-By Gaurav Babu
 Casting is one of the oldest manufacturing process
 Casting involves:
1. Preparation of mould & pattern
2. Melting & pouring
3. Cooling & solidification
4. Defects & Inspection
PATTERN
 Pattern is replica of final casting to be produced with some
modifications in terms of allowances & core-print.
 Types of Allowances:
1. Shrinkage Allowance:
 Liquid & solidification shrinkage
» Compensated by using riser.
» Expressed in terms of linear dimension
» Liquid shrinkage is maximum for Aluminium (Large size riser
used).
 Solid shrinkage:
» Compensated by increasing the size of pattern, and is called as
shrinkage allowance
» Expressed in terms of % of shrinkage volume of casting
» Shrinkage allowance is a positive allowance
» Solid shrinkage is maximum for Brass (Large size pattern used)
» Overall shrinkage is maximum for Steel.
2. Machining Allowance:
» It is positive for external dimensions & negative for
internal dimensions
3. Rapping/Shake Allowance:
» It is a negative allowance
4. Taper/Draft Allowance:
» Provided on vertical surfaces
» For easy removal of pattern
5. Distortion Allowance:
» Provided on weak structures of casting
» Amount of distortion depend on
𝐿
𝑇
ratio
i.
Single piece/Solid piece pattern
vi. Gated pattern
ii. Split pattern
vii. Match plate pattern
iii. Loose piece pattern
viii.Skeleton Pattern
iv. Sweep pattern
ix. Follow board pattern
v. Segmental pattern
x. Cope & drag pattern
Natural sand
Synthetic sand
Loam sand
 Natural sand:
» Composition: moisture(5-8%)+binding particles(5-20%)+organic matter
» Green sand
 Pitcher
» Composition: silica(70-85%)+clay(10-20%)+water(4-8%)+additives(4%)
1. Refractoriness: Ability to withstand higher temperatures of
liquid metal without breaking down or fusion
2. Permeability: Ability to allow gases to escape
𝑉∗𝐻
» Permeability number Pn =
𝑃∗𝐴∗𝑇
where, V = volume of the air passed through specimen (cm3)
H = height of the specimen (cm)
P = pressure difference (gm/cm2)
A = cross-section area of specimen (cm2)
T = time taken for air to pass through specimen (mins)
» With
increase in moisture content, permeability first
increases reaches maximum point and then decreases.
3. Flowability or Plasticity: Ability of sand to flow to each corner
& attain shape of pattern.
» With increase in moisture content, flowability first decreases
and then increases.
4. Strength: Ability to retain shape & size of mould cavity against
force applied by liquid metal.
» Green strength
» Dry strength
» Hot strength
5. Hardness: Ability of sand to prevent erosion due to forces
applied by liquid metal.
6. Adhesiveness: Property by which sand particles sticks to
sides of moulding box.
7. Cohesiveness: Property by which sand particles attract
each other.
8. Collapsibility: Ability of sand to break after solidification of
casting.
9. Durability: Ability to withstand cyclic heating & cooling
(reusable).
10. Moulding sand should be chemically inert.
11. Bench life: Ability to retain properties during storage.
12. Fineness: To improve surface finish, strength & hardness.
1
» Permeability∝
𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠
13. Co-efficient of expansion should be low.
14. Thermal conductivity should be high.
1. Pouring basin: as
reservoir of
liquid metal
2. Sprue:
to provide adequate
velocity to liquid metal
3. Runner: provides path for liquid
metal to flow into cavity
4. Ingate: entry to the main cavity
5. Riser: to compensate liquid and
solidification shrinkage
 Aspiration Effect: The constant cross section of sprue creates
negative/vacuum pressure in it, which will cause the atmospheric
air to flow in it. This is called Aspiration effect.
 To avoid aspiration effect, minimum area ratio
Where,
𝐴2
=
𝐴3
𝐻𝑇
𝐻1
HT = height of liquid metal above gate
H1 = height of pouring basin
H2 = height of sprue
A2 = area at sprue top
A3 = area at sprue bottom
 The ideal shape of sprue is parabolic, but for manufacturing ease
tapered cylindrical sprue is used.
 Fluidity: Ability of molten metal to flow inside gating system.
