ON THE ROLE OF GAS IONIZATION IN EXPLOSIVE WELDING 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 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 surfaces. 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 Тк, К δ М γ L50, cm L100, cm 3,5 5000 8,9 2,55 1,24 30 46 4,2 6500 9,44 2,63 1,23 16 22 4,5 8300 10,35 2,8 1,21 1,4 2,2 where: L50 и L100 – 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 At explosion welding according to Lysak and others Pervuchin and others 1. Formation of physical contact. 1. Activation and cleaning of surface. 2. Activation of surface. 2. Formation of physical contact. 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 –Vс coordinates, where  is the angle of collision and Vс the velocity of contact point

IONIZATION POTENTIAL OF DIFFERENT COMPONENT AIR Gas Ionization potential (V) Ionization potential (V) Ar 15,8 SO2 13,1 N2 15,6 H2O 12,6 H2 15,4 O2 12,5 CO2 14,4 NO2 11,0 CO 14,1 NO 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 = 3⁄2kТ, gas temperatures of 6000–12000 K must correspond to the energy of 0.52–1.03 eV that is acquired by a single species of ideal gas. At T = 3500 K, this is 0.3 eV. It follows that, at typical conditions of explosive welding, the amount of ionized gas within the weld gap 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]. 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.

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 Energy density, Watt/m2 Time of plasma influence, sec Thickness of moving off layer, μm Plasma-arc cleaning 103 5-10 200-300 Shock plasma 1010 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. 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  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 contact point undergo self-purification and some activation caused presumably by accumulation of lattice defects in both metals and followed by collectivization of electrons and transition to a plastic state, thus facilitating the formation of a weld seam. Self-purification of the 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 their oxides, the purification process is also facilitated by the heat released upon collision and friction of the plates. (4) Shock-compressed gas favors the removal of residual dirt and oxide films from the weld gap.

Conclusions 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.   Activation and self-purification of the metal surfaces take place at the moment of their collision, due to mechanoactivation and deformation.  Shock-compressed gas removes dirt from the gap but... the residual (unremoved) gas may cause faulty fusion.   

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Structure of a surface of metal Environment Metal Oil film Film of oxides 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.

ε= l-l0/ l0 P>>σд 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 Обычно методами.. измеряют пластическая деформация в околошовной зоне. Непосредственно в образовании соединения учавствуют только соединяемые поверхности. Деформацию этих поверхностей можно оценить как отношение разности длины линии соединения к ее проекции. Activation time at plastic deformation of a surface in a contact zone tа  10-8 … 10-7 с. [1] Pressure in a contact point Strain of surface L –length of the line of connection, L0 – projection length P – pressure σд – dynamic tensile strength ε= l-l0/ l0 P>>σд 1– Lysak V. I. Kuzmin S. V. Welding explosion. - M.: Mechanical engineering - 2005. 18 18

Thermal impact of the USG area on a surface - thermal stream from gas on a surface of plates; St – Stanton number; ср; ρ – heat capacity and gas density respectively; ТУСГ – temperature of shock -compressed gas; Т0 – reference temperature. - Stanton number at a turbulent flow of plates a gas stream. - 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 600 1010-5 3000 900 8,310-5 3500 1200 7,210-5 4000 1500 0,3 6,310-5 4500 2000 1,5 5,510-5 5000 6 510-5

The scheme of calculation of shock-compressed gas area ahead of a contact point υ υ Vk 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; Vk – the contact point velocity ; Р0 – atmospheric pressure; Р1 – pressure in the shock-compressed gas υ - flow out velocity of gas from a welding gap; l – length of shock-compressed gas area mgr - The grasped weight of air mex - The expiring weight of air

Dependence is determined by the size of the shock-compressed gas Dependence l = f(s) ρ0 – density of flowing gas, b – length of the contact line, P1 – 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 tа=l/Vk 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 Density energy W/м2 Time of influence of plasma, sec. Thickness of a deleted layer, мкм in a zone of stabilization of strength on other site Plasma and arc cleaning * 103 5-10 200-300 Shock plasma ** 1010 7,6·10-6- 1,2 ·10-5 2,4 ·10-5- 1,12 ·10-4 3-5

Scheme of joint formation 12 Scheme of joint formation Initial condition shock-compressed gas area The beginning of process, formation of the shock-compressed gas area heats gas Clearing and activation of welded surfaces t = 7,6·10-6- 1,12 ·10-4 s. Plasma Formation of physical contact Volumetric interaction with formation of connection behind a contact point.

Modeling the process of EXPLOSIVE WELDING (DMC)

DMC