RU2789971C2 - Method for welding homogenous porous materials - Google Patents

Abstract

FIELD: welding.

SUBSTANCE: invention relates to a method for welding homogenous porous materials. Butt welding is performed with a high-energy laser radiation source. Before welding, a welded zone of materials is purified, and an insert of compact material identical in its chemical composition to welded porous materials is placed between welded materials. Welding is carried out with addition to the welded zone of modifiers in the form of nano-powder materials selected from a number of refractory compounds. An insert thickness (δ) is specified with a ratio

, where b₀ = 1 is a correction factor;

is a temperature conductivity coefficient of compact material; τ is time of impact of a laser beam on the insert, while τ= d_F/ν, where d_F is a diameter of a focal spot; ν is a welding speed, and an insert height h is specified as exceeding a thickness h_s of welded materials by a value Δh=b₁m_ph_s, where b₁ = 0.78 is empirical constant; m_p is porosity.

EFFECT: prevention of sagging of a face surface of a welded joint and, respectively, increase in the quality of the welded joint, strength, and plasticity of the welded joint.

1 cl, 3 dwg

Description

The invention relates to the field of welding materials with high-energy radiation sources, such as laser, plasma or electron beam method, and can be used for laser welding of products for various purposes from thin-sheet materials in the chemical, electronic and radio engineering industries, as well as in the manufacture of pipes. It can be used for direct connection of two homogeneous porous metals or alloys to each other.

The technological task of welding porous metals is to improve the quality and strength of a permanent joint. To achieve these results, various filler materials are introduced into the welding zone in the form of wire, paste or an intermediate insert made of compact metal, which have a positive effect on the formation of the required properties of the weld. The successful solution of this problem can be facilitated by the use of a new technology for modifying metals and alloys with nanopowders of refractory compounds.

A known method of joining porous parts made of metal powders, including argon-arc welding with a non-consumable electrode using a filler wire made of metal, identical in composition to the material of the porous parts [1]. The disadvantages of the method are the need to use a filler material, the extreme complexity of implementation for welding parts made of porous titanium, which is associated with its high chemical activity at elevated temperatures and the need for additional use of a sealed chamber filled with argon, or additional blowing of argon from the side opposite the welding arc.

A known method of laser welding with a pulsed beam with a pulse duration Τ ≈ 5 ms, the slope of the leading and trailing edges of the pulse is not more than 1/20 T, the use of protective gas argon, carbon dioxide, etc., and the parts to be joined are preheated to a temperature of about 2/ 3 melting points [2]. The disadvantages of the method are the limited technological capabilities - the method is not applicable for porous parts due to the infiltration of the melt from the weld pool into the pores and the continuity of the seam, as well as the complexity associated with the need to use a protective atmosphere or a high-temperature heating chamber.

A method of laser welding of porous parts is known, including joining the edges of the parts to be welded and processing them with a laser beam with energy sufficient for their partial melting, and to prevent filling the pore volume with the melt, coaxial blowing into the welding zone of protective (argon, helium, etc.) and energy-carrying (hydrogen) gas, with additional supply of metal hydride, mainly titanium hydride, to the welding zone and compression of the edges towards each other with a force in the direction of the weld [3].

The disadvantages of the method are: the complexity of implementation associated with the need for preparation and introduction of dispersed filler material, providing coaxial injection of two gases; limitation of technological possibilities. The method is applicable mainly for flat parts; low quality of the weld, due to the possibility of infiltration of the melt from the weld pool into the pores and the formation of discontinuities (pores) in the pool, thereby interrupting the weld as a whole.

The closest to the claimed technical solution is a method of joining porous metal or cermet materials by heating a meltable insert installed in the root part of the connection with a recess in the porous metal of the part, with an electron beam, after which the joint is heated by a beam with heat input, which ensures the penetration of the insert material by 0, 7-0.8 of its height [4].