 Factors effecting fluidity of liq. metal are:
Factors
Fluidity
Pouring temperature of liq. metal ↑
↑
↓
↓
↓
Viscosity ↑
Surface tension ↑
Density ↑
Moisture content in sand ↑
Thermal conductivity of mould ↑
↓
↑
↓
Inclusions in liq. metal (impurities) ↑
↓
Pouring rate of liq. metal ↑
Mould design (compatible to fluid flow) ↑
↑
↑
Pattern of alloy solidification (freezing range) ↑
↓
Surface finish of Gating elements ↑
 Freezing range = TL - TS
1
 Freezing range ∝
𝐹𝑙𝑢𝑖𝑑𝑖𝑡𝑦
i.
Top Gating system
• Velocity at gate, Vg =
2 ∗ 𝑔 ∗ 𝐻𝑇
𝐴𝑚∗𝐻𝑚
𝑉𝑜𝑙 𝑚
• Mould filling time, ( Tm)top =
=
𝐴𝑔∗𝑉𝑔
𝐴𝑔∗𝑉𝑔
ii.
Bottom Gating system
• Velocity at gate Vg =
2 ∗ 𝑔 ∗ (𝐻𝑇 − ℎ)
• Mould filling time, (Tm)bottom =
2∗𝐴𝑚
𝐴𝑔∗ 2∗𝑔
∗ [ 𝐻𝑇 −
𝐻𝑇 − 𝐻𝑚 ]
where, Am = cross section(c/s) area of mould cavity
Hm = height of mould cavity
h = instantaneous height from bottom of mould cavity
Ag = c/s area of Ingate
2∗ 𝑉𝑜𝑙 𝑚
2∗ 𝑉𝑜𝑙 𝑚
• When HT = Hm, mould filling time (Tm)bottom =
=
𝐴𝑔∗𝑉𝑔
𝑄𝑚𝑎𝑥
iii. Parting Gating system
• Mould filling time, (TM)parting = (TM)top + (TM)bottom
NOTE:
• In Bottom gating system, for a given Ag & Am
when HT = Hm ,
Mould filling time (Tm)bottom = 2×(Tm)top
• Top gating system is recommended for casting of
ferrous materials.
• Bottom gating system is preferred for non-ferrous
materials.
• Mould filling time in top gating system is less
compared to that of bottom gating system.
 Core is used to produce hollow objects.
 Core Print: ‘Projection on pattern’ or
‘undercut inside mould cavity’.
 Chaplets:
Small metallic objects to
support core (same material as casting).
 Let,
F1
F2
= self weight of core
= (mg)core = (ρVg)core
= force of buoyancy
= weight of liquid displaced
= (mg)L = (ρVg)L
 Net Force/ Net Buoyancy force (Fnet):
Fnet
= F2 – F1
= (ρL – ρc)*Vc*g
 Chvorinov’s principle:
• Solidification time is directly proportional to the square
of modulus of the casting to be produced
𝝉s ∝ (M)2
𝑉𝑜𝑙𝑢𝑚𝑒
where, modulus (M) =
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎
𝝉s = K.
𝑽 𝟐
𝑺.𝑨
where, K = solidification factor
 Objective:- Riser must solidify after the solidification of
casting
(Solidification time)riser > (Solidification time)casting
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎
 Cooling characteristic =
𝑉𝑜𝑙𝑢𝑚𝑒
 Cooling characteristic of riser should be less than that of
casting
(Cooling characteristic)riser < (Cooling characteristic)casting
 For a given volume, sphere is having minimum surface area.
 Thus, ideal shape of riser is spherical. But for manufacturing
ease cylindrical riser is used.
1. Top cylindrical riser:,
» Condition for minimum surface
area is, D = 2H and
𝑫
» Modulus of riser, Mtop =
𝟔
2. Side cylindrical riser:,
» Condition for minimum surface
area is, D = H and,
𝑫
» Modulus of riser, Mside =
𝟔
1) General method:
This method is used when shrinkage volume of casting is
given.
» Step 1: Calculate VS.
Shrinkage volume, VS = 3% of VC
» Step 2: Calculate VR.
