Saturday, 29 April 2017

Metallurgy and Mechanics of Welding by Regis Blondeau Free Download PDF

METALLURGY AND MECHANICS OF WELDING

BY REGIS BLONDEAU

FREE DOWNLOAD PDF




CONTENTS
  1. Traditional Welding Processes
  2. High Density Energy Beam Welding Processes: Electron Beam and Laser Beam
  3. Thermal, Metallurgical and Mechanical Phenomena in the Heat Affected Zone
  4. Molten Metal
  5. Welding Products
  6. Fatigue Strength of Welded Joints
  7. Fracture Toughness of Welded Joints
  8. Welding of Steel Sheets, With and Without Surface Treatments
  9. Welding of Steel Mechanical Components
  10. Welding Steel Structures
  11. Welding Heavy Components in the Nuclear Industry
  12. Welding Stainless Steels
  13. Welding Aluminum Alloys
  14. Standardization: Organization and Quality Control in Welding




Introduction to Metallurgy and Mechanics of Welding by Regis Blondeau

Welding: The Permanent Bond Between Two Solid Bodies

What a long story welding is! Seeing the light of day at the end of the 19th century in the mind of scientists, it passed quickly into the hands of technicians, first
of all with the oxyacetylene technique, then with arc welding and resistance welding techniques. Other processes (we will not quote them all in this introduction) then
followed and the 20th century ended with laser welding which had its origins in the 1980s. However, it must be said that only since the 1950s has welding been the main
means of assembly, as riveting was the most used method up to that point. 

Traditional Welding Processes

Introduction

To avoid any misunderstandings, the definitions of the terms which appear in this text are those proposed in the document entitled “Terms and definitions used in
welding and related techniques” published by the “Publications of Autogeneous Welding and the International Council of the French Language” [COL 96]. It has been specified in the preface to this book that welding makes it possible to reconstitute metallic continuity between the components to be assembled. This reconstitution involves the re-establishment of the interatomic metal bonding forces which requires at the same time a connection of the nodes of the crystal lattices and the absence of any foreign body likely to constitute a screen. This chapter will successively cover the physical conditions necessary to create the metallic bond and the industrial processes which make it possible to establish this bond.

Conditions to create metallic bonding : Creating the metal bond consists, theoretically, of bringing the surfaces to be linked closer so that the surface atoms are at a distance of the order of the inter-nodal distances of their own crystalline system.

This operation, which would assume at the beginning that surfaces are chemically clean and in a specular state of polish, is not practically feasible. To mitigate this industrial impossibility, the surfaces to be joined will have to be activated with a view to eliminating the foreign bodies and elements likely to obstruct the creation of the bond. 


Activation of surfaces : The most effective surface activation is fusion which can simultaneously ensure their cleaning. The metallic bond is created by solidification. Different procedures can be employed:

--> the two parts to be assembled undergo a surface fusion and thus contribute to the formation of a molten metal pool (possibly with the addition of a filler) which
solidifies without mechanical action;

--> the two parts to be assembled undergo a surface fusion but an external mechanical action expels the molten metal and creates the assembly by placing the surfaces in contact at the solidus temperature; 

--> the two parts to be assembled undergo a localized fusion and take part in the formation of a captive molten metal core which during its solidification is compacted by the action of an external effort of compression.

The activation of surfaces can also be obtained by heating without fusion. In general it is then supplemented by a mechanical action which enables, moreover, cleaning and improvement in contact of the surfaces to be assembled. It is possible to distinguish between:

a) the case where the heating and the cleaning of surfaces to be assembled are simultaneously carried out by mechanical friction (which implies the assembly of axisymmetric parts) and is followed, after stopping the latter, by a crushing (“forging”) by axial compression; and 

b) the case where the heating is carried out by external heating and the close contact is ensured by an effort perpendicular to the joint plane. Finally, activation can result from a mechanical action without total heating of the parts to be assembled. This mechanical action causes a plasticization of the outer layer of each surface and generates a very localized heating which finally allows the establishment of the metallic bond. This process simultaneously requires a relative displacement of the surfaces to be assembled, parallel to the mating plane, coupled with a compressive force perpendicular to this same plane. It is necessary to carry

.

Elimination of obstacles to bond creation

Obstacles to the creation of the metallic bond can be of various kinds:

– geometrical surface irregularities,
– pollution of the surface (oxides, grease, moisture, etc.),
– chemical elements brought in by the surrounding air.


Surface irregularities are likely to disrupt the creation of metallic bonds in all the cases where there is not surface fusion of the parts to be assembled. It will then be necessary to carry out a surface preparation by mechanical means (grinding, machining, etc.).

All pollution of surfaces to be assembled will have to be eliminated by mechanical action (sanding, grinding) or by chemical means (solvents, scouring, drying, etc.).
It is necessary to neutralize the possible effects of chemical elements brought in by the surrounding air. Welding operations generally being carried out in atmospheric conditions, it is especially oxygen, nitrogen and hydrogen (carried in the air’s humidity) which can be harmful. Oxygen can react with the elements volatilized by the arc and in this way contribute to the creation of welding fumes. Furthermore, it can especially dissolve in the molten metal and, during solidification, contribute to the formation of: 

– metallic oxides which constitute inclusions in solidified metal;

– porosities in the molten metal due to the drop in solubility which accompanies cooling and solidification. This formation of porosities can be aggravated by a
reaction developing with an element contained in the metal and leading to the formation of a gas compound (for example, formation and release of CO during steel
welding without protection against the atmosphere).

