Enhancement of explosive welding possibilities by the use of emulsion explosive

1. EPNM -2014 Cracow, Poland, May 25-30, 2014 Enhancement of explosive welding possibilities by the use of emulsion explosive Boris Zlobin*, Victor Sil’vestrov**, Alexandr Shtertser*, Аndrey Plastinin**, Victor Kiselev* *Design & Technology Branch of Lavrentyev Institute of Hydrodynamics SB RAS **Lavrentyev Institute of Hydrodynamics SB RAS Novosibirsk, Russia *asterzer@mail.ru

2. Subject of research 1) To specify the boundaries of explosive welding (EW) window for the couple of very dissimilar materials; 2) To apply the emulsion explosive (EMX) developed in LIH SB RAS for EW of very dissimilar materials.

3. 5 4 2 3 D  h 1 6 EXPLOSIVE WELDING LAYOUT 1- base plate, 2- flyer plate, 3- explosive charge, 4- detonation front, 5- detonation products, 6- ground (anvil), D - detonation velocity, γ - collision angle, h - gap. Acceleration parameter: R = mass of explosive / flyer plate mass.

4. Search for new explosives for EW Presently there are many examples of explosive welding (EW) use in up-to-date technology. EW is mostly employed in production of steel-titanium, steel-aluminum, steel-copper, and carbon steel-stainless steel bimetal articles. Sometimes in the industry, especially when new equipment is being developed, there is the need to join together low-ductile alloys with ductile ones, or to clad some articles with thin protective or function layers from special alloys. Thin coatings are rather frequently needed on un-massive or thin-walled articles which may be destroyed or defected by explosion of too big high explosive charge. In these cases, the difficulties arise due to the fact that the utilized industrial HE cannot detonate steadily in thin (below 8 – 10 mm) layers with the low (2 – 3 km/s) detonation velocity needed for the EW. It makes researches seek for new explosives. For example, in [1] they studied the detonation characteristics of fine PETN mixtures with baking soda. Similar attempts, but on the base of another compositions, are undertaken in Lavrentyev Institute of Hydrodynamics (LIH SB RAS). [1] L. A. A n d r e e v s k i k h, Y u. P. D e n d e n k o v, O. B. D r e n n o v, A. L. M i k h a i l o v, N. N. T i t o v a, A. A. D e r i b a s, Explosive Mixture for Explosive Welding of Thin Foils, Propell Explos Pyrot 36, 48-50 (2011).

5. Emulsion Explosive developed in LIH SB RAS Emulsion explosive (EMX) detonates in low thickness layer with low detonation velocity suitable for EW. Hollow glass microballoons are included in composition of this explosive with the aim to increase its detonation sensitivity. Laboratory experiments have shown that EE can be successfully employed in explosive cladding with flyer plate thickness less than 1 mm [1, 2]. EMX density is 0.62  0.01 g/cm3. Detonation velocity has weak dependence on layer thickness and changes from 2.3 to 2.6 km/swhen thickness changes from 3.5 to 25 mm. Critical thickness of flat charge in polyethylene casing (t 0.5 mm) is less than 3.5 mm. EMX contains water solution of ammonium and sodium nitrate (oxidizer), paraffin (fuel), special emulsifying agent, and hollow glass microballoons. EMX oxygen balance is close to zero. Emulsion density is 1.41  0.01 g/cm3, oxidizer drop size is not greater than 2 µm. Microballoon average diameter is 58 µm, green density ~0.15 g/cm3. 1V. V. Sil’vestrov and A. V. Plastinin, Investigation of Low Detonation Velocity Emulsion Explosives, Combustion, Explosion, and Shock Waves, 2009, Vol. 45, No. 5, pp. 618–626 2Silvestrov V.V., Plastinin A.V., and Rafejchik S.I., Application of emulsion explosives for explosion welding, The Paton Welding Journal, 2009, N11, p. 61-64.

6. γ no waves waves 1 2 Critical angle for jetting jetting 3 no jetting 4 Vt Vc c Explosive welding window Explosive welding window on the Vc– γ plane: 1- low boundary, 2- upper boundary, 3- the boundary Vc = Vt between the ranges of smooth and wavy shape of the bonding zone in the welded couple (Vt is detected experimentally), 4- the boundary between the ranges of occurrence and absence of cumulative jet at the collision of flyer and base plates (jetting does not occur if Vc exceeds c); Vc – contact point velocity, γ – collision angle, c – compression wave speed (c2 = K/ρm, K-bulk modulus, K = E/3(1-2μ ), E- Young modulus, μ- Poisson ratio). Broken curves and the lines 1, 2 circumscribe the typical experimental area where reliable welding is achieved.