Volume of riser, VR = 3*VS
» Step 3: Apply limiting condition:
𝑆𝐴
≤
Riser
𝑉
» Step 4: Verification:
𝑆𝐴
𝑉 Casting
2) Modulus method:
𝜏𝑆 Riser > 𝜏𝑆 Casting
Mriser =1.2*Mcasting
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑅𝑖𝑠𝑒𝑟 (VR)
Volume ratio (V.R) =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑐𝑎𝑠𝑡𝑖𝑛𝑔(VC)
𝑀𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑅𝑖𝑠𝑒𝑟 (MR)
Freezing ratio (F.R) =
𝑀𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑐𝑎𝑠𝑡𝑖𝑛𝑔(MC)
3) Caine’s Method:
𝑎
X=
+𝑐
𝑌−𝑏
where,
X = Freezing ratio
Y = Volume ratio
a, b, c = constants
𝑎
F.R =
+𝑐
𝑉.𝑅−𝑏
4) Modified Caine’s Method:
 Here, F.R is replaced by shape factor (S.F)
𝐿𝑒𝑛𝑔𝑡ℎ+𝐵𝑟𝑒𝑎𝑑𝑡ℎ
Shape factor of casting =
𝐻𝑒𝑖𝑔ℎ𝑡
 Shape factor for different shapes of castings,
1. Cube:
Shape factor = 2
2. Solid cylinder:
𝐿+𝐷
Shape factor =
𝐷
3. Sphere:
Shape factor = 2
4. Hollow cylinder:
Shape factor =
𝜋𝐷𝑖+𝜋𝐷𝑜
𝐿+
2
𝐷𝑜−𝐷𝑖
2
Riser placement
 Chills: Metallic objects with high melting point & high thermal
conductivity used for directional solidification.
 For two risers, approximate length of casting,
i.
With end wall effect, L = 4.5T + 4T + 4.5T = 13T
ii.
Without end wall effect, L= 2T + 4T + 2T = 8T
where T = thickness of casting
Gating Ratio:
Gating ratio = AS : AR : AG
Choke area:
» It is the minimum area of cross-section amongst AS, AR, AG.
Achoke =
where,
𝑚
𝜌.𝐶𝑑.𝑇𝑓. 2.𝑔.𝐻𝑇
m = mass of casting
𝜌 = density of molten metal
𝐶𝑑 = Co-efficient of discharge
𝑇𝑓 = Mould filing time
 For pressurised gating system, Achoke = AG
 For unpressurised gating system, Achoke = AS
 Casting process can be classified as
1. Expandable mould casting:
Ex: sand casting, shell moulding, investment casting,
vacuum mould casting, ceramic mould casting etc.
2. Permanent mould casting:
Ex: centrifugal casting, die casting, slush casting,
squeeze casting etc.
Shell mould casting:
Advantages:
1) Very good surface finish
2) Fluidity is good
3) Close dimensional tolerance
4) Machining operation is reduced
Disadvantages:
1) Metallic patterns are expensive
Applications:
-gears
-bushings
-engine blocks
-valve bodies
-cam shaft,
-cylinder head of IC engine etc.,
Vacuum mould casting:
 Sand particles or sand mould are held together by vacuum.
 Advantages:
1) No binders used
2) No ramming needed
3) No moisture is added hence, free from moisture defects
 Disadvantages:
1) Relatively slow process
Full mould casting:
» Pattern: polystyrene, rubber, PVC, foam etc.
 Applications:
-fitting
-tooling
-lock components
-motor casing etc.
Plaster mould casting:
 Permeability increases
 Because of presence of POP, porosity is very less
 For casting light materials with lower melting point such
as aluminium, magnesium, copper based alloys.
Ceramic moulding:
 For casting of high melting point temperature materials
 Not for intricate shapes
Investment casting:
 Pattern: wood, metal, rubber,
foam, polystyrene, wax etc.
 For high melting point
temperature materials
 For intricate shapes
 High surface finish with close
tolerance
 Applications: turbine blades,
machine parts, jewellery, dental
etc.
 Metallic moulds such as steel, cast iron etc, are used.
 For casting lower melting point temperature materials like Al, Cu,
Mg, etc.
Advantages:
1) High surface finish
2) Close tolerance
Limitations:
1) For simple part geometry
2) For mass production
DIE
CASTING
Gravity
die
casting
Low
pressure die
casting
High
pressure
die casting
Gravity die casting:
Low pressure die casting:
 Pressure ≈ 0.1MPa
 Uses pressure to fill the
mould
 Reduces oxidation chances
High pressure die casting:
a) Hot chamber die casting:
 Furnace is a part of system
 For low melting point temp materials like Mg, Pb, Sn, Zn etc.
