Lecture No. 6
Gold alloys of the day solders
In the manufacture of jewelry and art products Soldering is used to make gold alloys.
Gold solders are marked in the same way as silver solders.
The gold content in solders must correspond to the grade of the alloy being soldered. Strict requirements are imposed on the color of the solder; it must strictly match the color of the metal being soldered. In addition to gold and silver-based solders, jewelry technology uses copper-based solders - copper-zinc and copper-phosphorus, which may additionally contain tin, manganese, iron, aluminum and other metals. These solders can withstand high mechanical loads.
Fluxes are used to reduce surface tension and improve solder flow. For soldering jewelry solutions of borax and boric acid.
Silver is a chemical element, metal. Atomic number 47, atomic weight 107.8. Density 10.5 g/cm3. The crystal lattice is face-centered cubic (fcc). Melting point 963°C, boiling point 2865°C. Brinell hardness 16.7.
Silver - metal white. It is considered the second noble metal after gold. Polished pure silver practically does not change its color in air. However, under the influence of hydrogen sulfide in the air, over time it becomes covered with a dark coating - silver sulfide AgS. Silver, compared to gold and platinum, is less stable in acids and alkalis.
Silver deforms beautifully in both cold and hot states. It polishes well and has high reflectivity.
The widespread use of silver in photography and electrical engineering is due to its unique physical properties: the highest electrical and thermal conductivity among metals.
Despite the fact that silver is a relatively rare element (its content in the earth’s crust is only 7x10 -6%, and in sea water even less than 3x10 -8%), it has been widely used in jewelry production for many centuries. This is primarily due to the high decorative properties of silver, as well as its unique ductility. Silver jewelry is often made using the “scani” technique - a pattern made from thin wire. Threads for silver embroidery are made from silver.
In jewelry manufacturing, as well as in the electronics industry, both pure silver and its alloys with copper and platinum are used.
Grades of silver and silver alloys are regulated by GOST 6836-80.
The standard applies to alloys intended for electrical conductors and contacts, jewelry, and strings of musical instruments.
According to this standard, silver alloys are designated by the letters Wed, followed by ligatures ( Fri- platinum, front- palladium, M- copper). The numbers after the letter designation of the alloy indicate the mass fraction of silver, expressed in ppm (tenths of a percent) for pure silver and silver-copper alloys (for example, Ср 999, СрМ 916, СрМ 950, etc.), or the mass fraction of the main alloying components , expressed as a percentage (in this case, the number is separated from the letter designation not by a space, but by a hyphen, for example: SrPl-12 (12% Pt, 88% Ag), SrPd-40 (40% Pd, 60% Ag), SrPdM-30 -20 (30% Рd, 20% WITH u , 50% Ag).
All silver alloys (GOST 6836-80) can be used in the electrical industry for the production of contact groups for various purposes. For the manufacture of strings of musical instruments, the alloy SPM 950 is used.
GOST 6836-80 establishes grades of silver and silver alloys with copper, platinum and palladium, intended for the manufacture of semi-finished products by casting, hot and cold deformation. Other silver alloys are regulated by industry standards or specifications.
Chemical composition silver and its alloys must comply with the standards specified in the tables (GOST 6836-80).
Silver alloys
In jewelry, in almost all cases, alloys are used in which the silver content is higher than 72%. The white color of silver becomes more and more yellowish with increasing copper content. If copper makes up 50% of the alloy, the alloy becomes reddish, and with 70% copper it is red. If the alloy needs to be soft after casting, then it should not be hardened; on the other hand, heating to a certain temperature can achieve a significant increase in hardness. For enameling, alloys with a high silver content or even pure silver should be used so that the product on which the enamel is applied does not melt.
The resistance of silver-copper alloys to acids is almost the same. Silver alloys are easily dissolved in nitric and concentrated sulfuric acid.
According to GOST 6836-80, 18 silver samples. The following alloys are used in the jewelry industry: 960, 925, 916, 875, 800 and 750.
All of them are silver-copper, have high ductility and malleability.
Platinum and palladium alloys
Platinum alloy is rarely found in modern jewelry; it has given way to white gold. Some jewelry uses a two-component 950 alloy, which contains copper and iridium in addition to platinum. The addition of iridium significantly increases the hardness of the alloy.
Palladium is not yet generally recognized as an independent metal for the production of jewelry, but it has good prospects, since it is cheaper than platinum and has a more intense White color, better machinability, and the same resistance to tarnishing in air as platinum.