The disadvantage of this method is the complexity of implementation associated with the preparation of samples, which requires a strict specification of the gap between the welded edges of the parts, the size of the insert and its recess into the porous material, the need for subsequent removal of the remaining unmelted lower part. The method is applicable mainly for connecting thin filter parts.

The objective of the proposed method of welding porous materials is to improve the quality of the weld, strength and ductility of the welded joint. When performing the task, it is necessary to take into account such requirements as the local nature of the thermal effect, the minimum thermomechanical deformation of the part, a wide range of adjustable energy characteristics of the beam, providing an optimal thermal regime with high heating and cooling rates.

A promising direction in the development of high-energy technologies for welding porous materials with penetration is the combination of a high-energy effect on the materials being welded with the modification of the melt in the weld pool with nanosized refractory compounds, called nanopowder inoculators (NPI) in metallurgy. The task is achieved due to the fact that the method of welding porous materials with high-energy laser radiation sources, according to which, before welding, an insert of a compact metal is placed between the welded ends, identical in chemical composition to the materials being welded, while its height h, (m) in order to compensate for the substance by infiltration into porous plates and preventing sagging of the front surface of the weld is set more than the thickness of the plates h _s , (m) by a certain value Δh≥b ₁ m _p h _s /100, (m) (“profitable part”), where b ₁ =0 ,78 is the dimensionless coefficient determined experimentally, m _p is the porosity (%), and the thickness, respectively, is equal to the layer of the compact metal to be melted during the time of the laser beam impact on the insert. These values are estimated by theoretical ratios with correction factors determined experimentally.

When welding materials (plates) with a porosity of ~ (33-35)%, the formation of a weld flush with the base material is carried out when determining the height of the insert according to the ratio Δh/h _s =0.25-0.28.

In FIG. 1 - photographs of one-piece joints made of stainless steel 12X H10T (a, b) and titanium grade VT 1-0 (c, d) are presented, characterizing the morphology of welds at various values of Δh/h _s a) 0.45; b) 0.28; c) 0.35; d) 0.25. It can be seen that by changing the height of the "profit" insert Δh, it is possible to control the size of the welding bead. So, to obtain a weld flush with the upper surfaces of the plates with thickness h _s =3 mm and porosity m _p ~ (33-35)%, it is necessary to take Δh / h _s = 0.78⋅ (0.33-0.35)≅0, 25-0.27 (Fig. 1b.d).

In FIG. 2 - micrographs of the structure of the central zone of welds of steel (a, b) and titanium (c, d} are presented: a, c - without nanomodifying additives; 6, d - with nanomodifying additives (TiN + Ti). in the central zone of the weld instead of a columnar finely dispersed globular structure, which significantly affects the mechanical properties of the joint.

In FIG. 3 - diagrams of bending strength of welded joints: a - titanium, b - steel; 1 - base metal; 2 - with nanomodifying additive (TiN+Ti), 3 - without modifier. When testing for temporary strength, the destruction of all experimental (modified) samples occurred only in the base metal.

The method of welding materials is carried out as follows.

First prepare the welded surfaces of the materials. When preparing the surfaces to be welded (plates), an oxide film is removed over a width of 10-15 mm along the entire length of the joint by etching in a NaOH solution (50 g per 1 liter of H ₂ O), followed by clarification in a 30% HNO ₃ solution. After etching, the plates are washed in hot water and dried. Immediately before welding, the surfaces to be welded are cleaned with a scraper. Helium is used as a protective medium in laser welding, or helium is used to protect the upper surface of the weld pool, and the lower surface is protected by argon. Helium consumption is 6-8 l/min, argon is 5-6 l/min.

где p - массовая доля нанопорошка, ρ - плотность жидкого металла,

- ширина сварочного шва, h_s толщина пластин.The mass consumption of nanopowder material per unit length of the treated surface is

where p is the mass fraction of the nanopowder, ρ is the density of the liquid metal,

- width of the weld, h _s thickness of the plates.

