FUSION JOINING SYSTEMS

Post 391 – by Gautam Shah

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Fusion welding is a common term to describe a process of joining by melting or softening with heat, energy or chemical action, two similar or dissimilar materials.

Fusion joining systems are used in fabrication of metals and thermoplastics. There are basically THREE categories of fusion joining systems. For soldering and brazing, the work pieces are not melted, yet joined by using a melt-able filler material. For welding the work pieces, and in some instances, the filler material, both are melted. The joint is created with or without application of pressure. Some plastics are joined by solvents that dissolve (soften) the surface areas of the work pieces; this is often termed as a solvent welding, but this is truly an adhesion fixing.

Forged wrought Iron joints

Fusion joining requires a heat source, such as a gas flame, an electric arc, a laser, an electron beam, friction, or ultrasound. Fusion joining sometimes requires a slight to very heavy pressure. Fusion joining is done under many different environmental conditions like open air, rain, frost, underwater in vacuum or space and sometimes under the shield of inert gases like Nitrogen, Argon, Helium, Carbon dioxide, etc. Fusion joining in spite of all care is essentially a hazardous procedure. It involves risks of high electric currents, high temperatures, sparks, fumes, and radiation.

Powder welding joint

Forge welding was the first fusion joining process. Blacksmiths used to beat the heated and overlapped metals for joining. Wood, charcoal and later mineral coals were used to heat the work pieces.

Soldering and Brazing use external metal softening at low temperature. For the purpose gold, silver, tin, lead, were common in craft work.

During 1800s, DC power sparking, Oxygen-fuel, and arc processes were developed for fusion welding processes, The results were unreliable as joints cracked and came of the surfaces. It was in the 20th C that AC power was consistently produced and distributed. After world war II, Plasma, laser and electron welding systems, along with X-ray technology for checking the integrity of welded joints were developed. Deep and narrow welding became possible with concentrated heat of Electron beam welding (1958). Later in the 1960s the laser beam welding helped high speed clean profile cutting and automated precision welding. Though both processes are expensive are used for special applications. Today industrial plants have the robotic welding systems, using these technologies.

Friction welding uses heat generated through friction and then pressure to achieve joining. Friction heating is generated by sliding the parts together, and as the surfaces soften, heavier pressure is applied.

Oxy-fuel welding is one of the best-known forms of fusion welding. This uses fuel gases, like acetylene, liquid petroleum, hydrogen, propane, natural gas or propylene with oxygen. Typically Acetylene gas and pure oxygen can generate flame temperatures of 3500 C. This flame is hot enough to melt most industrial metals.

Gas arc Welding

Gas arc Welding

Arc-welding uses electricity to generate an electric arc between an electrode and the pieces of metal to be joined. It is homogeneous welding. It is most widely used method of welding.

Electric resistance welding is similar to the arc-welding. In this process two electrodes are placed on either side but close to the piece to be welded. As the electrodes are brought closer under pressure to create resistance and thereby heat. One of the known such process is butt or spot welding.

Thermit welding is uses intense heat generated by igniting iron oxide and aluminum powder. This process is used for joining rails at remote sites where heavy equipment cannot be carried.

Tram Rail joining by thermit welding

Laser beam welding uses very precise heat source of a laser. The lasers penetrate deep and narrow areas without affecting the surrounding parts. It requires heavy industrial set-up.

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Heat fusion processes are good for heat fusible substances like metals and some plastics, but induce stresses in the mass. Electric resistance processes of welding require electrically conductive mass. In fusion joining processes the deposited substances are heat-hardened and these are very difficult to grind out. Joint testing procedures such as X-ray and sonography are elaborate and not always perfect.

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Fusion joining systems require power or energy input, a filler material, a flux and often a shield gas. Welding in certain situations are done without the use of a filler material. Filler materials where used, are invariably of compatible materials in terms of melting point temperature, alloying capacity, fluxing capacity. These are in the form of powders of pure metals, alloys, oxides and such other compounds, ceramics, granules, foils, wires, and rods. Flux or fluxing agents are required to dissolve the existing, and to be formed oxide. Welding rods are coated with borax and aluminum chloride. Stainless steel welding requires zinc chloride and hydrochloric acid in equal parts. Soldering of the Galvanized surfaces need hydrochloric acid. Brass soldering requires tallow or rosin. Soldering of tinned surfaces require hydrochloric acid or rosin. Copper, Brass and gun metal need aluminium chloride, hydrochloric acid or ammonium phosphate. Shield gases are inert or noble gases (constituting O group of a periodic table), such as Helium, Neon, Argon, Crypton, Xenon, Radon. These gases do not react with other materials so are used for forming an environment so that outside contaminants and other substances do not react with the weld. Some other non-inert gases are used for welding, are Carbon dioxide and Nitrogen.

Tablet strip

Fusion Joining has few inherent issues. Distortion occurs due to substantial thermal expansion and contraction. Oxidation occurs at high temperatures, which prevents either joining or results in poor bonding. Oxidation can be prevented by welding in protected environments (metal inert gas -MIG, and tungsten inert gas TIG). In oxyacetylene welding, the gases produced by combustion prevent oxidation. In certain cases the electrodes are coated with a fluxing agent. Cracking occurs for many reasons such as inadequate heat and moisture in air. Stresses in the welded zone and surrounding areas (heat-affected zone -HAZ) occur due to high heat. Prolonged cooling anneals the steel.