Alloys with similar compositions different countries may have different names, sometimes there are “outdated” names, and many alloys of non-ferrous metals are used in which the word “gold” may be used, but at the same time is not gold. Here are some of them.
Alloys of gold and platinum and their imitation
· Gerazoloto - the German name for 8-10 carat gold produced by the factory method.
· Gold “pink”” - English name for a very pale shade of gold.
· American False Gold- very finely gilded tombac.
· Candied gold- gold 980 and 1000 samples.
· Rolled gold- copper with thin (8 microns) gold plating.
· Electron- - natural alloy of gold and silver (39%).
· Gold “Musiv”- tin sulfide plates with a golden sheen.
· Garnet gold- an alloy of 250 and 1000 gold, used in the 19th century in the Czech Republic for products with garnets.
· Palau- North American name “ white gold" Alloy of gold and palladium (8:2).
· Oraide or French gold- 80% copper, 15% zinc, 5% tin, or 86.13% copper, 13% zinc, 0.4% tin, 0.6% iron.
· Pinchbeck or English gold- an alloy of copper (83-93%) and zinc.
· Half gold(German name) - an alloy of copper (83.7%), zinc (9.3%), tin (7%). As a rule, with gilding.
· Goldin- an alloy of copper and aluminum.
· Gold leaf- very thin brass sheets.
· Similor- alloy of copper (83.7%), zinc (9.3%), tin (7%), yellow color
· Sterrometal- brass alloy.
· Red brass- an alloy of copper (90%) and zinc (10%), there may be a different ratio.
· Oroton- a trade name for an alloy similar to tombak.
· Chrysocalc or golden bronze- copper alloy (95-98%), zinc (2-5%). Maybe a different alloy.
· Bashbronze- bronze containing 6% tin, suitable for gilding.
· Aluminum bronze- alloy of copper and aluminum (9: 1). English name aufin, aural, aufor; - French name for fire-gilded silver.
· Hamiltonmetal(chrysorin) - an alloy of copper (66.7%), zinc (33.3%). Well suited for gilding.
· Mannheim gold- alloy of copper (83.6%), zinc (9.4%), tin. Products are gilded.
· Mosaic gold- alloy of copper (66%), zinc (34%). Has a hint of native gold.
· Polixenes- the name of natural platinum with other metals.
· Platinin- the name of the alloy of platinum (67%) and silver (33%).
· Plakart- the alloy is similar in appearance to platinum, consists of palladium (78%), gold (15%) and silver (7%).
· Belgika- an alloy that imitates platinum, consists of iron (74.5%), chromium (16.6%) and nickel (8.9%).
· Durametal- an alloy of copper, zinc and aluminum.
· Platinor- an alloy consisting of copper (57%), platinum (18%), silver (10%), nickel (9%) and zinc (6%). It has a beautiful golden color.
· Platinum Bronze- an alloy of nickel and tin with a small addition of platinum, sometimes silver is added.
· Stellite- an alloy of chromium and cobalt, similar to platinum.
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Replacing silver with dielectric coatings makes it possible to obtain light filters with a maximum transmittance of 80 - 90% with a small bandwidth.
To replace silver, it is necessary to take refractory metals, and non-oxidizing ones, since widely used ceramic masses require firing in an oxidizing environment. Such metals are platinum and palladium; both of them are scarce and expensive, which increases the cost of monolithic capacitors and limits the development of their production.
Oxide catalysts for the oxidative dehydrogenation of methanol have been developed to replace silver. The most effective of them are oxides of molybdenum and titanium. To increase activity, up to 37% iron oxide is added to molybdenum oxides. Mixed catalysts are more active and selective; the process on them proceeds at a higher low temperatures(350 - 400 C) and with a large excess of air in the reaction mixture. These catalysts are gradually replacing the silver catalysts previously used in industry.
Copper is considered one of the most promising materials for replacing silver in fusible elements of high-speed fuses. Copper is at least 300 times cheaper than silver and is close to it in its electrical properties. The electrical resistivity of copper is 5-6% higher than that of silver, which is easily compensated by an increase in the cross-section of the fusible elements. The elastic modulus of copper is 1 to 3 times higher than that of silver, which is unfavorable for the cyclic operation of the fuse. The thermal conductivity of copper is approximately 6% less than that of silver, and the melting point is more than 120 C higher.
