Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences (ISMAN)

ON THE ROLE OF GAS IONIZATION IN EXPLOSIVE WELDING

M.I. ALYMOV, A.A. DERIBAS AND I.S. GORDOPOLOVA

We are going to critically revise the concept of gas ionization assumingly taking place at

surfaces.

6000–12000 K

within a stand-off (weld) gap, according to which thus formed plasma jet was suggested [1, 2] to play a key role in the activation and self-purification of weld 1. S.Yu. Bondarenko, O.L. Pervukhina, D.V. Rikhter, L.B. Pervukhin, Explosive welding: Parameters of shock-compressed gas in the weld gap ahead of the contact point, Avtomatich. Svarka, 2009, no. 11, pp. 46–48. 2. L.B. Pervukhin, D.V. Rikhter, O.L. Pervukhina, S.Yu. Bondarenko, Continuity defects in explosion welded large-sized sheets and their relation to the processes taking place in the weld gap ahead of the contact point, Svarochn. Pr-vo, 2009, no. 7, pp. 32–37.

EXPERIMENTAL DATA TESTIFIED TO FORMATION OF PLASMA INTO STANDOFF.

Experimental setup for measuring of gas temperature: 1-explosive charge, 2-clad plate, 3-light filter, 4- chink, 5-immovable plate, 6-condensed air Dependence of brightness gas temperature on detonation velocity

Curve 1 –

dependence of brightness temperature of gas clot on detonation velocity .

Curve 2 –

shock Hugoniot of air Dependence of distance of melting beginning on parameters of gas flow D, km/s 3,5 4,2 4,5 Т к , К 5000 6500 8300 δ 8,9 9,44 10,35 М 2,55 2,63 2,8 γ 1,24 1,23 1,21 L 50 , cm 30 16 1,4 L 100 , cm 46 22 2,2 where: L 50 * и L 100 – distances on which melt of plates surface appears at roughness of 50 and 100 μm accordingly.

Ишуткин С.Н., Кирко В.И., Симонов В.А. Исследование теплового воздействия ударно-сжатого газа на поверхность соударяющихся пластин // Физика горения и взрыва. – 1980. - №6. – С. 69-73 * Козлов П.В., Лосев С.А., Романенко Ю.В. Поступательная неравновесность во фронте ударной волны в аргоне // Вестник Московского Университета. Серия 3. Физика. Астрономия. 1998, №5, стр.46-51.

Pervukhina O.L., Rihter D.V., Pervukhin L.B., Denisov I.V., Bondarenko S.Yu.

SOME ASPECTS OF JOIN FORMATION DURING EXPLOSIVE WELDING

FORMATION OF WELDED JOIN.

At pressure welding supposition 1. Formation of physical contact. 2. Activation of surface. At explosion welding according to

Lysak

and others 1. Formation of physical contact.

Pervuchin

and others 1. Activation and cleaning of surface.

2. Activation of surface. 2. Formation of physical contact.

3. Volume interaction 3. Volume interaction

Pervukhina O.L., Rihter D.V., Pervukhin L.B., Denisov I.V., Bondarenko S.Yu.

SOME ASPECTS OF JOIN FORMATION DURING EXPLOSIVE WELDING

HERE THERE ARISE AT LEAST THREE QUESTIONS THAT HAVE TO BE ANSWERED.

•

Are the temperatures developed within the weld gap sufficiently high for plasma formation?

•

Are these temperatures sufficient for formation of the so-called cold plasma?

•

Even if such plasma is formed in reality, how significant are the consequences of its formation?

G.M. SENCHENKO, I.N. FEDOSENKO, A METHOD FOR MEASURING THE TEMPERATURE OF SHOCK-COMPRESSED GAS DURING EXPLOSIVE WELDING, RUSS. PATENT 2 009 454, 1994.