7. Formulas The low boundary of EW window: The upper boundary of EW window: Vp – flyer plate velocity, Vc – contact point velocity, D – detonation velocity, n – detonation products polytropic index, y = h/e, h – gap, e – explosive charge thickness, HV - Vickers hardness,Tm is the melting point,  - thermal conductivity, a - thermal diffusivity, m- density of the metal, 1- thickness of the thinner plate, 2- thickness of the thicker plate. k = 5.5·ξ 0.18, where ξ = s/ (s isthe surface film thickness,  - flyer plate thickness). In real practice, k normally varies from 0.6 to 1.2, for the materials with natural oxide films k = 1.14.

8.  + + + + + Vc EW window for dissimilar materials Intersection of EW theoretical windows built for two different materials. In theory welding is possible in the intersection area (crosses). Investigations show that successful welding can be ensured in the vicinity of low boundary of a lesser strong material.

9. Low boundary of EW window It would seem that EW of materials having very different physical and strength parameters is either impossible, either the reliability to get high bonding quality is low (as the needed collision parameters lie in the narrow zone in the EW window). However, to our joy, the experiments show that EW low boundary can be determined on the basis of the strength of lesser strong component [1-3]. As a rule, the lesser strong material is, at the same time, more ductile. [1] I. D. Zakharenko, B. S. Zlobin. Effect of the Hardness of Welded Materials on the Position of the Lower Limit of Explosive Welding, Combustion, Explosion, and Shock Waves, 1983, vol. 19, no. 5, pp. 689-692. [2] B. S. Zlobin. Explosive Welding of Steel with Aluminum, Combustion, Explosion, and Shock Waves, 2002, vol. 38, no. 3, pp. 374-377. [3] Lysak V. I., Kuz’min S. V. Explosive Welding. – Moscow: Mashinostrenie-1, 2005 (in Russian).

10. The necessity of intensive plastic deformation It can be explained by the reason that strong bonding takes place when only one of two welded plates becomes deformed intensively with high deformation rates and with generation of disperse particles jet from its surface layer. High-speed jet flow of the softer plate material results in the removal of surface films from the harder plate surface and in deformation and activation of its surface layer. For example when aluminum is welded to steel, high deformation level in the aluminum plate causes the high temperature of the particles in the jet. The jet mainly consists of aluminum particles. As a result, as the jet interacts with a steel, the steel surface is heated, its strength decreases and it becomes prepared to the following process of bond formation behind the collision point. When collision angle is high the jet gets solid and moves along the steel surface scrubbing it like a blade. This process is strikingly illustrated by the picture on the next slide.

11. Example of jet formation The bimetal made by explosive welding: 1 – steel plate, 2 – aluminum plate, 3 - aluminum jet coming out of collision point

12. Delicate explosive welding (DEW) So, to provide strong bonding between soft and hard materials there is no need to accelerate a flyer plate to a very high velocity. It is only necessary to stimulate intensive flow of soft material in the welded couple. When the base plate material has low plasticity and can crack under collision we must reduce a flyer plate mass (thickness) and velocity, and, correspondingly, reduce the explosive charge mass (thicness). EMX enables to do it. It was shown that use of EMX reesults in significant reduction of the residual deformation in the produced bimetal [1]. That is why we call “delicate explosion welding” (DEW) the explosive welding with the use of EMX, when a flyer plate is thin. [1] B. S. Z l o b i n, V. V. S i l’ v e s t r o v, A. A. S h t e r t s e r, A. V. P l a s t i n i n, in: A.A. Deribas and Yu. B. Sheck (ed), New Explosive Welding Technique for Production of Bimetal Plane Bearings, Abstracts of the XI Intern. Symp. on Explosive Production of New Materials: Science, Technology, Business, and Innovations (Strasbourg, May 2-5, 2012), Moscow, Torus Press, (2012), pp. 116-118.

13. Experiments In order to clarify the DEW potentiality, welding of materials with highly differing properties was studied experimentally. The parameters of flyer plate acceleration were chosen in such a way to make the collision process pass near the low boundary of the EW window; the position of this boundary was calculated on the base of the less strong material characteristics.

14. 3 4 1 2 3 D А A Experiments: copper/steel couple In the first series of tests, under study was the effect of the stronger material hardness on the forming of copper-steel bonding. Before the welding, the steel samples (23 х 25 х 125 mm) underwent mechanical treatment in order to reduce the welded surfaces roughness to Ra= 0.32 – 0.16 m. A copper plate of 1.5 mm thick was driven on 3 steel samples with different hardness: Н1 = 130 – 160 НВ; Н2 = 32 – 34 HRC;Н3 = 44 – 46 HRC. Schematic of the experiment on copper plate drive on the steel samples with different hardness: 1 - explosive charge, 2 – copper plate, 3 – steel samples, 4 – base plate, D – detonation velocity

15. Copper/steel couple The collision conditions can be assumed similar for all steel samples. EMX was used as an explosive. The first test was carried out at the drive parameter R = 0.5, which corresponds to the collision angle γ ≈ 7°, the contact point velocity was Vc = 2.6 km/sec. At the given collision parameters, the corresponding point on the Vc – γ diagram is located near but above the lower boundary of the EW window which is calculated on the base of the copper hardness. The tests showed that copper was welded similarly well to every sample. Evident, that the difference in the strength characteristics of the steel samples does not influence the bonding process at such collision parameters.

