advanced welding processes by hamid taghipour armaki
TRANSCRIPT
1
LOGO
Advanced Welding Processes By Hamid Taghipour Armaki
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Chapter 1 : Introduction in Welding Processes
Chapter 2 : Advanced Process development trends
Chapter 3 : Ceramic to Metal Bonding Processes
Chapter 4 : High-energy density processes
Contents
Chapter 5 : Newest Welding Processes
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Chapter 1Introduction in
Welding Processes
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Introduction
Welding and joining are essential for the manufacture of a range of engineering components, which may vary from very large structures such as ships and bridges, to very complex structures such as aircraft engines or miniature components for micro-electronic applications.
The basic joining processes may be subdivided into: mechanical joining; adhesive bonding; brazing and soldering; welding
This chapter will introduce some of the basic concepts which need to beconsidered and highlight some of the features of traditional welding methods.
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Classification of welding processes
Several alternative definitions are used to describe a weld, for example:
A union between two pieces of metal rendered plastic or liquid by heat or pressure or both. A filler metal with a melting temperature of the same order as that of the parent metal may or may not be used
A localized coalescence of metals or non-metals produced either by heating the materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone, with or without the use of a filler metal.
Many different processes have been developed, but for simplicity these maybe classified in two groups; namely ‘fusion’ and ‘pressure’ welding as shown in Fig. 1.1, which summarizes some of the key processes.
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Classification of welding processes
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Welding with pressure
Resistance welding
Mechanism : The resistance welding processes are commonly classified as pressure welding processes although they involve fusion at the interface of the material being joined. Resistance spot, seam and projection welding rely on a similar mechanism. Note : Use High Current10000 A
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Welding with pressure
Features of the basic resistance welding process include:
the process requires relatively simple equipment; it is easily and normally automated; once the welding parameters are established it should be possible to
produce repeatable welds for relatively long production runs.
Note : The major applications of the process have been in the joining of sheet steel in the automotive and white-goods manufacturing industries.
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Welding with pressure
Cold pressure weldingMechanism : If sufficient pressure is applied to the cleaned mating surfaces to cause substantial plastic deformation, the surface layers of the material are disrupted, metallic bonds form across the interface and a cold pressure weld is formed. The main characteristics of cold pressure welding are: the simplicity and low cost of the equipment; the avoidance of thermal damage to the material; it is most suitable for low-strength (soft) materials.Note 1: The pressure and deformation may be applied by rolling, indentation, butt welding, drawing or shear welding techniques. In general, the more ductile materials are more easily welded.Note 2 : This process has been used for electrical connections between small diameter copper and aluminium conductors using butt and indentation techniques. Roll bonding is used to produce bimetallic sheets such as Cu/Al for cooking utensils, Al/Zn for printing plates and precious-metal contactsprings for electrical applications.
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Welding with pressure
Friction weldingMechanism : In friction welding, a high temperature is developed at the joint by the relative motion of the contact surfaces. When the surfaces are softened, a forging pressure is applied and the relative motion is stopped. Material is extruded from the joint to form an upset.
The process may be divided into several operating modes in terms of themeans of supplying the energy:Continuous drive: in which the relative motion is generated by directcoupling to the energy source. The drive maintains a constant speed during the heating phase.
Stored energy: in which the relative motion is supplied by a flywheel which is disconnected from the drive during the heating phase.
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Welding with pressure
Features of the process include: one-shot process for butt welding sections; suitable for dissimilar metals; short cycle time; most suited to circular sections; robust and costly equipment may be required
Note :The process is commonly applied to circular sections, particularly in steel,but it may also be applied to dissimilar metal joints such as aluminum tosteel or even ceramic materials to metals.
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Welding with pressure
Diffusion bonding or diffusion weldingMechanism : In diffusion bonding the mating surfaces are cleaned and heated in an inert atmosphere. Pressure is applied to the joint and local plastic deformation is followed by diffusion during which the surface voids are eliminated.Features of the process include: it is suitable for joining a wide range of materials; it is a one-shot process; complex sections may be joined; a vacuum or controlled atmosphere is required; a prolonged cycle time may be necessary.
Note: The process can, however, be used for the joining of complex structureswhich require many simultaneous welds to be made.
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Welding with pressure
Explosive weldingMechanism : In explosive welding, the force required to deform the interface is generated by an explosive charge. In the most common application of the process, two flat plates are joined to form a bimetallic structure. An explosive charge is used to force the upper or ‘flier’ plate on to the baseplate in such a way that a wave of plastic material at the interface is extruded forward as the plates join.
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Welding with pressure
Note : For large workpieces, considerable force is involved and careis required to ensure the safe operation of the process.
Features of the process include: it is a one-shot process; it offers a short welding time; it is suitable for joining large surface areas; it is suitable for dissimilar thickness and metals joining; careful preparation is required for large workpieces; safety is an issue.
Note : The process may also be applied for welding heat exchanger tubes to tube plates or for plugging redundant or damaged tubes
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Welding with pressure
Magnetically impelled arc butt (MIAB) welding
Mechanism : In MIAB welding a magnetic field generated by an electromagnet is used to move an arc across the joint surfaces before the application of pressure.Note : Although the process produces a weld similar to that of frictionwelding, it is possible to achieve shorter cycle times and relative motion ofthe parts to be joined is avoided.