The current trends of increasingly widespread introduction of ceramic materials instead of petroleum-containing plastics for the manufacture of insulating parts of electrical installation devices and the replacement of silver and silver-containing metal-ceramic compositions with full-fledged contact materials (alloys) that do not contain precious metals, for the manufacture of breaking contacts of switches and switches will receive the greatest development in the future.
Nowadays, nickel silver and its related cupronickel (nickel silver has no zinc, but contains about 1% manganese) are used not only and not so much as a replacement silverware, how many for engineering purposes: cupronickel is the most resistant (of all known alloys. This excellent material for taps, valves and especially condenser tubes.
Nickel silver and its related cupronickel (nickel silver contains no zinc, but contains about 1% manganese) are used not only to replace silverware, but also for engineering purposes: cupronickel is the most resistant (of all known alloys.
S-electrons of the alloying metal fill the vacancies of the cf band of palladium, reducing % A, and the effect of the additive increases as one goes from Ag to Sb and especially sharply when replacing silver with cadmium.
Toning positive images in different colors, for example brown, blue, green and others, is based on the transformation of metallic silver in the emulsion layer into some colored compound, as well as by replacing silver with another metal or dye. When toning, only the image itself is colored, and the image may be slightly strengthened or weakened during the toning process. The composition of tinting solutions and their quantity are determined by the method used.
Large quantities of silver are used to produce photographic and film materials. Despite persistent attempts to replace silver in these materials with other metals or substances, the problem still remains unresolved.
Enhancement is used to increase the visual or copy density of an image, as well as correct its contrast. Strengthening the skins can be done by building on metallic silver present in the layer of any substance, forming a colored silver salt and replacing the silver with another substance. The amplification process is performed in one or more solutions.
In the case of phosphorus KS1 - Ag, the curve consists of two maxima at 575 and 450 tu. The latter coincides with the K-band in the spectrum of phosphorus KS1 - Ag and is undoubtedly caused by silver, and not by the base, since this peak is not observed when silver is replaced by thallium. Curve a Fig. 70 depicts the absorption spectrum of X-ray phosphorus NaCl - - Ag with a low concentration of activator.
Copper-tolo-po alloy plating, or bronzing, is used for both corrosion protection and decorative finishing surfaces of products. Coating with a low-tin alloy (10 - 20% tin) of golden yellow color is also used as a sublayer instead of copper and nickel coatings before chrome plating. A high-tin alloy (40 - 45% tin), the so-called white bronze, in some cases can serve as a replacement silver Despite the fact that the value of the electrical resistivity of the Cu-Sn alloy is significantly higher than that of silver, in an industrial atmosphere where there are impurities of sulfur compounds, it remains stable, while, like that of silver, it increases by tens of dollars. For this reason, white bronze coatings are recommended for application to electrical contacts.
Treatment with developer can be replaced by treatment with ammonia or sodium sulfide, which precipitate only mercury in the form of black oxide or sulfide. When exposed to ammonia, silver chloride dissolves simultaneously. Replacing silver with the mentioned mercury compounds increases the intensity of blackening. Reinforcement with a mixture of uranium nitrate and potassium iron cyanide is often used. Reacting with silver, this mixture deposits a dark precipitate on its grains from a mixture of ferric cyanide salts of uranium and silver.
Tantalum can also replace platinum in the manufacture of various chemical utensils. In the artificial silk industry, tantalum is used for the production of mouthpieces, in the chemical industry for lining equipment and parts of pumps that are subject to the greatest corrosion. Tantalum is recommended to replace silver in spark contact tips and as a cathode in the analysis of metal salts. High strength, thermal conductivity and acid resistance make possible use tantalum as a material for electric heaters for hydrochloric and sulfuric acids.
(Frolov V.V., Ermolaeva V.I.)
30.1. Physico-chemical properties of silver
Silver is a chemical element of group I B of the Periodic Table of D.I. Mendeleev with serial number 47 n and atomic mass 107.88. Silver crystallizes in a face-centered cubic lattice and does not undergo polymorphic transformations. Silver has the highest electrical conductivity, thermal conductivity and reflectivity among metals.