 Pressure range ≈ 7MPa – 35MPa
b) Cold chamber die cating:
 Furnace is not a part of system
 For high melting point temperature materials like Cu, Al &
their alloys
 Pressure range ≈ 14Mpa – 140Mpa
 Pressure used in die casting for thin section is ≈7MPa – 950MPa
Applications:
-carburettor
-valve bodies
-crank case
-connecting rod of small size
-fuel injection pump part
Injection Moulding:
 Metal replaced by plastic
 Thin section of 0.5mm can be produced.
Centrifugal casting:
 Material segregation takes place
 Impurities are forced towards centre of rotation
 Pure metal forced towards wall
 Fine grain structure obtained due to high rate of heat transfer
Applications:
-axisymmetric objects
-pipes
-gun barrels
-bush
-propeller shaft
 Gravitational factor (G-factor):
𝑪𝒆𝒏𝒕𝒓𝒊𝒇𝒖𝒈𝒂𝒍 𝒇𝒐𝒓𝒄𝒆
G=
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒄𝒂𝒔𝒕𝒊𝒏𝒈
Semi-centrifugal casting:
 For making non tubular parts like pulley, flywheel, spoke wheels etc.
Slush casting:
Applications:
-thin sections
-lamp shades
-toys
-hollow statues
-decorative items
a) Gaseous defects:
 Blow holes, scar, blister, pin holes
Remedies:
• Increase permeability
• Reduce moisture content
• Use dry sand mould
• Proper heating
b) Mould material & method defects:
 Penetration, mould crack & scab – due to improper ramming
 Swell & rate tail – due to excessive heat
 Dirt, drop – due to erosion
c) Gating design defects:
 Wash/cut – improper gate design
 Shrinkage cavity – due to improper riser design
d) Defects related to fluidity:
 Misrun – improper filling of cavity (single stream of liq. metal)
 Cold shunt – when two liq. streams cannot diffuse properly
Remedies:
• Increase pouring temperature
• Reducing thermal conductivity of mould
e) Other defects:
 Core shift – improper design, not using core print
 Mould shift – misalignment of cope & drag
 Machining is a negative manufacturing process
 Desired shape & dimensions are obtained by the action of
metal removal
Machining
Non-conventional
Conventional
WJM
Single point
Double point
Multi point
Turning
Drilling
Milling
AJM
Facing
Scissors
Broaching
EDM
Shaping
Nail cutter
Grinding
ECM
Honing
USM
AWJM
Cutting tool Vs Machine tool
 Cutting tool
» Body which removes extra material from work piece through direct
mechanical contact
» Ex: turning tool, broach, drill bit etc.
 Machine tool
» Machine that provides necessary relative motion between work
piece & cutting tool
» Ex: Lathe machine
 Generatrix & Directrix
 Line generated along cutting motion – Generatrix
 Line generated along feed motion - Directrix
TYPES OF MACHINING
1) Orthogonal cutting (two-dimensional):
• When chip & work piece(w/p) move in a plane parallel to
plane of paper.
• When cutting edge is straight & relative velocity of w/p &
tool is perpendicular to cutting edge.
2) Oblique cutting (three-dimensional):
• When relative velocity is not perpendicular to cutting
edge.
NOTE:
 Study & analysis of orthogonal is simpler than oblique
 Nature of metal removal is same for all basic machining
operations.
Ex: milling grinding, turning, broaching, facing etc.
Important terminology:
𝛼 = rake angle
𝛾 = clearance angle
t1 = uncut chip thickness
t2 = chip thickness
w = width of cut
L = length of chip
𝜑 = shear angle/shear plane angle
t1
 Chip thickness ratio/ cutting ratio (r) =
t2
• For ductile materials, r < 1
• For brittle materials, r ≈1 or r < 1
 Area of chip = t2×w
 Volume of chip = t2×w×L
Rake angle (𝛼):
• Angle of inclination of rake surface from the reference plane
• 𝛼 allows easy chip flow
• It reduces cutting force requirements & increases surface finish
• 𝛼 provides sharpness to cutting edge
Types of rake angle:
a. Positive rake angle:
» For machining ductile/low strength materials
» For low speed, low power machining
» Tool material – tough & ductile. Ex: HSS
b. Negative rake angle:
» For machining high strength materials
» For high speed, high power
machining
» Carbide & ceramic inserts (tips)
are used with ductile material
shank for tool
c. Zero rake angle:
» Form tools
» Thread cutting
» Zero rake is used to avoid digging
Ex. Brass, cast iron
Let,
πR = Reference plane (parallel to base)
πX / πL = Longitudinal plane (⊥r to base & ∥el to width)
πY / πT = Transverse plane (⊥r to base & ∥el to length)
Back rake angle (𝛼 b): Angle of inclination of rake surface from 𝜋R
& is measured in πT.