Features of the process are: it is a one-shot process; it is suitable for butt welding complex sections; it offers a shorter cycle time than friction welding.
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Welding with pressure
Note : The process has been applied fairly widely in the automotive industry for the fabrication of axle cases and shock absorber housings in tube diameters from 10 to 300 mm and thicknesses from 0.7 to 13 mm. It is also being developed for transmission pipeline welding, particularly for small diameter thin wall pipe.
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Fusion welding
Gas tungsten arc welding (GTAW)
Mechanism : In the gas tungsten arc welding process [also known as tungsten inert gas (TIG) in most of Europe, WIG (wolfram inert gas) in Germany, and still referred to by the original trade names Argon arc or Helium arc welding in some countries], the heat generated by an arc which is maintained between the workpiece and a non-consumable tungsten electrode is used to fuse the joint area.Note : The arc is sustained in an inert gas which serves to protect the weld pool and the electrode from atmospheric contamination
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Fusion welding
The process has the following features: it is conducted in a chemically inert atmosphere; the arc energy density is relatively high; the process is very controllable; joint quality is usually high; deposition rates and joint completion rates are low
Application Note : The process may be applied to the joining of a wide range of engineering materials, including stainless steel, aluminium alloys and reactive metals such as titanium. These features of the process lead to its widespread application in the aerospace, nuclear reprocessing and power generation industries as well as in the fabrication of chemical process plant, food processing and brewing equipment.
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Fusion welding
Submerged arc welding (SAW)
Mechanism : Submerged arc welding is a consumable electrode arc welding process in which the arc is shielded by a molten slag and the arc atmosphere is generated by decomposition of certain slag constituents
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Fusion welding
Note : The filler materials a continuously fed wire and very high melting and deposition rates are achieved by using high currents (e.g. 1000 A) with relatively small-diameter wires (e.g. 4 mm).
The significant features of the process are: high deposition rates; automatic operation; no visible arc radiation; flexible range of flux/wire combinations; difficult to use positionally; normally used for thicknesses above 6 mm.Note : The main applications of submerged arc welding are on thick section plain carbon and low-alloy steels and it has been used on power generation plant, nuclear containment, heavy structural steelwork, offshore structures and shipbuilding.
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Fusion welding
Gas metal arc welding (GMAW)
Mechanism : In gas metal arc welding [also known as metal inert gas (MIG) or metal active gas (MAG) welding in Europe; the terms semi-automatic or CO2 welding are sometimes used but are less acceptable] the heat generated by an electric arc is used to fuse the joint area. The arc is formed between the tip of a consumable, continuously fed filler wire and the workpiece, and the entire arc area is shielded by an inert gas.Some of the more important features of the process are summarized below: low heat input (compared with SMAW and SAW); continuous operation; high deposition rate; no heavy slag – reduced post-weld cleaning; low hydrogen – reduces risk of cold cracking.
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Fusion welding
Electron beam welding
Mechanism : A beam of electrons may be accelerated by a high voltage toprovide a high energy heat source for welding.Note : The power density of electron beams is high (1010 – 1010 W/m) and keyhole welding is the normal operating mode. The problem of power dissipation when the electrons collide with atmospheric gas molecules is usually overcome by carrying out the welding operation in a vacuum.Features of the process include: very high energy density; confined heat source; high depth-to-width ratio of welds; normally requires a vacuum; high equipment cost. Note : Applications of electron beam welding have traditionally included welding of aerospace engine components and instrumentation.
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Fusion welding
Laser Welding
Mechanism : The laser may be used as an alternative heat source for fusion welding. The focused power density of the laser can reach 1010 or 1012 W/m and welding is often carried out using the ‘keyhole’ technique. Significant features of laser welding are: very confined heat source at low power; deep penetration at high power; reduced distortion and thermal damage; out-of-vacuum technique; high equipment cost.Note : These features have led to the application of lasers for microjoining of electronic components, but the process is also being applied to the fabrication of automotive components and precision machine tool parts in heavy section steel.
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Appendix 1
Question 1 : What's your idea about difference in fusion welding and welding with pressure processes?Question 2 : What's the problem in joining of dissimilar material by the fusion welding?
Difference in melting temperature
Difference in thermal expansion
Difference in thermal conduction
Non – solubility
Wetting
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Appendix 2
Note : Significant features of welding with pressure.
joining of dissimilar materials / ceramic to metal
Reduce in residual stress/ Like FSW, FRW, DW..
Low HAZ/ Like FRW….
Cladding and composite / Like FSW
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Chapter 2Advanced Process
development trends
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Introduction
The incentives for welding process development
Primary Secondary
Improve total Cost effectiveness
the requirementfor new processes
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Introduction
Cost effectiveness
Note : The cost of labour in many traditional welding processes is mostly 70 to 80% of the total
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Introduction
Cost effectiveness
Note : In the past, it seems to have been assumed that the cost effectiveness of welding processes was totally dependent on deposition rate.