The main physicochemical and mechanical properties of silver are given below:
TOC o "1-5" h z Density, kg/m3 .................................... ........................................... 1049
Temperature coefficient of linear expansion,
■10e, deg"1.................................... ................................. 19
Thermal conductivity coefficient, W cm-1 deg-1 .... 4.18
Specific heat capacity, kJ/kg-deg.................................... 0.235
Electrical resistivity, µOhm-cm... 1.59
Melting point, °C................................................... ................. 960.5
Ultimate tensile strength, MPa.................................................... 180
Yield strength, MPa................................................... ......................... thirty
Relative elongation, % 50
Silver does not dissolve in dilute hydrochloric and sulfuric acids; it dissolves well in nitric acid, a mixture of nitrogen and hydrochloric acid, in hot concentrated sulfuric acid, does not interact with alkalis, silver oxides are unstable. The darkening of silver is associated with the formation of a film of Ag2S sulfide on its surface in humid air containing sulfur compounds. Therefore, it is impossible to use silver and its alloys in an environment containing hydrogen sulfide, wet sulfur dioxide, or in contact with rubber and hard rubber. Silver is used in instrument making mainly for the manufacture of contacts, in the chemical industry for the manufacture of welded structures operating in particularly aggressive conditions, in cryogenic technology, and in the jewelry industry.
Various impurities even in small quantities significantly reduce the conductivity of silver. Silver is susceptible to erosion and has low arc parameters compared to other metals; it lends itself well to all types of plastic processing, welding and soldering.
Silver is produced in two grades: Ср999.9 and Ср999 (GOST 6836-80), the silver content of which is 99.99% and 99.9%, respectively. Main impurities: Pb, Fe, Sb, Bi.
30.2. Main grades, structure and mechanical properties
Silver forms a continuous series of solid solutions with gold and palladium, the alloys of which are widely used
In the silver-gold system at average concentrations of components, resistivity, thermal conductivity, and ductility are maximum, mechanical strength is low, and corrosion resistance is high. Gold-silver alloys are strengthened with copper, they are marked ZlSrM990-5, ZlSrM980-15, etc. (GOST 6835-80), where the first digit indicates the gold content, the second - silver. The ZlSrM990-5 alloy contains 99.0% gold, 0.5% silver, and the rest is copper. Alloys of this system contain Ag from 0.5 to 33% (by mass).
Alloys of the Ag - Pd system are produced in two grades: SrPd20 and SrPd40 with a silver content of 80 and 60%, respectively. They have properties similar to those of gold-silver alloys.
Ag - Pd - Cu alloy SrPdM30-20 (GOST 6836-80) contains 50% Ag, 20% Cu, 30% Pd.
Ag-Pt alloys form a peritectonic phase diagram with limited solubility of components. Alloys with a Pt content of 10-45% (by mass) may be subject to aging. By heat treatment of these alloys, high hardness and strength can be achieved: up to 3600 MPa after hardening at 1000 °C and aging at 550 °C.
Ag - Cu alloys form a eutectic type phase diagram with regions of limited solubility. Aging can significantly improve the mechanical properties of alloys. Copper increases the hardness and reduces the erosion of silver especially in the field of eutectic alloys, but reduces the corrosion properties
30.3. Weldability of silver and its alloys
Welding silver and its alloys is difficult due to high thermal conductivity, which requires the use of concentrated heat sources and preheating to 500-600 °C. A high coefficient of thermal expansion can lead to significant stress and deformation of products. Liquid silver dissolves oxygen well; during crystallization of the metal, the formation of Ag20-Ag eutectic with a melting point of 507 ° C is possible, the release of which embrittles the metal, and the formation of pores is also possible. When melting and welding, silver evaporates intensely. The impurities Al, Cu, Si, Cd contained in silver alloys can be oxidized during welding, which will lead to a loss of ductility of the alloy. Due to its high fluidity, welding of silver and its alloys is recommended to be performed in a lower or slightly inclined position.
30.4. Welding technology for silver and its alloys
For welding silver and its alloys, gas welding, argon arc welding with a non-consumable electrode, and forge welding are used.
When gas welding, methane-oxygen and acetylene-oxygen normal flames are used, as well as filler wire deoxidized with aluminum, and flux prepared in ethyl alcohol from equal amounts of borax and boric acid. Flux is applied to the joined edges or filler wire. Flame power, l/h: №=(100-150)s, where s is the thickness of the metal being welded, mm. The “left” welding method is used, and the distance from the flame core to the surface of the weld pool should be 3-4 mm. The torch is positioned perpendicular or slightly inclined to the surface to be welded. Heating is carried out at the maximum possible speed, without breaks or repetitions. Assembly is carried out, as a rule, without tacks in special devices. The edges to be welded and the filler wire are melted simultaneously, and the wire is heated to more than high temperature. Seams are very prone to porosity.
