INTRODUCTION ATOMIC BONDING FREE ENERGY

INTRODUCTION ATOMIC BONDING FREE ENERGY

Definition 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.

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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.

Broad View for Motivation Advantages of Solid State Welding • 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.

Disadvantages of Solid State Welding • Surface preparation can be necessary. • Joint design is limited. • Elaborate and expensive equipment may be required. • Non-destructive inspection is very limited.

Materials Solid State Welding Materials • 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.

Applications 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. • Drill pipe. • Intake / exhaust automatic valves. • Bi-metallic pipe.

Applications Solid State Welding Explosion clad titanium steel tube sheet blanks 180 inch diameter dome of 3/16 inch type 410 stainless steel on 3 inch thick A387 steel formed from explosion weld. Courtesy AWS handbook

Types of Solid State Welds We Will Look At Each Linnert, Welding Metallurgy, AWS, 1994

Questions

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

Atomic Bonds • There are two major types of atomic bonds • Primary bonds • Secondary bonds • Primary bonds are much stronger than secondary bonds.

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

Ionic Bonding • Bonding takes place between metallic and nonmetallic elements. • Metallic atoms give up valence electrons to nonmetallic atoms. • Examples : NaCl, MgO, CaCl2. Na+ Cl Na+ Na+ Cl Cl Cl Na+ Na+

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

H H H C H Covalent Bonding • Bonding between two atoms takes place by cooperative sharing of electrons. • Examples: Gas - N2, O2, CH4. Solid - carbon (diamond), silicon, germanium. Methane (CH4)

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.

Questions

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

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

Dipole - Induced Dipole Interaction Kotz, “Chemistry & Chemical ReaCTIONS”, Saunders College Pub., 1999

Induced Dipole - Induced Dipole Interaction Kotz, “Chemistry & Chemical ReaCTIONS”, Saunders College Pub., 1999

Questions

Adhesion of Perfect Metal Surfaces 10 A • 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). From: Materials Science and Engineering: An Introduction by W.D. Callister, John Wiley & Sons, 1985

Free energy formation of a weld missing bond A B 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. Surface energy of A is greater than B

A B gAB Free energy formation of a weld • The welding of metal A to metal B results in a decrease in free energy (DGweld). • This negative energy difference (DGweld) creates a driving force which actually promotes welding. A B g0 g0 g0 and gAB are surface energies (surface tension) of the free surfaces and grain boundaries respectively.

A B Free energy formation of a weld Summary for Similar Metals A B g0 g0 gAB

Free energy formation of a weld Summary for Dissimilar Metals • A similar relationship can be developed for dissimilar metal welding showing a large negative (-) DG for all dissimilar metal combinations.

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End of Atomic Bonding

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INTRODUCTION ATOMIC BONDING FREE ENERGY