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Introduction
Cost effectiveness
General
Higher the deposition rate
Shorter the weld time
Lower the labour cost
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Introduction
Note & Question : Deposition rate may, however, give a misleading indication of cost effectiveness if, for example, quality is sacrificed and higher repair rates are required. Answer : Deposition rate is also an inappropriate way of describing ‘single shot’ high joint-completion rate autogenous processes such as explosive welding and laser welding.
For a more complete assessment of cost effectiveness, it is clear that thefollowing additional factors should be considered: control of joint quality; joint design; operating efficiency; equipment and consumable cost
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Chapter 3Ceramic to Metal
Bonding Processes
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Introduction
In crystalline form, ceramics are generally considered to be the inorganic compounds between metals (or semi-metals) and nonmetals. Ceramics are covalently and ionically bonded materials and have some unique and excellent properties, such as electrical insulation, chemical and size stability and resistance to the effects of heat. Due to ceramic materials’ wide range of properties, they are used for a multitude of applications. For example, alumina, aluminum nitride and beryllium oxide are the base materials most widely used in electronic packaging because of their high electrical resistance. Zirconia is used as the solid electrode of SOFCs due to its ionic conduction properties at high temperature and as the oxygen sensor material for its sensitive electrical properties. It is also an ideal biomaterial because of its chemical stability, biocompatibility, size stability and high strength and toughness. It is also the material of ferrules for optical fiber connectors because of its wear resistance and mechanical properties.
Joining a metal to a ceramic is required in many advanced applications, such as microchip substrates, capacitors and heat sinks. For example, in power amplifier packaging, ceramic aluminum nitride is required to be joined to a copper heat sink and lead frame. The application of ceramic zirconium oxide as biomaterial requires joining between the ceramic and metal, for example, to make implantable micro stimulators . In the miniature manufacturing field, the joining of ceramics and metals is necessary and unavoidable. This chapter gives an introduction to the characteristics of some typical ceramics used in miniature manufacturing, the major methods (mainly brazing and diffusion bonding) to join ceramics to metals and the main difficulties exhibited in the joining process.
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Introduction
Why Ceramic-Metal Joints?
Using ceramic for its unique properties
Using ceramic for its unique properties
Produced complex components
Ceramic-Metal joining
Problem
Solution
Frangibility and high tenacity
Ceramic-Metal joining
Problem
Solution
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Problem in Ceramic-Metal bonding
General Problem
Bond diversity Limitation Fusion-Weld
Note : Ionic bonds and covalent bonds are characteristic atomic bond configurations of ceramic materials. The peripheral electrons are extremely stable. Using the general joining method of fusion welding to join ceramics with metals is almost impossible, and the molten metal does not generally wet on ceramic surfaces. When joining ceramics to metals with the brazing method, metallization on the ceramic surface is necessary with general inactive brazing filler metal or the use of active brazing alloys in order to get a reliable joint.
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Problem in Ceramic-Metal bonding
General Problem
Tm diversity Limitation Fusion
Note : Ceramics have high melting points, high strength (in compression) and a high hardness value. Direct diffusion bonding of ceramics is very difficult, requiring the finished surface to be extremely smooth and clean, the bonding temperature high and a long bonding time. For example, when joining Si3N4 ceramic directly by diffusion bonding, the finish of the ceramic surface should be more than 0.1 µm and bonding temperature should be 1500~1750 °C. Therefore, joining ceramics by diffusion bonding is normally indirect, using soft, ductile metals as an interlayer which can decrease the bonding temperature considerably and the plastic deformation of interlayer metals can lower the requirements of processing the joining surface of the ceramic.
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Problem in Ceramic-Metal bonding
General Problem
diversity R-Stress Crack
Note : The thermal expansion coefficients of ceramics are generally much lower than metals. Stress will be generated in the ceramic/metal joint due to the thermal expansion mismatch and will degrade the mechanical properties of joint and can cause joint cracking immediately after the joining process. The thermal stress in the joint due to the thermal expansion mismatch should be carefully considered when joining ceramic with metal.
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Problem in Ceramic-Metal bonding
General Problem
𝑻𝒉−𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒊𝒕𝒚 Low Limitation-FW
Note : Many ceramics have low thermal conductivity and susceptibility to thermal shock. Using the fusion welding method to join ceramics by concentration heating or with a high energy density heat source, cracking in the ceramic easily occurs. It is necessary to reduce the temperature gradient in and around the fusion zone as much as possible and to carefully control the heating and cooling speed during the joining process.
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Problem in Ceramic-Metal bonding
General Problem
𝑬−𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒊𝒕𝒚 Weak Limitation -EW
Note : Most ceramics have weak or no electrical conductivity. It is hard to joinceramics by using electrical welding methods unless special techniques used.