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This fusion joining can be also done for metal, ceramic and other polymer surfaces with use of alternating magnetic field to heat and melt a nano-crystalline iron-aluminium powder, which then cools to bind the surfaces of two parts.

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MASTIC, PUTTIES and CAULKING COMPOUNDS

Post 231 –by Gautam Shah

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These are heavy consistency compounds used for several purposes: for filling in cracks and gaps, levelling the surface, fixing objects over surfaces and as interlining and pointing material. Due to their heavy consistency these compounds do not run. High viscosity is achieved by heavier phase formation (polymerization), foaming, or by addition of bulky filler materials. Some are designed to dry out completely slowly or quickly, whereas many are designed to remain green (ever wet or soft) so as to be removable, or to absorb the vibrations and movements.

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Sealants are very viscous, to very thin or fluid materials. Viscous materials stay put, except they have a capacity to level out, whereas fluid materials run or flow out to penetrate very thin cracks and crevices, often by designed reduced surface tension and by means of capillary action.

Caulking

One of the most common of such compounds is glass fixing putty or mastic. This is made by mixing china clay, water and alkyd resin or bodied linseed oil. In earlier periods asbestos and mica, were used in place of china clay. Other commercial compounds in this category include butadiene, poly-sulfide and silicone compounds. Some of these are two pack (resin + hardener) high strength and high bonding materials often used as industrial crack filler or sealant. These are also used for temporary bonding so that clamps may not be required for assembly work (e.g. truss erection), and also to eliminate mechanical fastenings like nut-bolts and rivets. Such industrial sealants offer greater assurance of the joint, better distribution of stresses.

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Wooden boats were constructed with overlapping wood planks. The joints were sealed by traditional caulking method, where hemp fibres soaked in pine-tar were pushed into V shaped seam joints. Specific head formed caulking mallet and a caulking iron, a chisel-like tool, were used. The caulked joint was then lined with putty.

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Natural Coal Tar

Epoxy putty compounds such as M-seal etc., are also of this category. Asphalt-based water proofing caulking compounds are widely used in crack filling over roofs or terraces, for sealing pipe and duct joints, and lining the joints in sheet structures. Epoxy based caulking compounds are also used in fixing composite aluminium sheets, frame-less glass facades, fibre glass sheets, PVC door frames, and aluminium sections to masonry faces. Polymer-based water soluble compounds help fix tiles and panels as floors or claddings. Caulking compounds of heavier viscosity give assured fixing, compared to low viscosity contact adhesives. Mastic are sold in cans, tubes, or canisters that fit into hand-operated or air-operated caulking guns.

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BRICKS SURFACES

Post -by Gautam Shah

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Bricks are fired clay blocks. Quality and Colour of bricks vary from region to region mainly due to different types of soils being used. Brick quality and colour also depend on the additives, firing technique and temperature. The colour of brick is a primary indicator of its soundness.

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Under burnt and low temperature fired bricks are more absorbent compared to over burnt and high temperature bricks. Hand-pressed bricks are less compact than machine-made bricks, and as a result absorbs more water. Hollow and perforated bricks are extrusion-machine cast by from wet and plastic mass. Roofing tiles and facing brick tiles are die mould-cast from slightly less wet mass.

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Exposed bricks surfaces are created for its decorative appeal and also as the inevitable surfacing. Exposed brick surfaces (un-plastered brick masonry surfaces) of walls, floors and occasionally the underside of roofs (Jack-arch roofs, domes and arches) are created with uniform brick colour and texture or sometimes with slight variegated shading.

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Bricks of exposed masonry surface, if permeable allow bacterial growth such as mould, fungi etc. on the surface. Soluble salts present in the clay, usually get decomposed during the burning but immediately after highest temperature of firing and while cooling, sulphate of sodium, calcium, potassium and magnesium are formed with the help of sulphur from the fumes of the fuels. These salts on contact with absorbed moisture leach out on the surface. Most of the sulphate gets washed away from the masonry surface, but magnesium sulphate does not leach out readily. It expands and causes crack in bricks. Calcium sulphate though not easily leached out, settles on the surface to form whitish scum.

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Porous and rough brick surfaces are better for mortar adhesion than an impervious smooth surface of a very vitrified brick, Over burnt or highly vitrified bricks have very low suction capacity for mortar binding. Over-burnt bricks are dimensionally deformed due to running of the mass and unsuitable for masonry work.

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Next to the Bricks self colour, texture and quality, the quality of the masonry surface is characterized by the joints. The colour of the joint material, its pattern gives a different look to the wall masonry or flooring. The joints are racked (deep grooved), flushed or projected out, all with selective emphasis on horizontal or the vertical. Flushed with string mark type of pointing is considered best, however for dramatic effect racked and projected joints are preferred. Unlike the projected or grooved pointing, flushed pointing does not retain dirt or water in its holds. The string marks are adequate guide path for any hair crack that may develop in the joints.

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The masonry inter joint material and the joint surface pointing material both must surpass the overall performance of the bricks. High adhesion, low permeability and suitable colour matching are some of the attributes of a good jointing-pointing material.

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