Experimental studies were carried out of the effect of intermediate inserts made of a compact material identical to the porous materials being welded, and nanomodifying powder additives on the quality and mechanical properties of permanent joints during laser butt welding of porous plates made of 12KhN10T stainless steel or titanium alloy of the VT1-0 grade 2 mm thick with an average porosity value of 35% and 33%, respectively. Welding of samples was carried out on a continuous CO ₂ laser in a helium atmosphere at various values of radiation power (0.9–1.5) kW and welding speeds (1.2–1.5) m/min. Specially prepared nanomodifying additives (TiN nanopowder clad with titanium) in the form of a suspension were applied to the pretreated surfaces of the insert and porous plates in the welding zone in an amount of 0.1% by weight of the material to be melted.

In FIG. 1 shows photographs of permanent joints made of stainless steel (a, b) and titanium, characterizing the morphology of welds at various values of Δh/h _s : a) 0.45; b) 0.28; c) 0.35; d) 0.25. It can be seen that a high-quality weld with a surface flush with the surface of the plates is formed at values of the relative profitable part of the insert Δh/h _s equal to 0.28, 0.25 for steel and titanium, respectively.

As can be seen from the micrographs in Fig. 2, and the weld structure of the unmodified joint has a coarse columnar structure from the periphery to the center of the weld. Nano-modifying additives reduce the size of dendrites and form a globular structure of crystalline grains (Fig. 2b), significantly increase the fineness of the structure and decrease the characteristic size of crystalline grains (Fig. 2d). According to the Hall-Petch law, this has a positive effect on the mechanical properties of materials.

Diagrams of bending strength of welded joints are presented (see Fig. 3): a steel, b - titanium: 1 base metal; 2 compounds with nano-modifying additive (TiN+Ti); 3-. without modifier. The strength of welded joints in bending without nanoadditives (BTI) is close to the strength of the base metal, for joints with NTI by 7%. higher. The microhardness also increases (by 10–20)% and, in contrast to the unmodified joint, is characterized by a more uniform distribution over the weld width. Tensile strength tests showed the destruction of welded specimens only along the base metal.

The conducted experiments on welding of porous materials showed a positive effect of intermediate inserts made of a compact material identical in chemical composition to the porous materials being welded using modifying refractory nanopowder materials on the morphology and microstructure of welded joints of porous materials and their mechanical properties. The use of an intermediate insert made of a compact material with a selected (calculated) thickness and height, depending on the porosity of the welded material, allows you to control the size of the welding bead, to obtain a weld flush with the surface of the plates being welded. The use of nanomodifying additives has a positive effect on the structure and mechanical properties of the metal in the area of laser radiation. The morphology and size of the crystalline grain in the area of the weld significantly changes, as a result of which the strength and plastic properties of the welded joint increase, increasing its operational reliability.

Information sources

1. Belov S.V. Porous metals in mechanical engineering. M.: Mashinostroenie, 1981. S. 3134;

2. Application JP 2842967, IPC7 B23K 26/00.1992;

3. Application DE 19859933, IPC7 B23K 33/006.1998;

4. Ovchinnikov V.V., Antonov A.A., Gureeva M.A. The method of connecting porous metal or cermet materials // Patent RU 2215629, published: 2003.11.10 - prototype.

Claims (1)

A method for welding homogeneous porous materials, which includes preliminary cleaning of the surfaces to be welded, placement of an insert between them made of a compact material identical in chemical composition to the porous materials being welded, and butt welding with a concentrated laser radiation source, while welding is carried out with the addition of modifiers in the form nanopowder materials selected from among refractory compounds, characterized in that the thickness of the insert δ is given by the ratio

, where b ₀ =1 - correction factor,

coefficient of thermal diffusivity of a compact material, τ = d _F /ν, where τ is the time of exposure of the laser beam to the insert, d _F is the diameter of the focal spot, ν is the welding speed, while the height of the insert h is set to exceed the thickness of the materials to be welded h _s by Δh= b ₁ m _p h _s , where b ₁ =0.78 is an empirical constant, m _p is the porosity.

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