This document discusses the micro-structure of welding, including the fusion zone, partially melted zone, and heat affected zone. It describes the changes that occur in these zones due to the welding process, including re-melting and solidification in the fusion zone, localized melting at grain boundaries in the partially melted zone, and solid-state metallurgical reactions like recrystallization and grain growth in the heat affected zone. It also discusses how welding parameters like heat input, welding speed, and electrode diameter affect the micro-structure and properties of the different zones.

1. MICRO-STURCTURE OF
WELDING
PRESENTED BY:-
SANJIT DHAR(17120065)
CONTENTS INCLUDE THE MICRO-STRUCTURE
OF:-
1. FUSION ZONE
2. PARTIALLY MELTED ZONE
3. HEAT AFFECTED ZONE

2. THE FUSION ZONE
Similar to a casting process , the micro-structure in a weld zone
is expected to significantly change due to re-melting and
solidification of metal at the temperature beyond the effective
liquidus temperature . Nucleation and Growth of new grains
occur at the surface of the base metal in welding rather than
at the casting mould wall .

3. EFFECT OF WELDING PARAMETERS
1. WELDING SPEED :-
The welding parameters have
much more effect on the pool
shape in stainless steel
welding than aluminum
welding . The much lower
thermal conductivity of
stainless steel makes it more
difficult for the weld pool to
dissipate heat and solidify
quickly .

4. 2. HEAT INPUT
Heat Input increases ; then weld bead size increases ; then heat-affected zone size
increases

5. NUCLEATION AND GROWTH AT THE FUSION
BOUNDARY
In fusion welding , the existing base-metal grains at the fusion
line acts as the substrate for nucleation . If the liquid metal ,
which is in intimate contact , wets the substrate grains
completely , crystals can nucleate from the liquid metal upon
the substrate without difficulties . During weld metal
solidification , grains tend to grow perpendicular to pool
boundary along the direction of maximum heat extraction .
In autogenous welding , new crystals nucleate by arranging
atoms from the base metal grains without altering their
existing crystallographic orientations .
CRYSTAL TYPE EASY GROWTH
FCC <100>
BCC <100>
HCP <1010>
BCT <110>

6. EPITAXIAL AND NON-
EPITAXIAL GROWTH

7. GROWTH AND TEMPERATURE
GRADIENT
The growth rate R , is low along the fusion line and increases towards
the centreline . Maximum temperature is at the centre and then
decreases towards the fusion line . Since the weld pool is elongated ,
temperature gradient , G , is highest at the fusion line and less at the
centreline ; (G/R)CENTRELINE << (G/R)FUSION-LINE .

8. EFFECT OF OTHER WELDING
PARAMETERS
1. HEAT INPUT INCREASES ; WELDING SPEED INCREASES ;
AMOUNT OF EQUI-AXED GRAINS INCREASES
2. ELECTRODE DIAMETER INCREASES ; WELD BEAD SIZE
INCREASES ; AMOUNT OF WELD BEAD DECREASES ; H.A.Z. SIZE
INCREASES
3. ELECTRODE DIAMETER INCREASES ; HEAT INPUT INCREASES
; ALSO THE COOLING TIME INCREASES ; LEADS TO COARSE
MICRO-STRUCTURE
4. COLUMNAR GRAINS FORM ALONG THE PERPENDICULAR
DIRECTION OF TEAR-DROP WELD POOL IN CASE OF HIGH
WELDING SPEED ;
5. AXIAL GRAINS ALIGN ALONG THE LENGTH OF THE WELD POOL
SHAPE , ESPECIALLY IN CASE OF ELLIPTICAL SHAPED WELD
SPEED , IN CASE OF LOW WELDING SPEED .

9. WELD METAL NUCLEATION MECHANISM
 DENDRITE FRAGMENTATION
 GRAIN DETACHMENT
 HETEROGENOUS NUCLEATION
 SURFACE NUCLEATION
 INOCULATION
 ARC OSCILLATION
 ARC PULSATION
 STIMULATED SURFACE NUCLEATION
 MANIPULATION OF COLUMNAR GRAINS
 GRAVITY

10. PARTIALLY MELTED ZONE
1.PMZ along with unmixed zone(UMZ) forms between the fusion zone
and the heat-affected zone .
2. UMZ is characterized by completely melted and re-solidified base metal ; and
no mixing with the bulk fusion zone (composite region) . It is most prevalent in
arc welding processes (GTAW , GMAW) , not observed in EBW and LBW .
Physical properties like melting point , fluid viscosity , miscibility affect their
formation .
3. PMZ is characterized by the transition from 100% liquid at the fusion
boundary to 100% solid in the HAZ . Localized melting normally observed at
grain boundaries , and constitutional liquation of certain particles . Grain
boundary liquation occurs in the PMZ due to segregation of solute/impurities
which leads to lowering of localized melting point .

11. MICRO-STRUCTURE EVOLUTION IN
THE PARTIALLY MELTED ZONE
- At T<T(eutectic) , alpha-phase and
theta-phase are present within grains
and along grain-boundary .
- At T=T(eutectic) , eutectic reaction
occurs .
- At T>T(eutectic) , liquation inside
PMZ intensifies . More alpha-phase
dissolves into liquid phase , changing
composition into hypo-eutectic liquid .
Cooling at this point gives Cu-depleted
alpha and Cu-rich eutectic .

12. PROBLEMS ASSOCIATED WITH THE
PMZ
PLANAR MODE CELLULAR MODE
(MODES OF G.B. LIQUID SOLIDIFICATION)

13. HEAT AFFECTED ZONE
SOLID-STATE METALLURGICAL REACTIONS IN HAZ:-
1. RECRYSTALLIZATION
2. GRAIN GROWTH
3. ALLOTROPIC/PHASE TRANSFORMATIONS
4. DISSOLUTION / OVER-AGING OF PRECIPITATES
5. FORMATION OF PRECIPITATES
6. FORMATION OF RESIDUAL STRESSES SOLID STATE JOINING
PROCESSES , WHICH
HAVE NO FUSION ZONE
BUT STILL HAVE HAZ :-
1. FRICTION WELDING
2. FLASH-BUTT
WELDING
3. DIFFUSION WELDING
4. EXPLOSIVE WELDING

14. RECRYSTALLISATION AND GRAIN
GROWTH IN WELDING
 HEAT-INPUT PER UNIT LENGTH INCRESES , THEN HAZ WIDTH
INCREASES , AND LOSS OF STRENTH/HARDNESS INCREASES .
 THE HARDER THE BASE METAL , THE GREATER IS THE STRENGTH
LOSS . EFFECT OF WORK-HARDENING IS COMPLETELY GONE IN
FUSION ZONE DUE TO RE-MELTING .
 MUTI-PASS WELDS HAVE LARGER HAZ , SINGLE-PASS WELDS HAVE
SMALLER HAZ .