Clearance angle/End Relief angle (𝛾/ 𝛾 e ): Angle of inclination of
end flank from πL & is measured in πT.
• To avoid rubbing of flank face with machined surface
• 𝛾 reduces wear, increases tool life & surface finish
• Always positive (3º-15º)
Side rake angle (𝜶s ): Angle of inclination of
rake surface from the πR and is measured in
πL.
Side relief angle (𝜸s ): Angle of inclination
of side flank from the πT and is measured in
πL.
Side cutting edge angle (𝜳s): Angle of
inclination of side cutting edge from πT and
is measured in πL.
End cutting edge angle (𝜳e): Angle of
inclination of end cutting edge from πL and
is measured in πT.
1) Tool in-hand system:
• No quantitative information
• Only salient features of cutting tool are identified
2) Machine reference/ASA (American standard association)
system:
• Tool signature is :- 𝜶b - 𝜶s - 𝜸e - 𝜸s – 𝜳e – 𝜳s – R
𝛼 b – Back rake angle
𝛼 s – Side rake
𝛾e – End relief angle
𝛾s – Side relief angle
Ψe – End cutting edge angle Ψs – Side cutting edge angle
R – Nose radius (inch)
3) Tool reference system/ORS (Orthogonal rake system):
• Tool signature is:- i - 𝜶 - 𝜸s - 𝜸e - 𝜳e – 𝝀 – R
i – inclination angle
𝛼 – orthogonal rake angle
𝜆 – principal cutting edge angle
• For oblique turning operation 𝜳s + 𝝀 = 90º
NOTE
 Orthogonal Turning: 𝜳s = 0
𝝀 = 90
i≠0
 Pure Orthogonal turning : 𝜳s = 0
𝝀 = 90
i=0
tan i = tan 𝜶b cos 𝜳s – tan 𝜶s sin 𝜳s
tan 𝜶 = tan 𝜶s cos 𝜳s - tan 𝜶b sin 𝜳s
NOTE
 Identification of tool signature can be done just by picking
up 6th component
» if it lies between 15 - 30º then it’s ASA
» otherwise it’s ORS
𝐭𝟏
𝐬𝐢𝐧 𝝋
 Chip thickness ratio (r) = =
𝐭𝟐 𝐜𝐨𝐬 𝝋−𝜶
𝟏
 Chip reduction ratio/ chip compression ratio 𝛏 =
𝐫
𝒓 𝐜𝐨𝐬 𝜶
 tan 𝝋 =
𝟏−𝒓 𝐬𝐢𝐧 𝜶
 Shear strain in machining
𝜸 = 𝐜𝐨𝐭 𝝋 + 𝐭𝐚𝐧 𝝋 − 𝜶
Note: If rake angle of tool is zero then the minimum shear strain =2
 Shear strain rate
𝐕𝐬
∈=
𝐭𝐡𝐢𝐜𝐤𝐧𝐞𝐬𝐬 𝐨𝐟 𝐩𝐫𝐢𝐦𝐚𝐫𝐲 𝐬𝐡𝐞𝐚𝐫 𝐳𝐨𝐧𝐞
Sine rule of triangles:
𝐕𝐜
𝐕𝐬
𝐕
=
=
𝐬𝐢𝐧 𝛗
𝐜𝐨𝐬 𝛂
𝐜𝐨𝐬 𝛗−𝛂
where,
Vc = Chip velocity/ velocity of chip w.r.t tool
Vs = shear velocity/velocity of chip w.r.t w/p
V = cutting velocity/velocity of tool w.r.t w/p
F = Friction force (Tangential force on tool face)
N = Normal contact force acting on chip-tool interface
FS = Shear force
FN = Normal force acting on chip-w.p interface
FC = Cutting force
FT = Thrust force (or) tool separating force
R = Resultant force
𝛼 = rake angle
𝛽 = friction angle
𝜑 = shear angle
 Friction force, F = R.sin 𝛽
 Normal force, N = R.cos 𝛽
 Cutting force, FC = R.cos 𝛽 − 𝛼
 Thrust force, FT = R.sin 𝛽 − 𝛼
 Shear force, FS = R.cos 𝜑 + 𝛽 − 𝛼
 Normal shear force, FN = R.sin 𝜑 + 𝛽 − 𝛼
 Co-efficient of friction,
𝑭
𝝁 = = tan 𝜷
𝑵
 Area of shear plane,
𝑾.𝒕𝟏
(Area)shear =
𝐬𝐢𝐧 𝝋
𝑾.𝒕𝟏
 Shear force, FS = 𝝉𝑺.