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Problem in Ceramic-Metal bonding
General Problem
𝑫𝒊𝒇𝒇𝒖𝒔𝒊𝒐𝒏 DW Problem
Metal
CeramicWith interlayer
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Problem in Ceramic-Metal bonding
Metallurgical Problem
𝑾𝒆𝒕𝒕𝒊𝒏𝒈
𝑭𝒐𝒓 𝒉𝒊𝒈𝒉sl
cossv sl lv
𝑹𝒆𝒅𝒖𝒄𝒆 𝒊𝒏𝒄𝒐𝒏𝒕𝒂𝒄𝒕 𝒂𝒏𝒈𝒆𝒍𝑰𝒏𝒄𝒓𝒆𝒂𝒔𝒆 𝒊𝒏𝒄𝒉𝒄𝒆𝒎𝒊𝒄𝒂𝒍 𝒂𝒏𝒅𝒑𝒉𝒚𝒔𝒊𝒄𝒂𝒍𝒘𝒐𝒓𝒌
𝑪𝒉𝒆𝒎𝒊𝒄𝒂𝒍 𝒓𝒆𝒂𝒄𝒕𝒊𝒐𝒏 :𝒎𝒆𝒕𝒂𝒍𝒊𝒄 𝒃𝒐𝒏𝒅𝒕𝒐𝒘𝒂𝒓𝒅 𝒊𝒐𝒏𝒊𝒄𝒃𝒐𝒏𝒅
𝑭𝒐𝒓𝒎𝒂𝒕𝒊𝒐𝒏𝒏𝒊𝒕𝒓𝒊𝒅𝒆𝒐𝒓 𝒐𝒙𝒊𝒅𝒆𝒃𝒂𝒔𝒆𝒅𝒐𝒏𝒎𝒂𝒕𝒆𝒓𝒊𝒂𝒍𝒔
Problem in Ceramic-Metal bonding
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Problem in Ceramic-Metal bonding
Mismatch of the physical and mechanicalproperties
Note : Another difficulty in joining ceramic to metal is the difference of thermal expansion coefficients between them. The thermal expansion coefficient of ceramics is generally around 10-6k-1 for instance, the thermal expansion coefficient of Si3N4 ceramic is near 3* 10-6k-1, while the average thermal expansion coefficient of common metals such as carbon steel or stainless steel is about 14-18 10-6k-1. Such mismatch of thermal expansioncoefficients leads to residual stresses in the joints after cooling to room temperature from the joining temperature, the joint performance is therebyaffected.
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Problem in Ceramic-Metal bonding
The residual stresses produced in the ceramic metal joint could be estimatedaccording to following equation for fully elastic conditions :
If the thermal stresses in the metal exceed its yield strength, the residual stresses in thejoint could be determined by equation :
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For reduce in the residual stress
Using soft filler metals
Soft filler metals have low yield strength andcould release the residual stress.
Using soft interlayer
Residual stress could be reduced by the elastic andplastic deformation of an interlayer, e.g. when using Al or Cu as interlayer,
the residual stress is decreased.
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For reducing the residual stress
Using hard metals of which the thermal expansion coefficient is close toceramics as the interlayer.
Using hard metals such as W, Mo or Invar asthe interlayer, could reduce the residual stress
Using composite interlayer
Composite interlayers often constitute hardmetals and soft metals, like Cu/Mo-Cu/Nb, have a noticeable effect on
reducing residual stress, with a combination of merits of those two kindsof metals.
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For reducing the residual stress
Joining under low temperature
Joining ceramic to metal at a lowtemperature is good for reducing the joint deformation and effectively
decreasing the residual stresses.
Heat treatment after joining
Proper heat treatment post-joining sometimesreleases the stress and the strength will vary based on the heat treatment.
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For reducing the residual stress
Optimized design of joint configuration
Appropriate configuration ofthe joint could decrease the stress concentration extent and reduce the
residual stress.
Note : There are many ways to make ceramic-metal joints, such as mechanicalbonding, electrostatic bonding, hot isostatic pressing bonding, brazing anddiffusion bonding. Each method has its own features and specific applications.Among them, brazing and diffusion bonding are the most commonly usedmethods, for the high reliability and good repeatability characteristics ofjoints.
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Classification of ceramic-metal bonding
MechanicalChemical
Solid Solid-Liquid Liquid
Diffusion
Ultrasonic
Sintering
FRW
Electrostatic
TLP
PTLP
Eutectic
Brazing
Sintering
Adhesive
Microwave
Laser
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Ceramics to Metal processes
Brazing ceramics to metal
𝑫𝒊𝒓𝒆𝒄𝒕 𝑰𝒏𝒅𝒊𝒓𝒆𝒄𝒕
Direct brazing : uses filler metal containing active elements, oxides, fluorides or solder to join ceramic and metal.
Indirect brazing : is used first to achieve ceramic surface metallization, then conventional brazing filler metals are used to join them.
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Indirect Brazing Process
Metallization of ceramic surface
at low or high temperature
such asTi/Pd/Au
𝒄𝒉𝒆𝒎𝒊𝒄𝒂𝒍 𝒑𝒍𝒂𝒕𝒊𝒏𝒈 such as plating Ni
?
?
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Indirect Brazing Process
Thick-film metallization
Note 1: it is a method of producing a metal layer for sealing, conduction and resistance to the ceramic substrates by screen printing, forming the layer for brazing, electric circuits and connection points.
Note 2: Thick film slurries are generally made by mixing and milling the metal powders at a size of 1.5 microns, adding a percentage of permanent adhesive, organic solvents, thickeners and surfactant, etc.
Note 3: Metal slurries used for thick-film could be Cu, Ag and Au. Adhesives areUsually Si2O3-B2O3-ZnO, Si2O3-B2O3-BaO-Al2O3(forthe AlN ceramic thick-film metallization).
