RU2779439C1 - Mixed powder solder for soldering aluminum and alloys based on it - Google Patents

Abstract

FIELD: aluminum powder metallurgy.

SUBSTANCE: invention relates to the field of aluminum powder metallurgy and can be used to obtain brazed structures from aluminum and alloys based on it. Mixed powder solder for soldering aluminum and alloys based on it contains powders of Al-Si, Al-Cu, Al-Zn, Al-Mg and Al-Sn alloys and has the following chemical composition, wt.%: silicon 9-12, copper 0.005 -4.0, zinc 0.005-4.0, magnesium 0.005-1.5, tin 0.005-1.0, aluminum - the rest. The total content of copper and zinc in the solder does not exceed 7.0 wt.%, and the total amount of copper, zinc, magnesium and tin is from 1 to 9 wt.%.

EFFECT: invention provides a decrease in the solder melting temperature and the possibility of carrying out the soldering process at temperatures below 570°C while increasing the strength of the solder joint.

7 cl, 1 dwg, 3 tbl, 1 ex

Description

The invention relates to the field of aluminum powder metallurgy and can be used to obtain brazed structures from aluminum and alloys based on it.

Known solder for soldering aluminum alloys composition (wt.%): germanium 14-52, at least one of the components of the group: silicon, magnesium, bismuth, strontium, lithium, copper, calcium, zinc and tin 0-10, aluminum - the rest (international application WO/1992/019780, B23K 35/28, C22C 21/00), analogue.

The disadvantages of solder are the low corrosion resistance of solder joints and the high cost due to the high content of germanium.

A known method of soldering aluminum and material for soldering aluminum (patent RU 2164460, 2001), taken as a prototype.

The disadvantages of the method and material are that the composition of the material uses powders of elemental silicon, zinc, copper and germanium. The authors casually mention binary and ternary alloys of the Al-Si, Al-Cu, Al-Zn, Al-Si-Cu, Al-Cu-Zn, Al-Si-Zn and Zn systems, but do not give specific concentrations.

Indeed, silicon, zinc, copper, and germanium have significant solubility in aluminum, even in the solid state (see Table 2), but the resulting composition is variable and unpredictable. An alloy with such concentrations of elements can be formed, the melting temperature of which will be higher than the melting temperature of the main component of Al-12% Si solder (577°C), which will worsen the conditions for soldering structures.

For example, deterioration in solderability will occur if, when using elemental silicon powder and dissolving it in aluminum, an alloy with a silicon content that goes beyond 9-13 wt. %. The same applies to copper (permissible limits 24-42 wt.%), and zinc (53-100 wt.%) and germanium (10-70 wt.%).

The present invention is aimed at correcting this shortcoming.

Offered mixed powder solder for soldering aluminum and alloys based on it contains the following components in the following ratio, wt. %: silicon 9-12, and at least one of the elements from the group including copper 0.005-4.0, zinc 0.005-4.0, magnesium 0.005-1.5, tin 0.005-1.0, aluminum - rest. The total content of copper, zinc, magnesium and tin is 1.0-9.0 wt. %.

The objectives of the invention are to improve performance (strength, corrosion resistance), increase the range of brazed joints made of aluminum-based alloys and the service life of the structures obtained, reduce the cost due to the absence of germanium in the alloy (which is twice as expensive as silver).

The technical result consists in reducing the melting temperature of the solder and the possibility of carrying out the soldering process at temperatures of 550-570°C, improving the technological properties (wetting of the base metal with solder and spreading) of the solder, in enabling the soldering of structures made of high-strength aluminum alloys with a solidus temperature below 550°C , obtaining high solder strength and corrosion resistance of brazed joints, increasing the service life of the resulting structures.

The specified technical result is achieved by the fact that the solder for soldering aluminum and alloys based on it, containing aluminum, silicon and at least one element from the series: copper, zinc, magnesium, tin with the following content of components (wt.%): silicon 9-12; copper 0.005-4.0, zinc 0.005-4.0, magnesium 0.005-1.5, tin 0.005-1.0, aluminum - the rest, and the total content of copper, zinc, magnesium, tin is 1.0-9.0 wt. %.