16. Copper/steel couple In the next test, due to reduced explosive charge, the drive parameter was reduced to R = 0.3, the collision angle decreased to γ ≈ 5°. As the contact point rate is Vc = 2.6 km/sec, the collision parameters coincide, or are located a bit lower than the lower boundary of the EW window. Visibly the welding took place on all three samples. The tests with copper layer peeling (with hammer and chisel) showed that on the samples with the hardness of Н1 = 130 – 160 НВ; Н2 = 32 – 34 HRC, strong bonding took place on more than 95% of the samples area. On the sample with the hardness of Н3 = 44 – 46 HRC, the copper plate separated from about 25% of the area in the place of welding process ending. On the peeling site, the steel surface retained a thin copper layer, which proves that the bonding process took place indeed. We believe that the described experiments vindicate that, as the pressure in the collision zone exceeds the copper strength, and the deformation processes develop extensively in this area, varying hardness of the steel sample does not influence significantly on the welding process.

17. 1 2 3 4 5 Experiments: stainless steel/carbide composite couple (CC is in a copper matrix) Experimental layout for stainless steel welding with copper and carbide composite МС 221: 1 –stainless-steel plate (0.8 mm thick); 2 – copper matrix; 3 – carbide composite insert; 4 – explosive charge; 5 – detonator The composite consists of 82% WC; 3% TiC; 7% TaC; 8% Co. Stainless-steel hardness - 150 HB, copper - 70 НВ. The hardness of the composite МС 221 is of 89 HRA, which is much higher than the strongest steels have.

18. The welding parameters According to evaluations, to start the bonding process on the copper – stainless steel interface at Vc = 2.6 km/sec, the minimal necessary collision angle is γ = 8°, whereas for the carbide composite – stainless steel couple this angle must be γ = 11°. Taking into account almost complete absence of carbide composite plasticity, the shock action must be minimal to avoid cracking. In these very cases, EW is not applicable, and positive results can be achieved with the DEW. In the first experiment, the EMX charge thickness was 7.5 mm, density of explosive ρe = 0.63 g/cm3, detonation velocity D = Vc = 2.6 km/sec. At such parameters R = 0.75, the collision angle γ ≈ 10°. These values of γ and Vc guarantee the presence of the point (Vc,γ) inside the EW window for the couple stainless steel / copper. As for the area of the stainless steel and carbide composite, the point under consideration (Vc, γ) falls on the low boundary of the EW window if it is calculated on the base of the stainless steel hardness.

19. 3 1 2 The view of the welded sample The composite resulting from the explosion welding: 1 – stainless-steel plate; 2 – copper matrix; 3- carbide composite

20. Results In the obtained composite sample there are the waves typical for the explosion welding on the copper / steel interface, whereas the interface of the stainless steel and carbide composite is flat, there are no waves, which is natural for the welding of the materials with highly differing strength parameters. The experiment demonstrates that, aside for naturally predicted welding on the copper / steel interface, the bonding also took place on the carbide composite / steel interface. The 2nd experiment was organized in the same way, its parameters were the following: detonation velocity D = Vc = 2.6 km/sec, R = 0.6, γ = 8.5°. In this case, the point (Vc, γ) lies almost on the lower boundary of the EW window calculated by the copper hardness. The welding took place over the stainless steel / copper interface, whereas, opposite to the 1st experiment, there was no joining on the carbide composite / stainless steel interface.

21. Stainless steel/carbide composite couple (CC is in a steel matrix) To be sure that the start of the bonding process in the 2nd experiment depended on the level of deformation processes development on the copper surface, the following experiment was carried out. The copper matrix was replaced with a steel plate quenched to the hardness of 46 HRC. The rest test parameters coincided with the test No. 2. As was expected, there was no welding on the stainless steel / quenched steel interface. The signs of bonding on the stainless steel / carbide composite area are missing, too. So, it can be suggested that the role of the “special” preparation of the welded surfaces is played by the deformation processes occurring in the less strong and ductile material. The hardness of the stronger material is not of essence providing that its surface roughness influence is minimal. For additional verification of this concept the experiment on explosive welding of aluminum with a carbide composite were performed. For this couple, the hardness difference exceeds 20 times, density difference is 5 times, the aluminum melting point is about two times lower than the carbide composite’s one, and nevertheless, the bonding was obtained.

22. Summary • To realize the conditions needed for bonding, it is enough to provide the well-developed deformation process and intensive material flow in only one of two collided plates. • The low boundary of EW window for materials with highly differing properties can be found on the base of less strong material strength. • The use of EMX enables to plate without destroying and cracking strong low ductile alloy with a thin layer of ductile material. This extends the possibilities of EW.

23. Thank you for your attention!

24. Formulas