Note 4: The sintering atmosphere for Cu slurry is nitrogen, with a sintering temperature of 850 °C. For Ag slurry in the air, the sintering temperature is 920 °C, For Au slurry also in air, the sintering temperature is 850 °C.
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Indirect Brazing Process
Thin-film metallization
Note: Thin-film metallization, by using PVD methods like sputtering orevaporation, is used to produce a ceramic surface coated with thin metalfilms, such as NiCr/Pd/Au, Ti/Pd/Au, Ti, Ti/Ag, Ti/Ni, etc.
Direct Copper Bonding
Note: The DCB technique is based on the presence of an eutectic betweencopper and Cu2O which wets alumina.
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direct Brazing Process
Vacuum brazing with active filler metals (direct)
Note: The chemical bonds of ceramic materials are mainly ionic bonds and covalent bonds, which means ceramics are very stable in chemical reactions. Filler metals with metallic bonds can wet ceramic surfaces when chemical reactions occur between filler metal and ceramic.
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direct Brazing Process
Vacuum brazing with active filler metals
Note 1: In active brazing alloys (filler metals) Ti is usually used as the active element. Among the commercial filler metals, such as Ag-base, Cu-base or Ag-Cu eutectic filler metal, the content of Ti is 1~5wt%.
Note 2: Sometimes an amount of In is added to the brazing alloys to improve the fluidity and increase the activity of active elements.
Note 3: Besides Ag-base and Cu-base filler metal, there are some other active brazing alloys based on Sn or Pb, their melting point below 300 °C. The working temperature of the ceramic-metal joint bonded with Ag-base or Cu-base brazing alloys are generally not above 400 °C, however, with high melting temperature and noble metals such as Pt, Pd, Ni, Co, Au, that could reach to about 800 °C.
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direct Brazing Process
Vacuum brazing with active filler metals
Note 1: In active brazing, the protection of active elements is very important. These active elements are easily oxidized and if that were to occur they would be unable to react with ceramics. Active brazing is usually carried out in a vacuum or an inert atmosphere. In vacuum brazing the vacuum under brazing temperature is usually better than 10-4 mbar.`
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Brazing Process
The factors that may influence the brazed joint quality :
the properties of ceramic and metal (including the interlayer metal)
the brazing alloy system
the amount of brazing filler metal used
the activeelement type and its content
the surface structure, atmosphere, brazingtemperature and holding time
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Brazing Process
Note : When ceramics have high fracture toughness, the corresponding joint has high tension strength. The properties and thickness of the interlayer metal have a great influence on the joint strength. Soft and easily yielding interlayer metals favor releasing the stress generated in the joint because of the mismatched heat expansion behavior of ceramics and metals, the joint performance can be improved when the thickness of the interlayer metal is correct
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Brazing Process
Note : The content of active elements in filler metals affects the joint strength by its influence on the interface reaction and the microstructures of the brazing seam.
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Brazing Process
Note : There are favorable ranges of brazing temperature and holding time forthe joint strength.
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Brazing Process
Note 1 : Brazing temperature (usually above the liquidus temperature about 50~100 °C) and bonding time are key parameters for joint quality. The temperature of brazing ceramics to metals is usually between 800 °C and 1100 °C. Sn base or Pb-base filler metal with low melting points should also be brazed at high temperature to achieve sufficient thermodynamic activity to make chemical reactions.
Note 2 : ZrO2, Al2O3, Si3N4, SiC and AlN are widely used in applications. Commercial filler metals based Ag, Cu or eutectic Ag-Cu could wet those ceramics and aid in joining them reliably.
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Ceramic-Metal Bonding Processes
Vacuum diffusion bonding
Note : The solid state diffusion bonding technique was originally used to join dissimilar materials. It is now one of the general methods used to join ceramics, with direct bonding or with an interlayer.
Advantages: Compared with fusion welding, the advantages of solid state diffusion bonding include high joint strength, small shrinkage and deformation, accurate dimension control and good fit for joining dissimilar materials. Why?
Answer: Because bonding is usually carried out in a vacuum atmosphere at high temperature and with a long bonding time.Disadvantages: The disadvantages of diffusion bonding are expensive equipment, high production costs and limited sample dimension.
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Vacuum diffusion bonding
1st stage deformation forming
interfacial boundary.2nd stage
Grain boundary migration and pore elimination.
3rd stage Volume diffusion and
pore elimination.
asperities come into contact.
2nd stage grainboundary migrationand pore elimination
1st stage deformationand interfacial boundary formation
3rd stage volumediffusion poreelimination
Principles
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Vacuum diffusion bonding
The factors influencing diffusion bonding
Bonding temperature
Holding time
Pressure applied
Atmosphere
Surface structure
Chemical reactionsSimilarity of physical
properties
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Vacuum diffusion bonding
Effect of bonding temperature
Note 1 : Temperature is the most important parameter in diffusion bonding. The temperature of solid state diffusion bonding ceramics to metals generally needs to be over 90% of the melting point of the metal.
Note 2 : In solid state diffusion bonding, the inter diffusion of elements allows chemical reactions to occur and form sufficient interface combination. The formation of the reaction layer and its thickness affects the joint strength remarkably. The thickness of the reaction layer could be estimated accordingto equation:
where k0 is a constant, n is the exponential of time, Q is the activation energy, related to the diffusion mechanism and R is the gas constant.