A distinctive feature of the proposed solder is that it consists of powders of binary alloys based on aluminum (wt.%), namely: powder Al-12Si - the main component (known); powder of the Al-Cu system; and/or powder of the Al-Zn system; and/or powder of the Al-Mg system; and/or powder of the Al-Sn system. The last four alloy powders improve the physicochemical and service properties of the mixed material when used as a solder.

A mandatory requirement for powders of all components is their production by high-speed cooling. The processes of gas atomization of the melt provide cooling rates of particles with a particle size of 40-140 microns at the level of 10 ⁴ -10 ³ °C / s, with a particle size of 10-40 microns - at the level of 10 ⁵ -10 ⁴ °C / s, with a particle size of 0.01-10 microns - at the level of 10 ⁶ -10 ⁵ °C/s, which is necessary for the formation of a uniform structure of the "microingot".

Silicon, copper, zinc, magnesium and tin are the main components of the proposed mixed solder, which narrow the interval between solidus and liquidus and reduce the solder melting temperature. Silicon, zinc, copper provide an increase in service characteristics - the strength and manufacturability of the solder.

Silicon forms with aluminum the eutectic Al-12Si (wt %) with a melting point of 577°C. The final content of silicon in the mixed solder is 9-12 (wt.%). The lower limit of its content provides sufficient fluidity and prevention of hot cracks during soldering; the upper limit allows a significant part of the solder to crystallize near the eutectic point, only low-melting phases (eutectics with the participation of copper, zinc, magnesium and tin) remain liquid. At this point, the solder allows the formation of a sufficiently strong structure that can withstand shrinkage until it solidifies completely without the formation of hot cracks.

To improve the mechanical (increase in plasticity) and technological properties of the solder, additives of metal modifiers are used, the essence of the influence of which is reduced to the grinding of silicon grains in the composition of the eutectic.

Magnesium belongs to highly effective modifiers, zinc - to medium effective ones, tin - to ineffective ones. Copper is not a modifier for eutectic silumins; its main role is to destroy the oxide films of particles of Al-Si, Al-Mg, Al-Zn, Al-Sn alloys, being a part of a low-melting eutectic with aluminum, and thereby facilitate the difficult task of sintering aluminum alloy powders with the formation of a single strong junction.

In this case, a complex alloying of the solder with several modifiers was used, which makes it possible to achieve an enhancement of the modification effect.

Copper and zinc together with silicon and aluminum form a multicomponent eutectic.

To a greater extent, copper affects the decrease in the liquidus temperature. However, due to the large difference in standard electrode potentials compared to the aluminum base, copper significantly worsens the corrosive properties of solder joints and reduces technological properties (wetting of the base metal). In this regard, the doping of solder with copper is limited to 4.0 wt. %.

Alloying with zinc, reducing the melting point of the solder, affects the corrosive properties of the solder joint to a lesser extent. The content of zinc in the alloy is also limited to 4.0 wt. % in order to avoid a decrease in the strength characteristics of the junction.

To ensure high mechanical and satisfactory corrosion properties of brazed joints and prevent the formation of hot cracks during soldering, the total content of copper and zinc in the alloy does not exceed 7.0 wt. %.

In the case of using solder for flux-free soldering in a vacuum, small additions of magnesium are introduced into its composition. The saturated vapor pressure of the aluminum oxide film in a vacuum is extremely low, and under normal soldering conditions it does not collapse, which does not allow obtaining high-quality solder joints. To destroy the oxide film on aluminum alloys during vacuum soldering, activator metals are used, which, interacting with aluminum oxide, loosen it and allow liquid solder to reach the soldered surface. Easily evaporating (with the temperature of the beginning of evaporation below the melting temperature of aluminum-based solder) activator metal is magnesium. Magnesium supplements in an amount up to 1.5 wt. % into the solder on heating above 400°C allow to obtain high-quality solder joints.

During air brazing using fluoride-based fluxes, magnesium additives can form refractory magnesium fluorides (eg MgF ₂ , KMgF ₃ , K ₂ MgF ₄ ). But, since the magnesium content is limited to 1.5 wt. %, and not all magnesium will react to the formation of fluorides, and the content of low-melting flux composition 46 KF - 54 AlF ₃ (wt.%) in the solder paste is significant (20 wt.% or more), then the formation of 1.0-1.5 wt . % magnesium fluoride will not be able to significantly increase the melting point of the flux - only 2-3°C, than in the claimed conditions can be neglected.