𝐬𝐢𝐧 𝝋
𝑾.𝒕𝟏
 Normal shear force, FN = 𝝈𝒖𝒕.
𝐬𝐢𝐧 𝝋
where,
W = width of cut
𝑡1 = uncut chip thickness
𝜏𝑆 = ultimate shear strength of w.p
𝜎𝑢𝑡 = ultimate normal strength
NOTE
 Case I: If 𝛼 = 0 & 𝜇 ≠ 1
» FC ⊥ F and FT ⊥ N
» FC, FT, F & N form a rectangle
» FC = N & FT = F
 Case II: If 𝛼 = 0 & 𝜇 = 1
» FC ⊥ F and FT ⊥ N
» FC, FT, F & N for a square
» FC = N = FT = F
1. Cutting power = FC.V
2. Friction power = F.VC
3. Shear power =FS.VS
∴ Cutting power = Friction power + shear power
FC.V = F.VC + FS.VS
MRR = A.V
= uncut area × cutting speed
MRR = t1 × w × V
I
 In case of turning, t1.w = f.d
where,
t1 = uncut chip thickness
w = width of cut
f = feed rate
d = depth of cut
∴ MRR = f.d.V
 Another way,
(Area)chip = w × t2
MRR = w × t2 × VC
I = II
t1 VC
=
t2
V
VC
∴r=
V
II
 Specific power consumption
𝑝𝑜𝑤𝑒𝑟
S.P.C =
𝑀𝑅𝑅
i.
𝐽/𝑠𝑒𝑐
𝐽
= 3
= 3
𝑚 /𝑠𝑒𝑐
𝑚
𝑒𝑛𝑒𝑟𝑔𝑦
𝑣𝑜𝑙
Specific cutting power consumption
𝑐𝑢𝑡𝑡𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟
FC × V
FC
S.C.P.C =
=
=
𝑀𝑅𝑅
t1 × w × V
t1 × w
ii.
Specific shear power consumption
FS ×VS
S.S.P.C =
t1 × w × V
iii. Specific friction power consumption
F × VC
S.F.P.C =
t1 × w × V
 Cutting pressure / Specific cutting pressure
FC
FC
PC =
=
𝑢𝑛𝑐𝑢𝑡 𝑎𝑟𝑒𝑎
t1 × w
 It is based on some assumptions
i.
Material behaves like ideal plastic
ii. τS is assumed to be constant
iii. 𝛽 is assumed to be constant
Cutting power
𝐜𝐨𝐬 𝛃−𝛂
= FC.V = FS .
.V
𝐜𝐨𝐬 𝛗+𝛃−𝛂
𝐖.𝐭𝟏
𝐜𝐨𝐬 𝛃−𝛂
= 𝛕𝐒.
.
.V
𝐬𝐢𝐧 𝛗 𝐜𝐨𝐬 𝛗+𝛃−𝛂
 For minimum cutting power, denominator should be maximum
𝑑 sin φ.cos(φ+β−α)
i.e.,
=0
𝑑𝜑
∴ 2𝝋 + 𝜷 − 𝜶 = 90º
1) Cutting speed / Tangential speed:
𝛑𝐃𝐍
𝐕=
𝟔𝟎
N
𝜳s
FT.sin 𝜳s
FT.cos 𝜳s
FC
V
FC
FT
2) Feed: axial/ radial movement of tool w.r.t rotation of w/p
𝑚𝑚
• feed is measured in
𝑟𝑒𝑣
3) Feed velocity:
Vf = f.N
𝑚𝑚 𝑟𝑒𝑣
𝑚𝑚
.