Information about the temperatures of shock-compressed gas attained in explosive welding can be found in the literature. In case of steel–Al sheets welded at detonation velocity D = 2500 m/s, the measured gas temperature T was about

3500 K

, which is close to a theoretically predicted value of 3400 K

THE WELDABILITY DIAGRAM PLOTTED IN THE



– V С COORDINATES, WHERE AND V С



IS THE ANGLE OF COLLISION THE VELOCITY OF CONTACT POINT

IONIZATION POTENTIAL OF DIFFERENT COMPONENT AIR Gas Ar N

2

H

2

CO

2

CO Ionization potential (V) Gas Ionization potential (V)

15,8 15,6 15,4 14,4 14,1

SO

2

H

2

O O

2

NO

2

NO

13,1 12,6 12,5 11,0 9,5 http://www.ionization.ru/issue/iss59.htm

The tabulated values of ionization potential I for air components are known to range between 9,5 and 16 eV. Given that 1 eV = At T = 3500 K , this is 0.3 eV. is extremely low and hence cannot play a key role in the self-purification and activation of metal surfaces, as it was declared in [1,2].

3 ⁄ of 0.52–1.03 eV that is acquired by a single species of ideal gas. 2 kТ, gas temperatures of 6000–12000 K must correspond to the energy It follows that, at typical conditions of explosive welding, the amount of ionized gas within the weld gap

1. S.YU. BONDARENKO, O.L. PERVUKHINA, D.V. RIKHTER, L.B. PERVUKHIN, EXPLOSIVE WELDING: PARAMETERS OF SHOCK-COMPRESSED GAS IN THE WELD GAP AHEAD OF THE CONTACT POINT, CONTACT POINT, AVTOMATICH. SVARKA, 2009, NO. 11, PP. 46–48. 2. L.B. PERVUKHIN, D.V. RIKHTER, O.L. PERVUKHINA, S.YU. BONDARENKO, CONTINUITY DEFECTS IN EXPLOSION WELDED LARGE-SIZED SHEETS AND THEIR RELATION TO THE PROCESSES TAKING PLACE IN THE WELD GAP AHEAD OF THE SVAROCHN. PR-VO, 2009, NO. 7, PP. 32–37.

FORMATION OF WELDED JOIN

Another objection is that the processes of dissociation and associative ionization assumed in [*] can hardly be expected to happen from kinetic considerations. At D = 2500 m/s, the detonation wave passes a 1-m distance in 4·10 –4 s, while the free path in air is around 10 –7 m, and hence the time allowed for collision and chemical transformations is about 4·10 –11 s, which is too short for any kind of chemical transformations.

* L.B. Pervukhin, O.L. Pervukhina, S.Yu. Bondarenko, Theoretical and technological backgrounds for industrial-scale production of clad metals,

Izv. Volgogr. Gos. Tekh. Univ., Ser. Svarka Vzryvom Sv-va Svarn.

Soedin., 2010, nos. 4–5, pp. 75–82

Analogy with the action of plasmatron suggested in [**] seems inappropriate, because its effect in cutting/soldering of metals is exclusively thermal in its essence and directed (focused). In our case, the motion of minor amounts of electrons and ions is chaotic and short timed. In addition, the specific action of plasma becomes pronounced only in the presence of electromagnetic field, which can hardly be expected to exist in conditions of explosive welding. This is our answer to question (3).

** L.B. Pervukhin, O.L. Pervukhina, S.Yu. Bondarenko, Self-purification from oxides and dirt and surface activation during explosive welding, Avtomatich. Svarka, 2010, no. 7, pp. 46–49.

Convincing evidence against the key role of plasma jet in explosive welding was obtained in the experiments by Deribas et al *. Explosive welding in vacuum was found to give the same results as that in air; that is, the presence/absence of air in the weld gap had no influence on the quality of weld seam.

* А.А. Deribas, Fizika uprocheniya i svarki vzryvom (Physics of Explosion-Aided Strengthening and Welding), Novosibirsk: Nauka, 1980

PLASMA CLEANING

Method Plasma-arc cleaning Energy density, Watt/m 2 10 3 Time of plasma influence, sec Thickness of moving off layer, μm 5-10 200-300 Shock plasma 10 10 0,02 3-5

General view of surface after plasma-arc cleaning

* Сенокосов Е.С., Сенокосов А.Е., Плазменная электродуговая очистка поверхности металлических изделий, "Металлург", №4, 2005 г.