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Vacuum diffusion bonding
Effect of bonding temperature
Note 1 : The influence of bonding temperature on joint strength has the same tendency with reaction layer thickness. According to the experimental resultsof tension strength tests, the influence of bonding temperature on joint strength could be expressed by equation :
Where B0 is a constant (MPa), Q app is the appearance activation energy, the sum of various activation energy.Note 2 : According to the equation, the increase of temperature favors an increase in the joint strength.Note 3 : Joint strength may decrease with higher temperature, because the residual stress will also increase and reduce the joint strength.
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Vacuum diffusion bonding
Effect of bonding temperature
Note 1 : Moreover, increasing the temperature may result in a change in ceramic performance or cause the brittle phase to occur and the joint to fail. In addition to these factors, the strength of ceramic-metal joints is also related to the melting point of the metal. Among ceramic-metal joints, the joint strength increases linearly with increasing of melting point of metals
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Vacuum diffusion bonding
Effect of holding time
Note 1 : The holding time not only affects the thickness of the reaction layer, but it also affects the interface reaction products. The effect of time on the thickness of reaction layer (X) could be approximately shown by
X = k (Dt)1/2
(k is a constant and D is the diffusion coefficient). It can be seen that increasing time will make the reaction layer grow.
Note 2 : In general, the relation between bonding time (t) and tension strength is
BS = B0×t1/2
, where B0 is constant.
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Vacuum diffusion bonding
Effect of holding time
Example: Bonding SiC to SUS304 with Nb as the interlayer metal , the phase NbSi2 that has low strength and a large heat expansion coefficient compared with SiC occurred and made the joint shear strength decrease after the long bonding time.
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Vacuum diffusion bonding
Effect of pressure
Note 1 : Pressure applied in the solid phase diffusion bonding process creates plastic deformation, which could reduce the size of surface asperities and provide oxide breakdown, increasing the surface contact area, to offer the best condition for the diffusion of atoms. For preventing the macroscopic deformation of components, the bonding pressure is usually within the range of 0~100 MPa, while under that pressure sufficient deformation cannot be created.
Note 1 : As well as the bonding temperature and bond time, there also exists an optimum pressure range for joint strength. The best pressures for bonding Al to Si3N4 and Ni to Al2O3 are 4 MPa and 15–20 MPa respectively. Other factors, like the bonding material type and the surface oxide thickness, will change the effect of pressure on joint strength.
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Vacuum diffusion bonding
Effect of chemical reactions
Note : Generally, when joining ceramic to metal with solid diffusion bonding either directly or using a metal interlayer, compounds will be formed at the interface between ceramic and metal. Reaction compounds change with the diffusion bonding conditions, resulting in a variation of joint performance.
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Vacuum diffusion bonding
Effect of bonding atmosphere
Note 1 : Usually, the joint strength bonded in a vacuum is higher than that in an argon atmosphere or in air. In the bonding of Si3N4 using aluminum, the strength in the vacuum is highest. The fracture locates both in the Al layer and the ceramic. The fracture in the Al layer is plastic and in the ceramic it is brittle. While bonding in air, the fracture occurs at the interface between Al and Si3N4 because of the oxidation of Al. Although the pressure could break down the surface oxidizing film, the new oxidizing film forms continuously when the partial pressure of oxygen in the atmosphere is higher. joint
Note 2 : If diffusion bonding of ceramic Si3N4 is conducted at high temperature (1500 °C), the ceramic Si3N4 will decompose forming cavities. When bonding in an N2 atmosphere, the decomposition will be limited. The higher N2 pressure is favorable to joint strength. The flexural strength of the joint bonded in 1 MPa N2is higher by one-third than that in 0.1 MPa N2.
74
Vacuum diffusion bonding
Effect of surface state
Note : The joint strength may be significantly influenced by the roughness of the faying surfaces. High surface roughness may introduce high local stress concentrations with subsequent initiation of brittle fracture. The effect of the surface roughness on the joint strength can be seen in Figure for Si3N4-Al joints. Joint strength decreases from 470 MPa to 270 MPa when the surface roughness value varies from 0.1 µm to 0.3 µm.
75
Vacuum diffusion bonding
Effect of interlayer
Note 1 :The use of interlayer in diffusion bonding is required for many applications in order to reduce the bonding temperature, bonding pressure and bonding time, to enhance diffusion, to eliminate impurity elements, and to reduce the residual stresses generated at the bond interface if the interlayer is ductile. When bonding stainless steel to alumina, the lower residual stress caused by the interlayer. The interlayer can be in the form of a powder, foil or metallization film.
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Chapter 4High-energy density
processes
80
Introduction
High-Energy Density Processes
Note : The power density of these processes is significantly higher than that of the common arc welding processes and normally above 10 9W m–2.
High-current plasma
Electron beam
Laser welding
Engineering materials
81
Introduction
Mechanism
Note 1 : As a consequence of the high energy concentration, the mechanism of weld pool formation is somewhat different from that normally found in other fusion welding processes. The material in the joint area is heated to very high temperatures and may vaporize, a deep crater or hole is formed immediately under the heat source and a reservoir of molten metal is produced behind this keyhole’. As the heat source moves forward the hole is filled with molten metal from the reservoir and this solidifies to form the weld bead. The technique has been called keyhole welding.
