`earlier report on the technique and its mechanism is available
`(Bahrani 1967; Bahrani 1978), and another has appeared in 1987
`(Hardwicke l987L The phenomenon seems to have been noted much
`earlier than 1957 in military circles when shells and metal frag~
`ments were seen to bond with metallic surfaces when impacted at
`certain angles.
`
`the modes of the
`Fig.6—24a,b illustrate schematically one of
`process operation(Bucklow l983L Bonding can be achieved by an
`oblique, or parallel high velocity collision between two plates
`to be joined. Shear and plastic flow are the instantaneous reac-
`tions as the shock wave speeds over the clad plate either at
`supersonic or sub—sonic velocities.
`
`The Method of Explosive Cladding (Bahrani 1967; Hardwicke 1987):
`
`The substrate plate is called the backing plate and the flyer
`plate is the cladding material. The backing plate must be at
`least twice the thickness of the flyer plate, preferably not less
`than 10 microns. Three basic requirements are as follows:
`
`1. The flyer plate is spaced at a ‘stand~of£ gap‘ parallel to the
`plate at a distance greater than its own thickness, or touching
`its edge to the substrate at an oblique angle. When explosion
`occurs the components must be brought
`together over
`this gap to
`collide progressively over the surface area.A collision front
`traverses this surface area.
`
`285
`
`116
`
`

`
`23.. Starting position of fiyer plate
`2. Fiyer—p|ate
`1. Backing plate
`- parallel
`to backing plate 3. Buffer
`4. Explosive
`5. Ftyenpiate after
`deformation by detonated expiosive. The two plates are interlocked
`during the instant of contact.
`
`xx
`’
`xxx
`a(1zv'4'4-F-'—'///2
`
`:'a’J/A‘/3'1’/(‘r'/o’¢‘
`\\‘s\‘x’~.'».
`12/2122
`-.-.
`
`Melt flow and interlocking configuration at
`
`the impact region
`
`Fig.6—24 a,b; Schematic diagram of Explosive Cladding
`
`117
`
`

`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`is placed on the flyer plate
`2. A protective (buffer) sheet
`(usually rubber) and the explosive is spread on it as a slurry or
`a sheet. The quantity of the explosive must be controlled to
`result in an optimum detonation velocity of, est 2000 m/s for a
`thick plate. Slow explosions fail to secure a good bond and
`larger magnitude could shatter, distort, spell or melt the clad-
`ding. The velocity of the collision front must be below sonic
`velocity in the metals being joined. Shock waves travel
`through
`the materials at the velocity of sound. The subsonic velocity
`level of the collision front ensures that the shock wave precedes
`the bond being formed at the collision front.Failing this pro-
`duces non—bonding or immature bonding in the composite.
`The
`explosion initiates this progressive shock wave which shear
`im-
`pacts the flyer plate to the backing plate.At Suchaahigh speed
`and pressure,
`the surfaces also get heated up and squirt out in a
`scouring action as if they are liquid. The surfaces of both
`contacting faces are pushed out along with any contaminants, e.g.
`oxide layers and atomically clean surfaces look into each other.
`
`3. The interfacial pressure at the collision front must signifi-
`cantly exceed the material yield strength to enable plastic
`deformation.
`
`The collision angle of the flyer and backing plates determines
`the interface wave vortex morphology of the resulting composite.
`The volume of entrapped melt at the interface and the associated
`bond strength depend on the collision angle. A low collision
`angle results in an interlocking with a large vortex and a high
`degree of entrapped material as if it is a frozen melt. If the
`angle is very low the vortices link up and become continuous,
`instead of being discretely isolated,
`and the resulting bond is
`highly undesirable, since, effectively there are three layers of
`uncertain bond strength. The higher the collision angle,
`the more
`undulated the wave form will be with reduced vortices and entrap-
`ped melt. A very steep angle could result in a completly flat
`surface with little or no entrapped melt. The steep angle detona-
`tion configuration was thought commercially noneviable but has
`been achieved and patented (Szecket et al 1985;
`lnal et al l985L
`
`It seems that this process is only suitable for large masses of
`metal, but
`in principle ought to be applicable to any metal—metal
`bond as long as they are not Very brittle, and have simple shapes
`such as sheets,
`tubes, and low convex~conCave radius components -
`crucibles etc. Direct cladding of large areas, for instance 30
`sq.m., have been produced and thin sheet composite cladders
`fusion welded together from 2 metre wide strips. Double sided
`clads, with the cladders bonded simultaneously have also been
`reported. Cup forgings and tube forgings have been streamlined. A
`better understanding of the process has enabled bonding of many
`metals and alloys. Laminar composites of alternating material
`layers of widely divergent mechanical properties have unique
`resistance to fatigue failure. Transition composite joints can be
`produced in tubular or flat form with two dissimilar metals.
`bonding of strictly identical materials (for multi~layer lami-
`
`287
`
`118
`
`