* * Ишуткин С.Н., Кирко В.И., Симонов В.А. Исследование теплового воздействия ударно-сжатого газа на поверхность соударяющихся пластин // Физика горения и взрыва. – 1980. - №6. – С.69-73

Pervukhina O.L., Rihter D.V., Pervukhin L.B., Denisov I.V., Bondarenko S.Yu.

SOME ASPECTS OF JOIN FORMATION DURING EXPLOSIVE WELDING

We suggest that the sequence of the events taking place during explosive welding (instead of that suggested in [1]) should be considered as happening in the following order. (1)Initiation of detonation and its propagation over the layer of bulk-density HE. (2) Gaseous detonation products accelerate the flyer plate and, at low collision angles  , its impact collision with the base results in the formation of strong wave-like weld seam. Collision at higher presumably by accumulation of lattice defects in both metals and surface can also be associated with mechanical activation. At the moment of collision, all oxides and dirt keep moving in the direction of wave propagation and are carried out of the gap by a shock wave. Due to different thermal expansion of metals and released upon collision and friction of the plates. films from the weld gap.  contact point undergo self-purification and some activation caused their oxides, the purification process is also facilitated by the heat gives rise to the appearance of a cumulative jet, but this is accompanied by a decrease in the amplitude in wavelength of the wave structure of the seam. (3) At the moment of impact collision, the metal surfaces ahead of the followed by collectivization of electrons and transition to a plastic state, thus facilitating the formation of a weld seam. Self-purification of the (4) Shock-compressed gas favors the removal of residual dirt and oxide

Conclusions

1.The strength and quality of the seam strongly depend on the parameters of high explosive: detonation velocity, weight and thickness of charge layer, and uniformity of HE composition. These parameters define the inclination angle and conditions of gas expulsion from the gap. 2.Activation and self-purification of the metal surfaces take place at the moment of their collision, due to mechanoactivation and deformation.

3.Shock-compressed gas removes dirt from the gap but... the residual (unremoved) gas may cause faulty fusion.

THANK YOU FOR ATTENTION!

Structure of a surface of metal

Environment Metal Film of oxides Oil film Welding in a solid phase complicate:

Films of oxides Thin boundary layers of oils, fatty acids

For formation of connection in a firm phase it is necessary:

before the introduction of welded surfaces in contact to make their cleaning and activation then connection in a point of contact will occur instantly

Mechanisms of cleaning and activation of welded surfaces

Mechanically during removal from a surface of part of the metal (an exposure of the so-called pure juvenile surfaces) or chemically connected with it alien substance, (for example, oxides); at movement of the dislocations accompanying plastic deformation; thermally at the heating accompanied by noticeable diffusion and self-diffusion, movement of vacancies and other processes changing the provision of atoms in a crystal lattice; surface bombing by ions or fast-moving particles with rather high energy.

Gelman A.S. Bases of welding by pressure. M, "Mechanical engineering", 1970, 312 pages.

Mechanical removal of a blanket Cumulative (return) stream - at a speed of detonation from 2000 to 3000 m/s and asymmetrical impact the stream isn't formed 2. In the course of plastic deformation at formation of connection

Activation time at plastic deformation of a surface in a contact zone

t а



10

-8

… 10

-7

с. [1]

Pressure in a contact point Strain of surface

P>>σ

д P

σ

д – pressure – dynamic

tensile strength

ε= l-l

0

/ l

0 L –length of the line of connection, L0 – projection length

1 – Lysak V. I. Kuzmin S. V. Welding explosion. - M.: Mechanical engineering - 2005.

q



T c

Thermal impact of the USG area on a surface

St

  

u



c p

(

T УСГ



Т

0 ) - thermal stream from gas on a surface of plates;

S t

– Stanton number;

Т УСГ с р; ρ

– heat capacity and gas density respectively; – temperature of shock -compressed gas;

Т 0

– reference temperature.

