Joining vs assembly

Joining techniques

Welding

Soldering

Brazing

Adhesive bonding

welding

Coalescing parts that are in contact by using heat and or pressure

→ Works by locally changing microstructure of grains -> i.e. melting

soldering

Melting and flowing a filler metal into the joint

• Filler = low melting point

• Base metal not melted during the project

• E.g. roof flashing

brazing

Soldering but using filler metal that has a higher melting point

• Better strength than soldered joints

• Higher temp can cause problems

adhesive bonding

Filling gap with an adhesive (polymer) which forms a surface bond with the metal

Assembling techniques

Bolts and nuts

Rivets

Stitching

Stapling

Molding inserts

Integral fasteners

bolts and nuts

Two connecting parts that are easy to assemble on site

• Components can be dismantled for repair/modifications

rivet

Permanent connection

• E.g. skin panels of aircraft, harbour bridge

• Needs lots of maintenance

stitching

Fastening operation using a stitching machine to form U-shaped stiches from a steel wire

• Immediately drive them through the two parts to be joined

Stapling

Preformed U-shaped staples are pinched through two parts

molding inserts

The placement of a component into a mold before plastic molding or metal casting

• So it becomes a permanent part of molding or casting

• E.g. internally threaded nuts, externally threaded bearings

integral fasteners

Use deformation of component parts so that they interlock

How to decide which technique to use

Consider

• Strength

• Ease of manufacture

• Cost

• Permanency

• Corrosion-resistance

• Appearance

 

E.g. Bridge

• Joints in girders carry varying loads of vehicles

• Constraints

  • Joints have to be made on site

  • Time is not important parameter

  • Repair is hard

  • Welding requires lots of experience

types of welding

Fusion welding

Solid-state welding

fusion welding + steps

Fusion welding

• Heat to melt base metals

• Use filler material most of time

• Most welding process is this

 

Steps

1. Parent metal melted on each side of joint line

2. Molten metal combines to form liquid pool between two components

3. Molten metal solidifies to create a bridge

types

• arc welding

• gas welding

• resistance welding

• high energy welding

what is Solid-state welding

• Pressure alone OR combination of pressure and heat

• If heat involved -> below melting temp of base metals

• No filler metal used

types

• diffusion welding

• roll welding

• friciton wleding

• forge welding

pros of welding

• Permanent joint

• Can be a lot stronger than parent material if filler material has better strength properties

• Usually most economical way to join components regarding material use and fabrication cots

• Not restricted to factors -> i.e. can be onsite

cons of welding

Limitations in operations	Limitations on metals
Most operations are performed manually Expensive labour cost -> skilled labourers needed Dangerous since they use high energy No disassembly since permanent Defects can be difficult to detect  = loss of material properties	Warping and distortion from high residual stresses by uneven heating Cracking from loss in ductility + increased HAZ hardness Reducing joint toughness at HAZ Destroying favourable effects of heat treatment + work hardening done before welding

Parts of Weld

Fusion zone

Heat-affected zone (HAZ)

Unaffected base metals

what is fusion zone

• Mixture of filler and base materials that have completely melted

• Inhomogeneous grains

• Preferred grain growth orientation

  • Columnar grains

  • Perpendicular to boundaries of weld interface

• Coarse grains

what is Heat-affected zone (HAZ)

• Metal experiences below melting temperature -> still changes microstructures

• How much metal is changed depends on

  • Heat input

  • Peak temp

  • Distance from fusion zone

• Medium grains

what is Unaffected base metals

• Remaining parent metal in original structure

• Not enough temperature to change properties

• Can be high residual stresses due to expansion and shrinking of fusion zone

• Fine grains

Factors affecting distortion and residual stress in welding

• Coefficient of thermal expansion

• Melting point

• Phase changes

• Difference of thermal expansion between two metals

• Energy input

• Heat treatment history

• External restraints during welding

How to reduce residual stress in welding

• Peening

  • Can induced distortion

  • Difficult to control

• Heat treatment

  • Most controllable

  • More effective than peening

  • Metal needs proper heat treatment

when does weld induced distortion occur

Due to non-uniform heating of weld and base metal

what is weldability

= capacity to be welded under fabrication materials

• Depends on carbon content

• More carbon = harder to weld

Microstructure evolution

Peak temperature = can change microstructure

• Low carbon steel weld easily

  • Strength can be higher than base material

  • Fine pearlite forms during cooling step

• Welding of medium and high carbon steels is more difficult

  • Martensite can form in HAZ easily

  • Decreases toughness of material

what is carbon equivalence

• Over 0.76% carbon is hard and difficult to weld

• Add other elements to improve weldability and strength

• If CE < 0.4%

  • Very easy to weld

• If CE between 0.4 - 0.6

  • Prone to hydrogen cracking so need to add some low-hydrogen electrodes

  • Called hydrogen embrittlement

• If CE > 0.6

  • Need to add things to prevent hydrogen cracking and formation of martensite

  • Martensite is hard but very brittle -> we want tempered martensite to have ductility

    • We don't like brittle because we want plastic deformation

    • Done through pre-heat and post-heat treatment

    • Slower cooling rates

    • Higher heat welding input

ce%

are residual stresses always bad

Residual stresses are not always bad

• Compressive stress = crack don't grow