Tin additives are used to modify the structure of the solder. To obtain an optimal structure, a finely dispersed interdendritic distribution of tin is necessary. The solubility of tin in aluminum and zinc is extremely low (0.05-0.06 wt.%), the mutual solubility of tin and silicon is absent. The maximum content of tin in mixed solder is limited to 1.0 wt. %, because with a higher content, a decrease in the corrosion resistance of the solder is possible due to the appearance of low-melting eutectics such as Al-Sn (228.3 ° C), Al-Zn-Sn (198.5 ° C), Al-Si-Sn (232 ° C). When the tin content is less than 0.1 wt. % its modifying effect is not manifested.

Based on the above considerations, the total amount of copper, zinc, magnesium and tin in the proposed mixed solder is limited to a minimum value of 1.0 and a maximum of 9.0 wt. %.

Table 1 provides information on the temperature-concentration ranges of the doping region of binary alloys and on the position of the eutectic points.

According to Table 1, in order to obtain binary alloys in a liquid state at 577°C, their chemical composition should be as follows (wt.%):

Al-(24-42)Cu; Al-(54-95)Zn; Al-(16-85)Mg; Al-(84-95)Sn.

Upper value 95 wt. % (rather than 100) for zinc and tin is established from the consideration that these should be alloy powders containing at least 5% Al, and not pure Zn or Sn powders.

Table 2 shows the values of the limiting solubility of various elements and phases in aluminum.

Based on the information in Table 2, the most soluble in solid aluminum (and, accordingly, Al-12Si alloy) are Mg→Zn→Cu→Sn (arranged as solubility decreases). The following mechanism and chemistry of alloy formation is presented - see FIG. 1 (Appendix 1), where particles are schematically shown a) in a cold state b) at a temperature above 550°C c) at a temperature above 5770 C. Description of the scheme:

a) fine, 0.01-40 µm in size (preferably 0.01-10 µm), particles of Al-Me binary alloys are uniformly distributed over the surface of large (40-140 µm) Al-12% Si alloy powder particles;

b) Al-Me particles melt, the alloy spreads over the surface of Al-12Si particles and wets them;

c) Al-12Si particles melt, and the Me element, which enters the Al-Me melt, begins to diffuse deep into the Al-12Si alloy, forming a boundary, more fusible Al-Si-Me alloy of variable composition.

Since the particles of the base alloy are relatively large, and the particles of binary alloys are relatively small, and the total content of powders of binary alloys is small (up to 18%, see Table 3), the elements Cu, Mg, Zn and Sn will not diffuse deep into the Al-Si alloy particles, but they will create a near-surface low-melting layer enriched with these elements while maintaining the core of the particle of the initial composition Al-12Si.

Such a mechanism of non-equilibrium alloy formation is similar to brickwork, when a liquid cement mortar is placed between solid bricks. The difference lies in the fact that brick and mortar have only mechanical adhesion, and in the case of the mentioned alloy formation, the particles are also chemically bonded due to the formation of a single, more fusible than Al-12Si, Al-Si-Me alloy of variable composition.

Example.

The following alloy powders (wt %) sprayed with nitrogen were used for the experiments: Al-12Si, Al-33Cu, Al-80Zn, Al-50Mg, Al-80Sn.

A fraction of 40–140 µm was isolated from the Al-12Si powder by sieving on sieves 004 and 014 according to GOST 6613-86. A fraction of 0–29 µm was isolated from powders of other alloys by sieving on a 0029 sieve according to TU 1276-003-38279335-2013.

The powders of these binary Al-Me alloys are added to the main Al-12Si solder powder so that the content of the Me element does not exceed: 4.0 wt. % Cu and Zn each, 1.5 - Mg, and 1.0 - Sn, and 9.0 wt. % - all in total. Examples of the composition of powder mixtures, the chemical compositions of the obtained non-equilibrium powder alloys and the results are shown in Table 3.

An example of the composition of a mixture of Al-12Si, Al-33Cu, Al-50Mg with the expectation that the Cu content is 4.0, and Mg is 1.5 (wt.%) - Sample No. 9.

