Hot Sale Aluminum Coil for Nocolok

• Model NO.: AACB1001
• Panel Thickness: 0.05-4 mm
• Function: Heat Transfer
• Certification: ISO
• Width: 12 to 1300 mm
• Temper: O-H112
• Surface Finish: Original or Custom
• Alloy or Not: Is Alloy
• Applicaiton: Heat Exchanger and Industrial Fields
• Transport Package: to Be Packed in Wooden Crate
• HS Code: 7606920000

• Surface Finishing: Original
• Aluminum Thickness: 0.05-4 mm
• Usage: Heat Exchanger Fields
• Alloy Number: AA4004,4104,4045,4343
• Thickness: 0.05 to 4 mm
• Max Weight Per Packing: 1.5 Ton
• Place of Origin: Jiangsu,China
• Type: Plate
• Trademark: YB
• Origin: Jiangsu, China

7. Introduction of brazing

Brazing is a joining method which provides a permanent bond between the parts to be
joined with the help of a brazing filler metal. The composition of the filler alloy is such
that its melting point is slightly below the melting range of the parent metal of the parts.
Brazing is distinguished from welding by the fact that the parent metal does not melt
during the process. It differs from soldering by the facts that
a) the brazing filler metal is an aluminium-base alloy and
b) the working temperatures for soldering are appreciably lower.
By accepted general definition of DVS (Deutscher Verband für Schweißtechnik,
Düsseldorf) the working temperature for brazing is above 450 °C, whereas it is below
450 °C in the case of soldering (Note: 800 °F, resp. 426 °C, by definition of the
American Welding Society).
At the brazing temperature the molten filler metal ?wets" the surfaces of the joint and is distributed by capillary action. During "wetting" the filler and parent metals will come into tight contact (about 0,4 nm) and the inter-atomic attraction will bind them together in permanent metallic bond (brazing / soldering). Due to rapid diffusion kinetics at the high brazing temperatures the metallic bond is also characterised by an interchange of atoms between filler and parent metals.
Diffusion, preferably along the grain boundaries, will result in a solid solution or
precipitation of intermetallic phases which are not desirable if the mechanical strength is not to be impaired. However, due to diffusion during brazing especially designed alloys ("long-life-alloys") for heat exchanger applications, will develop a surface layer with different electrochemical potential, thereby providing better corrosion properties by cathodic protection.
The oxide surface of aluminium, although very thin, prevents the tight contact of the
metals. During brazing, therefore, the oxide has to be removed and, at the same time, the bare surface has to be protected against oxidation. It has to be remembered that oxide will immediately be formed on aluminium upon exposure to air and that the oxide melting point is very high (2038 °C). Aluminium oxide is neither molten nor reduced at the melting temperatures of the aluminium alloy itself.
Generally, brazing of the various aluminium alloys is restricted by the available filler
alloys which are based on the binary system Al - Si.
Alloys of type 2XXX and 7XXX are, therefore, not brazeable as their solidus/liquidus
temperature are too low for the filler metals.
Alloys with higher magnesium content (> 1 - 2 %) are difficult to braze due to increased oxide layer formation which cannot effectively be removed by fluxes. Alloys with higher Mg content (5XXX series) can be brazed by vacuum techniques.
Modern techniques of brazing aluminium have been established as an important mass
production method over the past decades. Some of brazing's advantages are
summarised below:
? joining of components of very small thicknesses
? joining of aluminium alloys to dissimilar metals
? if welding temperatures required which are not permissible
? if small distortion of components is required
? if large scale joint areas
? compact components containing many junctions per unit of area
? less personnel training required
? meniscus surface formed by the filler metal is ideally shaped for good
fatigue properties
? finishing costs are low.
 
Vacuum and Controlled Atmosphere Brazing
Many of those techniques which have been developed to reduce or eliminate flux in
brazing of aluminium are protected by patents. In one patented approach the furnace
atmosphere is replaced by a non-oxidising gas in order to limit or remove oxygen from the surface. Optimum quality joints can be obtained with a minimum of flux. Another approach is the use of vacuum. Generally, vacuum brazing does not need any flux.
Controlled atmosphere means a dry, inert gas atmosphere. The dew point has to be kept below - 60 °C. Gases used are argon, helium, nitrogen or carbon dioxide. Parts to be brazed in controlled atmosphere have to be prepared as usual.
Vacuum brazing is attractive as it produces higher quality joints than any other known
brazing technique, the process is non-polluting, the as-brazed-surface is free of any
contaminants that are considered corrosive to aluminium and the processing costs are
commercially attractive (no flux, post-cleaning). The preparation of the assemblies to be brazed are generally the same as discussed before. However, more emphasis has to be put on surface control and pre-cleaning (e.g. aqueous type chemical cleaning of clad vacuum brazing sheet must be avoided). In principle, the process works as follows: the thin oxide layer cracks during heating as the coefficient of thermal expansion of aluminium is three times greater than the coefficient of the oxide. Due to a vacuum of between 10-4 and 10-6 Torr the nascent aluminium surface will not oxidise again. The process is supported by magnesium containing filler metals, since magnesium vaporizes and reacts with traces of oxygen ("getter effect").

High Rib Formwork Mesh Also known as reticulated undismantled mould net is as consumption of fixed template. When the concrete pouring back in the template, so the mesh sheet embedded in concrete, the formation of a concrete block is connected with adjacent mechanical wedge. The quality of the joints is strictly controlled. The strength of the bonding and shear can be comparable to that of the well processed rough cut suture. Two when pouring to be perforated, picking process can shorten the construction period, and the increase in the strength of the casting body.

High Rib Formwork Mesh

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