Protection against oxygen in the air can be ensured by the interposition of a neutral gas, a molten slag or by fixing in the form of oxides by the addition of oxygen hungry elements (silicon especially). In the vicinity of the molten metal, the surface of the parent metal raised to a high temperature can also react with oxygen and be covered with oxides, which is a further justification for using protective means, including at the back of the weld. 


Aluminothermic welding

In this process the welding is carried out by running a metal in fusion (filler) into a mold built around the two faces of the parts to be assembled, placed face to face, at a specified distance. These two faces are often pre-heated with a flame via holes provided in the mold. The molten metal is created on the spot by aluminothermy, i.e. exothermic reaction between oxides (of the metal filler) and powdered aluminum. This operation is carried out in a crucible placed at the top of the mold. As the molten filler is run in, the surface of the parts to be assembled melts, preceding solidification of the assembly. The protection of the molten metal is ensured by the slag which is formed during the aluminothermic reaction. Afterwards, it is necessary to remove the mold and grind the assembled parts, so as to eliminate any excess deposits.

Resistance welding with containment of the molten metal 

This process combines the Joule effect and a mechanical pressure applied to the outside of the assembly, perpendicular to it and right where the molten metal zone
is. This force has the aim of ensuring a good electrical contact between the parts to be assembled, thereby confining the molten metal in the zone where it is formed and applying pressure to it after its solidification in order to improve its compactness by avoiding shrinkage (an operation known as spot forging). Generally the electrodes carrying the current apply this effort. These considerations show that they are overlapping assemblies of products of limited thickness. The containment of the molten metal within the joint avoids any contact with the air; the problem of its protection thus does not arise . The effectiveness of this process is related to the localization of the zone heated by the Joule effect, which depends on the electric contact resistance between the parts; precautions must be taken so that, on the one hand, other resistances in series in the electric circuit are much lower and that, on the other hand, there is no possibility of the welding current being diverted to one or more parallel circuits.

High Density Energy Beam Welding : 

Processes: Electron Beam and Laser Beam

Welding processes using high density energy beams result from the application, in the second half of the 20th century, of work conducted by physicists in the fields
of x-rays and vacuum techniques for the process of electron beam welding and optronics for laser beam welding. The possibility of concentrating these beams on points having a very small surface area led engineers to use this property to melt materials to achieve welds or cuts. 

Compared to traditional arc welding processes, these two processes are characterized by a very high energy density at the impact point on the work piece. Rykaline [RYK 74] gave a representation comparing for several processes the heat flows at the center of the heat sources and the diameters of these sources. We can observe that the energy density measured at the focal point of a laser beam or an electron beam is 10,000 times higher than that reached in an oxy-fuel flame 


Laser beam welding

History

Einstein surely did not suspect the technological revolution to which his stimulated emission theory, established in 1917, would give rise. The technological adventure could only really start in 1954 when Professor Townes and his team developed the first stimulated emission amplifier and oscillator, which they baptized “MASER” (Microwave Amplifier by Stimulated Emission of Radiation). This discovery was followed by many others. In 1958, Schawlow and Townes demonstrated the theoretical possibility of producing coherent light by stimulated emission of radiation. The first laser source was a ruby laser produced by Maiman in 1960. It was very quickly followed by the development of the first gas laser by Javan (helium-neon laser). Many mediums were then studied and used for the manufacture of lasers: doped crystals, semiconductors, ionized gases, molecular gases, liquids, dyes. Laser is currently experiencing an extraordinary development. It is used in many spheres of activity.


Principle Laser is an acronym formed by the initial letters of Light Amplification byStimulated Emission of Radiation :

An atom can pass from a fundamental state E1 to excited state E2 by the absorption of a certain energy quantity . This energy contribution can be mechanical or kinetic in origin. This atom will revert to its fundamental state by the restitution of this energy quantity, it is “the spontaneous emission”

These randomly emitted photons produce a light known as “incoherent”. There is no relationship of phase, direction and polarization between all these photons. If an incident photon causes a return to the fundamental state of the excited atom, there is “stimulated emission”. The two emerging photons are in phase, they have the same direction and same polarization as the incident photon: there is light amplification by stimulated emission of radiation. The excitation of the medium is called “pumping”. It allows the population inversion between excited and non-excited atoms. Pumping requires an external energy source which can be assured by electrical discharge or radio frequency in the case of gas lasers, and by lamps or laser diodes in the case of solid lasers. A laser source must thus be made up of three principal elements:

– an active medium made up of particles (atoms, ions, molecules),

– an energy source to carry out the pumping of the medium and thus to obtain the population inversion,

– a resonator cavity made up of two mirrors ensuring photon oscillation.

 If one of the mirrors is partially reflective, it will allow some of the photons to escape which will constitute the coherent light beam. The laser beam thus obtained will have certain characteristics of divergence, polarization and energy distribution which will define the quality of the laser beam. 

Fatigue Strength of Welded Joints

Fatigue strength

Introduction

The observation of many failures in welded structures generally points the finger at fatigue as the main cause. Moreover, it has become apparent that for welded structures, admissible service stresses were very low in respect of the static stresses (yield strength, breaking strength) and that it was not enough to apply a safety coefficient based, for example, on a fraction of the yield strength to be certain of avoiding failure. Indeed, welds can introduce severe stress concentrations and which differ from one structural element to another.



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