St Т

 8 ( 2 lg

a p k

1

- Stanton number at a turbulent flow of plates a gas stream.

  

q

2   6

at q

 1   

r

1  1 , 74 ) 2 

T

0

- Law of heating metal of plates.

λ

и

а

– thermal conductivity and heat diffusivity of a material of plates.

- depth of penetration metal.

Speed of a point of contact Vк, m/s Maximum temperature of heating of a surface metal, Тс, К Depth of penetration metal.

, ζ, мкм.

Heating time  , с 2500 3000 3500 4000 4500 5000 600 900 1200 1500 2000 2500 0 0 0 0,3 1,5 6 10  10 -5 8,3  10 -5 7,2  10 -5 6,3  10 -5 5,5  10 -5 5  10 -5

The scheme of calculation of shock-compressed gas area ahead of a contact point

υ υ V k

dm

gr

dt



dm

ex

dt



const

Two problems are in common solved :

-Problems about the moved piston, define of gas parameters for shock wave ; -Problems about flow out velocity of gas from a welding gap ;

l

V k Р 1 υ – – the contact point velocity ; Р 0 – atmospheric pressure pressure in the shock-compressed gas - flow out velocity of gas from a welding gap ; ; – length of shock-compressed gas area m

gr - The grasped weight of air

m ex -

The expiring weight of air

Dependence is determined by the size of the shock-compressed gas

l

  Dependence

l = f(s)

1

b

 2

s v к s

 0

b

 2   1

P 1

 1    2  1    2  1

ρ

0

– density of flowing gas, contact line ,

P

1 –

b

– length of the pressure in the shock compressed gas , 

Vк –

contact point velocity

,

velocity of the gas

, l -

extent of the zone of shock-compressed gas ,

s

– distance from the contact point Activation time

t а =l/V k

The dependence of the extent of the zone of shock-compressed gas (l) of the distance traveled by the contact point (s) and the width of the welded sheets (b)

Shock plasma

Nonequilibrium shock plasma is formed in an interface when welding explosion of large-size sheets in a welding gap at a flow of welded surfaces with a hypersonic speed (more than 5M) of VUSG Shock plasma has much in common with usual digit plasma, but there are some features: lack of external electric field, high temperatures (T=3000-20000 K) and existence fast the hemoionization of reactions with participation of the excited atoms and molecules.

Nonequilibrium physicochemical processes in gas streams and the new principles of the organization of burning / Under the editorship of A.M. Starika. - M.: TORUSS PRESS, 2001. 864 pages: silt.

Interaction of shock plasma with welded surfaces

Shock plasma interacts with a solid body, thus there is a destruction and evaporation of blankets of a solid body, dissociation of oxides and saturation of VUSG by them; 2. Shock plasma interacts with liquid which is formed at an reflow first of all tops of microasperity. The liquid layer is involved in a stream and sates VUSG with particles and vapors of melted metal, thereby changing its parameters.

3. Shock plasma at the same time interacts with a solid body and a liquid layer.

Method Plasma and arc cleaning *

Shock plasma **

Density energy W/м 2 10

10

3

10

Time of influence of plasma, sec.

in a zone of stabilization of strength on other site 5-10 5-10

7,6·10 -6 - 1,2 ·10 -5 2,4 ·10 -5 - 1,12 ·10 -4

Thickness of a deleted layer, мкм 200-300

3-5

12

Scheme of joint f ormation

Initial condition

shock-compressed gas area Plasma heats gas

The beginning of process, formation of the shock-compressed gas area Clearing and activation of welded surfaces

t



l V k

t =

7,6·10 -6 - 1,12 ·10 Formation of physical contact -4

s.

Volumetric interaction with formation of connection behind a contact point.

MODELING THE PROCESS OF EXPLOSIVE WELDING (DMC)

DMC