82
Introduction
The features of High-Energy Density
they are normally only applied to butt welding situations
they require a closed square butt preparation with good fit-up
the weld bead cross sections have high depth to width ratios
they allow full penetration of a joint to be achieved from one side
they can be used to limit distortion and thermal damage
Note : The mechanism of keyhole welding is illustrated in Fig. 8.2. Very stable operating conditions and, in particular, travel speed, are required to maintain a balance between the keyhole-generating forces (gas velocity, vapour pressure and recoil pressure) and the forces tending to close the keyhole (surface tension and gravitational). The need for consistent travel speed as well as safety considerations dictate the requirement to operate these processes automatically.
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Plasma keyhole welding
Introduction
Note 1 : PLASMA ARC WELDING (PAW) can be defined as a gas-shielded arc welding process where the coalescence of metals is achieved via the heat transferred by an arc that is created between a tungsten electrode and a workpiece. The arc is constricted by a copper alloy nozzle orifice to form a highly collimated arc column. The plasma is formed through the ionization of a portion of the plasma (orifice) gas. The process can be operated with or without a filler wire addition. Note : The exact current where the keyhole mode is initiated will depend on the torch geometry and the joint material and thickness, but currents of over 200 A and plasma gas flows of 3 to 4 1 min–1 are typical with a 2 to 3 mm diameter constricting orifice. The thermal efficiency of the process is high and it has been estimated that the heat transferred to the workpiece from a 10 kW plasma arc can be as high as 66% of the total process power.
WeldingHigh Energy
High Penetration
CuttingLow Energy
Low Penetration
85
Plasma keyhole welding
Current
Note 1: With high currents, very high arc forces are generated and undercut or humping may occur. The current must, however, be controlled in conjunction with welding speed to produce the required bead profile.
Note 2: At high currents, it is therefore necessary to maintain the plasma gas flow rate at a reasonably high level, but the upper limit will be determined by the occurrence of undercut and decreased thermal efficiency.
Plasma gas flow rate
86
Plasma keyhole welding
Welding speed
Note 1: If the speed is too high undercutting and incomplete penetration will result, whereas at low speeds the keyhole size may become excessive and the weld pool will collapse.
Note 2: The operating range of the process is therefore determined by a combination of mean current, welding speed and plasma gas flow.
87
Plasma keyhole welding
Orifice diameter
Note 1: Decreasing the orifice size increases the arc force and voltage. For keyhole welding small, 2–3 mm diameter, orifices are normally used.
Electrode geometry
Note 2: The electrode geometry and its position within the torch are critical due to their effect on gas flow within the torch. It is suggested [149] that tolerances of 0.1 to 0.2 mm on electrode position are required. Concentricity of the electrode is also important, since any misalignment may result in asymmetrical arc behaviour and poor weld bead appearance; the electrode should either be adjustable or fixed by a ceramic insert inside thetorch.
88
Plasma keyhole welding
Multiport nozzles
Note 1: The multiport nozzle may be used to enhance constriction and produce an elliptical arc profile which is elongated along the axis of the weld. Recent work [150] has shown that a second concentric nozzle may also be used to provide an increase in arc pressure, although excessive focusing gas flow rates may reduce the thermal efficiency of the process.
Shielding gases
Note 2: The most common shielding and plasma gas is argon. From 1 to 5% hydrogen may be added to the argon shielding gas for welding low-carbon and austenitic stainless steels.
Note 3: The effect of these small additions of hydrogen is quite significant, giving improved weld bead cleanliness, higher travel speeds and improved constriction of the arc. Helium may be used as a shielding medium for high-conductivity materials such as copper and aluminium. It will tend to increase the total heat input although it may reduce the effect of the constriction and produce a more diffuse heat source. With 30% helium/70% argon shielding gas mixtures, keyhole welding speeds 66% higher than those achieved with argon shielding have been reported for aluminium.
89
Plasma keyhole welding
Arc characteristics
Note 1: Constriction of any arc causes an increase in arc voltage. In the plasma process, increasing the plasma gas flow, decreasing the diameter of the constriction, increasing current and adding hydrogen or helium to the gas will all increase the arc voltage.
90
Plasma keyhole welding
Reverse polarity plasma
Note 1: The plasma process is normally operated with the electrode negative (DCEN) and the workpiece positive. This polarity may be reversed to allow cathodic cleaning to occur when welding aluminium alloys. It is usually necessary to increase the electrode size and limit the maximum current due to the additional heating effect within the torch, but helium shielding gas may be used to extend the thickness range weldable in the keyhole mode up to 8 mm.
Pulsed keyhole plasma
Note 2: Pulsed operation improves the resistance to undercut and generallyproduces a wider, flatter bead.
91
Plasma keyhole welding
Applications
Carbon–manganese ferritic steel: The plasma keyhole welding of carbon– manganese steels has recently been evaluated for circumferential root runs in pipes for power generation and offshore applications. The use of the process enables thick root sections (6–8 mm) to be welded in a single pass from one side and significantly improves productivity. Pulsed plasma keyhole has been used for these studies [154] to improve operating tolerance and positional performance.