`
`lm_mWmm_____,l__
`
`M.
`
`L
`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`nates) which are almost impossible to diffusion bond, e.g. Al—Li
`alloys, are very successfully explosion bonded.
`Imploded Zr—a1loy
`to outer steel tubes have proved their worth in drilling tube
`service in sour gas oil wells.
`
`The method has been found especially useful to bond normally
`‘mismatched'alloy bi—, tri- and quad—laminates,e.g.a V~20Ti
`bonded to M0 or Nb which itself was bonded to Cu (Kaminsky 1980);
`Pt, V, or Mo bonded to Ni—Cr-Fe~, or Fe—22Cr- alloys for high
`temperature use. Most of the clad materials listed by Gregory
`(1980) can be explosively bonded. A number of alloy combinations
`have emerged. Conversion rolling,
`formerly strictly reserved for
`stainless steel are now common for copper—nickel, all of Ti~base
`alloys, Hastelloys, many nickel alloys and Zr-clads. Zr-alloys
`which used Ti as interlayer, now are clad without it.Aerospace
`transition joints of Ti6Al4V to stainless steel, Al~alloys to Ti-
`alloys, multi-layer laminates of Al-Li alloys rank amongst the
`advances made by explosive cladding (Hardwicke 1987). Particular
`mention may be made of the straight waveless interface of Al
`to
`steel and Ti6Al4V to mild steel (szeoket et al 1985; Inal et al
`1985).
`
`6.9- DIFFUSIO BONEING
`
`A majority of coating processes include heat treatment as an
`intermediate or an ultimate step to ensure a sound substrate to
`deposit bond. Thus, almost all “non~overlay" deposition can be
`termed as diffusion bonds. This section discusses diffusion bond-
`
`ing employed as a primary process unlike its secondary role in
`other deposition techniques.
`
`Diffusion bonding is a solid state method where component parts
`are pressed together and heated to form a bond by interdiffusion.
`Diffusion bonding requires high temperature data of all the
`components used in metallurgical technology for the concerned
`substrate—coating complex. Yet, the least characterized of all
`metallurgical parameters seems to be in the diffusion reaction
`area, e.g. diffusion constants for elements in multi-component
`alloys. Often, simplified or estimated values are extrapolated.
`This is perhaps, one of the important areas where futher fundam-
`ental research is needed.
`
`Diffusion bonding is a slow process because a metallurgical bond
`by elements is achieved between two surfaces in Contact by a
`solid-state diffusion process giving rise to a three~zone compo-
`site. Composition, microstructure, physical, chemical and mecha-
`nical properties are affected and controlled by the duration of
`heat treatment,
`the temperature and the pressure at which the
`process is carried out and the rate and type of cooling given
`subsequently.§me physical, chemical and mechanical properties
`for the composite, such as hardness, stress/strain, oxidation,
`corrosion effects, microstructure etc” help in material selec~
`
`288
`
`119
`
`

`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`tion and use. These are the parameters which decide diffusion
`bonding as a cladding process. Elliott and Wallach (1981) consi-
`der five main process parameters to be time,
`temperature, pres-
`sure, surface condition and environment. Unless otherwise stated
`diffusion bonding is carried out under vacuum in order to prevent
`inclusions and oxidation.
`
`Diffusion bonding is also termed as solidvstate welding.A few
`useful sources of information are appended: Tylecote 1968; Bartle
`1969; Gerken & Owczarski 1965; Bartle 1978; Ohashi & Hashimoto
`1976; Fielding l978. Drewett
`(l969a,b) discusses diffusion coat~
`ings on steel on a wider context; Derby and Wallach (1982)
`present a theoretical model for solid~state diffusion bonding.
`Diffusion bonding is a suitable technique for joining materials
`which are difficult to join by conventional welding methods (e.g.
`Ti and its alloys) and to achieve bonding on a more commonly used
`material,e.g.stee1.
`
`6.9.1“ PBIERX CHARACTERISTICS OF DIFFUSIG BONDIRG
`
`Two material layers can be bonded when brought into elemental
`contact with an applied interfacial pressure at high temperature
`in the region 0.5— 0.8 Tm where Tm is the melting point in deg-
`rees Kelvin, of
`the material being bonded. Often additional
`layers in the fornlof coatings of a few microns, or loose shims
`are interposed to facilitate bonding or
`to act as diffusion
`barriers to specific elements.
`
`The advantages of diffusion bonding are (Derby & Wallach 1982M
`
`(i) Large areas can be joined.
`
`(ii) Metal joining is possible under unconventional situations,
`e.g. space environment, or as part of a superplastic and bonding
`sequence.
`
`(iii) Bonding is possible with minimunadeformation.
`
`(iv) Minimum microstructural damage and stress results while
`keeping a thin diffusion zone and smaller thermal gradients occur
`than in processes like fusion welding.
`
`The disadvantages are:
`
`(i) Poor adherence over large areas due to impurities such as
`oxides or grit at the interface.
`
`(ii) Poor bonding due to interface separation via vacancy concen-
`tration and void formation during diffusionr
`
`(iii) Unknown.therma1 effects on the properties of the bonding
`elements.
`
`289
`
`120
`
`