For aluminum-copper:

For aluminium-magnesium:

For aluminum-silicon: [Al-Si]=100-12.1-3=84.9%;

For silicon [Si]=12⋅0.849=10.19%

Mixed solder powder was used for soldering two plates 3 mm thick from alloy D16 in a sealed container installed in a Nakal furnace, model PKM 4.8.4/12.5 in an atmosphere of high-purity nitrogen ([O ₂ ] ≤ 7 ppm) using a flux composition (wt.%) 54AlF ₃ -46KF. The soldering temperature was 577°C. The container was purged with thirty times the volume of high purity nitrogen (no worse than 99.999% purity). The plates were overlapped, and a layer of paste approximately 1 mm thick was applied over the entire contact area of the two plates.

The exposure at the soldering temperature was 8 min.

To study each solder sample, 5 pairs of plates of solder samples were made, the soldering of which was carried out in one charge.

When soldering solder-prototype marked lower solder spreading compared to the claimed.

After soldering, the flux was washed in an ultrasonic bath. After washing off the flux residues, the formation of a high-quality solder joint was visually established.

The next two stages of research are mechanical tensile tests of solder joints and corrosion tests of solder joints in salt fog using an accelerated method.

Mechanical tests of brazed samples at work were carried out according to ISO 6892-84 on a standardized testing machine of the R-10 brand. Test specimens were made according to GOST 5264-80 (lap joint). The test results are presented in Table 3. During the tensile test, the specimens failed at the solder joint, while the solder joint was weakened by lateral radii of 5 mm with a shank width of 20 mm and a thickness of 3 mm. At the place of the soldering seam, the thickness was 10 mm.

Tests found that the claimed solder allows you to increase the strength of the joints in comparison with the prototype by 2-12%. Strength tests were carried out on samples according to GOST 5264-80 (lap joint).

There was no solder, while some of the samples soldered with the prototype solder were destroyed by seam defects (“non-solders”), due to insufficient wettability of the base metal solder and solder spreading.

After testing for corrosion resistance of samples soldered with a prototype solder, local centers of corrosion damage in the form of small white spots were revealed on the surface of the brazed joint. On the surface of the samples brazed with the claimed solder, no corrosion lesions were visually detected.

Thus, the claimed solder provides a higher strength compared to the solder prototype (not lower than 130 MPa), and improves the processability of the solder, provides soldering at temperatures from 550 ° C, which will allow the use of solder for soldering most modern structural aluminum alloys.

Claims (7)

1. Powder solder for soldering aluminum and alloys based on it, containing Al-12Si alloy powder as the main component, characterized in that it additionally contains powders of Al-Cu, Al-Zn, Al-Mg and Al-Sn alloys. 2. Powder solder according to claim 1, characterized in that the copper content in the Al-Cu alloy powder is 24-42 wt.%, zinc in the Al-Zn alloy powder is 54-95 wt.%, magnesium in the Al- Mg is 16-85 wt.%, tin in Al-Sn alloy powder is 84-95 wt.%. 3. Powder solder according to claim 1 or 2, characterized in that Al-12Si powder particles have a particle size of 40-140 microns, and Al-Cu, Al-Zn, Al-Mg, Al-Sn powder particles have a particle size of 0.01 -40 µm, preferably 0.01-10 µm, all powders obtained using high-speed cooling, for example, gas atomization of the melt, to form a uniform particle structure. 4. Powder solder according to any one of paragraphs. 1-3, characterized in that the powder content of the Al-Si alloy is 80-99 wt.%, Al-Cu - up to 10 wt.%, Al-Zn - up to 10 wt.%, Al-Mg - up to 10 wt. %, Al-Sn - up to 5 wt.%. 5. Powder solder according to any one of paragraphs. 1-4, characterized in that it has the following chemical composition, wt.%: Si 9-12, Cu 0.005-4.0, Zn 0.005-4.0, Mg 0.005-1.5, Sn 0.005-1.0 , aluminum - the rest. 6. Powder solder according to any one of paragraphs. 1-5, characterized in that the total content of copper and zinc in it does not exceed 7.0 wt.%, and the total amount of copper, zinc, magnesium and tin is not less than 1 and not more than 9 wt.%. 7. Powder solder according to any one of paragraphs. 1-6, characterized in that it provides a brazed joint, which is an Al-12Si alloy particles bonded by an interlayer in the form of a low-melting Al-Si-Cu-Zn-Mg-Sn eutectic of variable composition.

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