92
Plasma keyhole welding
Applications
Austenitic stainless steel: Austenitic stainless steel may be readily weldedusing the keyhole technique and the process has been applied to the longitudinal welding of pipe as well as the fabrication of components for cryogenic service. Welding speeds of around 1.0 m min–1 are achievable with keyhole welds in material up to 2.7 mm thick, whilst welds in 6.0 mm thick material may be made at 0.35 m min–1 . The use of hydrogen additions to the shielding gas or proprietary mixtures containing from 1 to 5% hydrogen provide improved bead appearance and increased travel speed. Undercut may be limited by careful control of welding parameters, but, if this is not possible, pulsed operation or the use of cosmetic passes is recommended.
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Plasma keyhole welding
Applications
Titanium: Steady and pulsed keyhole plasma welding techniques havebeen applied to titanium and its alloys [156, 157] and, providing adequateprovision is made for gas shielding, high-integrity welds may be made. Theshielding requirement may be met by carrying out the welding operation ina glove-box, which is vacuum purged and backfilled with argon, or using atrailing shield of the type shown in Fig.
94
Plasma keyhole welding
Applications
Note : The major problem experienced with this material is undercut; this may be alleviated by:
• careful selection of welding parameters on thicknesses up to 3 mm;• pulsed operation with controlled current decay on each pulse;• cosmetic runs with plasma or GTAW and filler;• magnetic arc oscillation along the axis of the weld.
95
Plasma keyhole welding
Applications
Aluminium: Plasma keyhole welding of aluminium is possible if electrode positive polarity is used as discussed above. It is also possible to use advanced power sources with the capability of variable-polarity operation and significant improvements in quality and cost have been reported using these techniques. It has been found that acceptable results are achieved with 15–20 ms of DC electrode negative operation and electrode positive pulses of 2–5 ms duration.
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Laser welding
Note 1 : The laser may be used as a welding heat source and consists of a high-energy coherent beam of light of an essentially constant wavelength.
Note 2 : LASER is an acronym for Light Amplification by Stimulated Emission of Radiation and the medium in which it is generated may be either solid, liquid or gaseous.
Helium/neon and CO2
ruby and neodymium doped yttrium aluminium garnet (Nd:YAG)
Note 3 : In welding applications, the two most commonly used lasers are the CO2 gas laser and the Nd:YAG solid state laser. Recent developments include the availability of high-power diode lasers (HPDL) and fibre lasers suitable for welding applications.
98
Laser welding
Laser Co2
Note 1: The principles of operation of the CO2laser are illustrated in Fig. 8.7. An electrical discharge within the gas (N2 , He, Co2) is used to stimulate the emission of radiation.
Excitation of Co2 molecule and Produce Photon
Note 2 : The initial low-level radiation is ‘trapped’ within the laser cavity by mirrors placed at either end. The internal reflection of the beam causes an increase in the energy level (amplification). A fraction of the laser beam generated in this way is allowed to escape from the resonant cavity via a partially reflective mirror. In the case of the CO2laser, the emergent light beam has a wavelength of 10.6 µm and is delivered to the workpiece by a series of mirrors and lenses.
99
Chapter 5Newest of Welding
Processes
100
Introduction
Newest of Welding Processes
MIAB
MIAF
Solid state
Liquid state
MIABThe components to be welded are rigidly clamped in the MIAB welding equipment, leaving a predetermined, small gap between their ends. In the first stage of the process, a DC arc is struck between the component ends. A static, radial magnetic field causes this arc to travel at high speed around the joint circumference, heating the component ends to a high temperature (see Fig). Heating continues for a few seconds then, in the second stage, the components are brought together under a predetermined forging pressure (see Fig. 2). Any molten material is expelled from the joint and a solid phase weld is produced.
101
Introduction
Newest of Welding Processes
MIABThe components do not rotate during the process. No filler material or shielding gas is used and MIAB welding produces very little fume. Low and medium carbon steel and low alloy steel are routinely MIAB welded. Production applications include drive shafts, propeller shafts, beam axles, axle casings, refrigerator condenser tubes, pressure accumulators and petrol tank filler spouts. Wall thickness from 0.6mm to 10mm and cross sections up to 2000mm 2 have been successfully MIAB welded at TWI. MIAB welding studies have included aluminum alloys, stainless steels and titanium alloys.
102
Introduction
Newest of Welding Processes
MIAFMagnetically impelled arc fusion (MIAF) welding is a variation on MIAB (magnetically impelled arc butt welding) which is widely used to join steel pipes and tubes. In MIAB, an arc is struck between the two components to be joined, MIAF uses a non-consumable electrode as an arc initiator. Illustrations A and B show a MIAF set-up for edge-welding the periphery of a flat, circular component. The component to be welded is located on a central spigot which is connected to a pulsed arc welding power supply, similar to that used for micro-TIG welding. The annular electrode completes the electrical circuit. When the power supply is activated, an arc is struck between a point on the annular electrode and the edge of the component. The arc is then propelled around the annular electrode by a series of electro-magnets (see illustration B) which are switched on and off in a high-speed sequence.Rotation of the arc continues for a pre-set period until the edge of the component is fusion welded. No additional filler material is used.
103
LOGO
By Hamid Taghipour Armaki