`
`Hm
`
`“.3
`
`COATINGS: LST:RSP:SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`6.9.2. BONIEG EECHAEISH
`
`Derby and Wallach summarise earlier approaches in their 1982
`review in their model. The presence of an oxide film on the
`substrate is considered an inhibitor but not as rate determining
`to bonding since except for metals with insoluble oxides, e.g.
`Al,
`the oxide dissolves rapidly in the parent metal as bonding
`proceeds. The diffusion layer formed at the interface could be a
`solid—state bonding, purely mechanical, or a phase reaction in a
`solid—state exchange or a eutectic substrate—coating phase trans—
`formed to a solid microstructure on cooling.Surface roughness
`requires data specific to the material considered and generaliza-
`tion is not possible. The authors propose a broader based mecha~
`nism by considering diffusion bonding analogous to pressure sin~
`tering. The proposed six mechanisms of mass transfer are:
`
`l. Surface diffusion from surface sources to a neck.
`2. Volume diffusion from surface sources to a neck.
`
`3. Diffusion along the bond interface from interfacial sources to
`a neck
`. Volume diffusion from interfacial sources to a neck.
`
`4 5
`
`law creep deforming the ridge.
`. Power
`6. Plastic yielding deforming the ridge.
`
`1 and 2 above are surface source related, driven by surface
`curvature differences across the surface of an interfacial void.
`
`3 and 4 consider chemical potential along the bond line as dri-
`ving force while 5 and 6 are gross deformation mechanisms driven
`by applied pressure with some surface tension effects. Five
`process parameters have been interwoven to apply the above six
`mechanisms, namely,
`temperature, pressure, initial surface rough-
`ness,
`initial surface aspect ratio and time.
`
`Computer mapping yields the theoretical data which ironically
`offer limited verification for lack of adequate high temperature
`diffusion and creep data. Almond et al
`(i983) envisage a pressure
`—sinter mechanism in the diffusion bond of hard metal joints,
`lending partial support to the Derby~Wallach model. They consider
`an evaporation—condensation mechanism in the case of WC—Co wear
`resistant bonds.Further experimental work in high temperature
`creep, diffusion and most other physico-mechanical properties is
`needed.
`
`6.9.3.. ROLE OF I YERS IE DIFFUSION BOWING
`
`The practical side of diffusion bonding has side—stepped techni-
`cal difficulties in straight metal—to—metal bonding by using
`intermediate layers.
`
`these prevent oxidation of the substrate
`1. Applied as coatings,
`prior to bonding (Crane 1967; Kammer et al 1969),
`
`290
`
`121
`
`

`
`COATINGS: LST:RSP.'.SPRAYING:WELDING:CLADDING & DIFFUSION METHODS
`
`1 They assist micro-deformation and establish contact between
`faying surfaces (Bartle 1978; Hauser et al 1967),
`
`3. They obviate bonding inhibition caused by contaminant films by
`increasing the rate of their dissolution,
`
`4. They enhance the diffusion rates (Kharchenko 1969; Davis &
`Stephenson l962; Lehrer & Schwartzbart l96l) and,
`
`5. They can minimise Kirkendall void formation and prevent or
`reduce intermetallic phases at the interface (Crane l967L
`
`The above five causes have been inferred in the context of diffe-
`rent diffusion bond pairs. Much more work is required to asso-
`ciate any of these with a proven mechanism (Elliott & Wallach
`1981).
`
`.6.9.4. DIFFUSION BONDED MAT
`
`Aluminium on steels has been an industrial diffusion bond techni-
`
`que for nearly four decades. Aluminizing and chromizing on steels
`over forty years and later on Ni~ base alloys over
`the last
`twenty years have been, consequently, the most investigated of
`all diffusion bond systems. High temperature and/or corrosion
`resistant materials such as Co—, Ti—, Mo—, and W— alloys have
`been investigated more recently. Materials listed by Gregory
`(1980) provide a reasonable cross section. Table 6:14 gives a
`list of elements which may or may not form good diffusion coa