Mechanical properties of joints made by oxygen acetylene welding: av 98-127 MPa, bend angle 30-180°.
Arc welding with a tungsten electrode in an argon environment is carried out with direct current of straight polarity. The filler wire is selected according to its composition close to the metal being welded. Manual and automatic welding is possible. Manual welding is carried out at a “forward angle” without transverse vibrations, the angle of inclination of the torch to the surface to be welded is 60-70°, the filler wire is fed at an angle of 90° to the tungsten electrode. Welding of silver butt joints is performed in a downward or slightly inclined position. High-quality formation of the seam is ensured by the use of forming linings. The mechanical properties of silver joints made by argon arc welding with a tungsten electrode are higher than those made by gas welding. In table Table 30.1 shows the mechanical properties of joints made by argon arc welding on silver sheet of grade Ср999.9 with a thickness of 2 mm. The original metal had a tensile strength sm = 161.9 MPa, relative elongation 6 = 28.5%, bend angle a = 180°.
The most stable properties, close to the properties of the original metal, are possessed by welded joints made in a chamber with a controlled atmosphere, which is associated with reliable protection of the weld pool.
When bimetallic sheets low-carbon steel - silver is observed a large number of pores, therefore in some cases it is recommended to use an intermediate cladding layer of nickel, copper or silver. At
Silver has been known to mankind since ancient times, but continues to be in demand today. His physical properties differ sharply from all other noble metals.
Silver is very ductile, easily forged and extremely malleable. The degree of softness is lower than that of gold, but higher than copper. The metal has the highest electrical and thermal conductivity, excellent reflectivity, does not react with other metals and is highly polished.
Jewelers have long used silver to make jewelry. However, in pure form it is not used. Due to its softness, the product is easily deformed, scratched and loses clarity relief patterns. Silver is afraid of hydrogen sulfide and ozone and quickly darkens, becoming covered with a black coating that is difficult to remove. To enhance the strength characteristics, silver is combined with certain metals: copper, aluminum, cadmium, nickel, zinc and rhodium. Such additives are called ligatures.
They give silver hardness and wear resistance. From metal with the obtained qualities, jewelers make highly artistic products of the most complex technique.
To estimate the silver content in an alloy, use the sign try, which shows how many grams of silver are contained in one kilogram of alloy. The most well-known to the general consumer are 875, 925, 960 and 999 samples.
When alloying with multiple metals, more complex technology is used. So, to obtain a silver-copper-zinc-cadmium alloy, each metal is first rolled into the thinnest plates. These plates are then wrapped in silver sheets, bagged, pressed, beaten and melted.
However, introducing an inappropriate amount of alloy into silver or an alloy may not improve the properties of silver, but sharply worsen it. For example, when 1% nickel is introduced into an alloy, its strength increases, but already at 2.6% the alloy becomes brittle. If you add more than 9% tin to an alloy of silver and copper, the alloy will become brittle and will begin to melt and oxidize.
To avoid such problems, jewelers add the most suitable metal to silver - copper. The usual rate of copper administration is from 5 to 50%. The products have excellent appearance and look like pure metal.
Alloy Shibuichi , obtained in Japan, consists of only ¼ silver, and ¾ copper. An alloy with the addition of 5% gold also has the same name. The alloy is very popular nowadays. Products are usually patinated to give beautiful shades. Widely used in the manufacture of bracelets, knife handles, rings, earrings and brooches.
In Russia, metal alloys are regulated by GOST. According to him, silver has a short designation - Sr, gold - Zl, palladium - Pd, copper - M.
Alloy of silver and copper, formulas A it is easy to read and understandable in its simplicity.
So the alloy ZlSrM585-80 (called red gold) contains 585 parts of gold, 80 parts of silver, the remaining parts are copper (1000-585-80=335). That is, an alloy bar of this brand weighing 100 grams contains 58.5 g of gold, 8 g. silver and 33.5 g of copper.
The most famous and widely used alloys: Ag 960, Ag 925, Ag 875, Ag 830, Ag 800
- It is also worth noting the so-called technical silver alloy
Silver grade metal is contained from 49.5 to 50.5%. Iron no more than 0.13%, lead – 0.005%, antimony and bismuth – 0.002% each. The rest is copper.
At the same time, to protect silver from environmental influences, galvanic coatings by rhodium, nickel plating or layering are also used clear varnish. When long-term storage the product is passivated with wax.