INTRODUCTION ATOMIC BONDING FREE ENERGY

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INTRODUCTIONATOMIC BONDING

FREE ENERGY

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Definition of Solid State Welding

A group of welding processes that produces coalescence at temperatures essentially below the melting point of the base metal. Pressure may or may not be used.

Definition

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Introduction to Solid State Welding

History of solid state welding dates back to very ancient time.

Gold was hammered together by the ancients earlier than 1000 B.C.

The iron framework of the Colossus of Rhodes was forge welded in 280 B.C.

Versatility of fusion welding eclipsed solid state welding in the first half of the 20th century.

Solid state welding experienced a rebirth in the 60’s and 70’s, especially in the field of micro-electronics.

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Eliminates liquid phases. Makes the joining of many dissimilar metal

combinations possible. Can be applied at different temperatures and under

different stresses At high temperature, where the atomic interaction range is

relatively large and solubility of contaminants is high, parts can be joined together with less deformation.

At low temperature, where the atomic interaction range is relatively small and solubility of contaminants is low, more stress is needed to join two parts together and thus more deformation is expected.

Broad View for Motivation

Advantages of Solid State Welding

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Surface preparation can be necessary. Joint design is limited. Elaborate and expensive equipment may be

required. Non-destructive inspection is very limited.

Disadvantages of Solid State Welding

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Both similar and dissimilar metals can be welded. Similar metal welds include:

Titanium-to-titanium alloy (aircraft rivets) by friction welding.

Ultrasonic welding of fine aluminum wire to aluminum metallization in microelectronics.

Examples of dissimilar metal includes Aluminum to steel, titanium to aluminum, and titanium to

stainless steel (tubular transition joint) by explosion welding.

Materials

Solid State Welding Materials

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Solid State Welding Applications

Bonding of stainless steel liners in aluminum fry pans.

Aluminum cladding bonded to uranium fuel rods.

Ultrasonic and thermo-compression bonding in the microelectronics industry.

Friction welding in aero-space and automotive applications.

Applications

Drill pipe. Intake / exhaust

automatic valves. Bi-metallic pipe.

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Solid State Welding

Applications

Explosion clad titaniumsteel tube sheet blanks

180 inch diameter dome of 3/16 inch type410 stainless steel on 3 inch thick A387 steel

formed from explosion weld.Courtesy AWS handbook

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1998/SSW1/41/15Linnert, Welding Metallurgy, AWS, 1994

Types of Solid State WeldsWe Will Look At Each

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Basic Principles

In solid state welding, joining of two surfaces takes place by atomic bonding between the atoms on the surfaces.

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There are two major types of atomic bonds Primary bonds Secondary bonds

Primary bonds are much stronger than secondary bonds.

Atomic Bonds

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Primary Bonding

Primary bonds include three types: Ionic bonds Covalent bonds Metallic bonds

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Ionic Bonding

Bonding takes place between metallic and nonmetallic elements.

Metallic atoms give up valence electrons to nonmetallic atoms.

Examples : NaCl, MgO, CaCl2.

Cl

Cl

Cl

Cl

Na+

Na+

Na+

Na+

Na+

Kotz, “Chemistry & Chemical ReaCTIONS”,Saunders College Pub., 1999


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Covalent Bonding

Bonding between two atoms takes place by cooperative sharing of electrons.

Examples: Gas - N2, O2, CH4.

Solid - carbon (diamond), silicon, germanium.

C HH

H

H

Methane (CH4)



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Metallic Bonding

Valence electrons are not bound to any particular atom and are free to drift throughout the metal.

Remaining non-valence electrons and atomic nuclei form ion cores.

Free electrons act as a glue to hold the ion cores together.

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Secondary Bonding Van der Waals bonds ( Ar, Kr, Ne). Polar molecule-induced dipole bonds

(HCl, HF). Hydrogen bonds ( H2O, NH3). Bond energy only about 1/10 of

primary bonds. Can cause adhesion of contaminants

to metal surfaces.


CoCl2 - 6 H2O

Ion - Dipole InteractionKotz, “Chemistry & Chemical ReaCTIONS”,

Saunders College Pub., 1999

Dipole - Dipole InteractionKotz, “Chemistry & Chemical ReaCTIONS”,

Saunders College Pub., 1999


Dipole - Induced Dipole Interaction


Induced Dipole - Induced Dipole Interaction

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1998/SSW1/41/37From: Materials Science and Engineering: An Introduction

by W.D. Callister, John Wiley & Sons, 1985

Adhesion of metal surfaces occurs by inter-atomic forces. For this to happen, the two mating surfaces must be brought together within a very close distance. For most metals, this distance is within a range of approximately 10 angstroms (A).

Adhesion of Perfect Metal Surfaces10 A

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The potential energy of atoms at the free surface is higher than that of atoms within the bulk of the solid.

The energy per unit area possessed by atoms near the free surface constitutes the free surface energy.

The average surface atom has about half the bonding energy of an interior atom.

B

A

Surface energy of A is greater than B

Free energy formation of a weld

missing bond

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The welding of metal A to metal B results in a decrease in free energy (Gweld.

This negative energy difference (Gweld) creates a driving force which actually promotes welding.

A B

0 and AB are surface energies (surface tension) of the free surfacesand grain boundaries respectively.


A B

AB

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Gweld AB

- 2 0

AB 0.3

0G

weld

170

.

A B

AB


Summary for Similar Metals

A B

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Summary for Dissimilar Metals

A similar relationship can be developed for dissimilar metal welding showing a large negative (-) G for all dissimilar metal combinations.

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Link to Bonding Demo

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INTRODUCTION ATOMIC BONDING FREE ENERGY