=
𝑟𝑒𝑣 𝑚𝑖𝑛
𝑚𝑖𝑛
a) Main cutting force (FC): Tangential component of force
b) Axial force / feed force(FF) = FT.cos 𝜳s
c) Radial force (FR) = FT.sin 𝜳s
 These three forces are mutually perpendicular
 For turning operation,
FC > FF > F R
t1
cos 𝜳s =
 In △ABD,
𝑓
⇒
t1 = f.𝐜𝐨𝐬 𝜳s
I
 For orthogonal turning, as 𝜳s = 0 ⇒ t1 = f
⇒ FR = 0
⇒ FF = FT
𝑑
 From △ABC, cos 𝜳s =
𝑤
I = II
t1.w = f.d
II
 Sudden death: premature end of tool
 Gradual / Slow death: gradual wearing
Important Note:
 Sudden death is,
» unpredictable tool failure
» It may include,
⇒ excessive plastic deformation
⇒ brittle fracture
⇒ fatigue fracture
⇒ edge chipping
Slow death
Flank wear
Crater wear
Flank wear
 Flank wear is the most imp. parameter in tool design than
crater wear
 It includes wear on both flank faces, cutting edge & nose
 It directly affects component dimensions
 Reasons of flank wear:
i.
Abrasion with particles
ii.
Shearing of micro welds
iii. Rubbing of fragments of BUE against end flank surface
iv. At slow cutting speed flank wear is predominant
 Stages in flank wear
i.
Primary wear:
 Zone where sharp cutting edge is quickly
broken
 Finite wear land is established
ii.
Secondary wear:
 Zone where wear takes place at constant rate
 This is working zone of tool
iii. Tertiary wear:
 wear progresses at highly increasing rate
because, area of contact is much larger
 Tool wear becomes more sensitive to increase
temperature of machining zone
 Regrinding needed
Crater wear
 It occurs on rake surface
 By changing chip-tool interface geometry it affects the
cutting process
 Most significant factors of crater wear
» Temperature at tool-chip interface
» Chemical affinity of tool-chip material
 Crater wear is predominant at higher speeds
i.
Abrasion wear/Mechanical wear
ii.
Adhesion wear
iii. Diffusion wear
iv. Oxidation wear (High temp, high cutting speed)
 High toughness
 Wear resistant
 Refractoriness
 Low co-efficient of expansion
 High thermal conductivity
 Low c-efficient of friction
 Corrosion resistant
 Chemically inert
 Formability
 High hot hardness
Tool life is defined in following ways
 Actual cutting time
 Length of the cutting
 Volume
 No. of parts produced
Taylor’s tool life eqn.
VTn = C
where, V = cutting speed (m/min)
T = tool life (min)
n = tool life exponent (depends on tool)
C = constant (depends on tool, w.p & cutting conditions)
T=
1
𝐶𝑛
1
𝑉𝑛
⇒
as V
↑ T↓
n = 0.08 to 0.2 ⇒ For HSS
n = 0.1 to 0.15 ⇒ For cast alloys
⇒ For carbides
n = 0.5 to 0.7 ⇒ For ceramics
n = 0.2 to 0.4
Modified Taylor’s equation:
V.Tn.fa.db = C
T=
1
𝐶𝑛
1 𝑎 𝑏
𝑉 𝑛 𝑓𝑛 𝑑𝑛
. .
⇒V ↑ T ↓
⇒f↑ T↓
⇒d↑ T↓
1
𝑎
𝑏
> >
𝑛
𝑛
𝑛
⇒ V > f >d (orderof parameters effecting tool life )
Edge chipping/Chipping:
 Breaking away of small pieces from cutting edge
 Chipping can be very small or relatively very large
 Unlike wear, chipping results in sudden loss in tool material
 It directly affects surface finish, surface integrity & cause
poor dimensional accuracy
Main causes of chipping
 Variable dimensions of cut; mechanical shock
 Thermal fatigue (cyclic temperature variation)
 High positive rake angle
 Surface irregularities
 Material irregularities (inter-composition)
 Tool wear depends on
1. Tool material
2. Work piece material
3. Cutting fluid
4. Process parameters like V, f, d etc.
1) Fixed cost/Non-productive cost
2) Machining cost
3) Tool regrinding cost
4) Tool changing cost
5) Total cost
VO = optimum speed for min. cost
CO = min. cost
Total cost = C1 + C2 + C3 + C4
Speed Vs cost graph
I.
For maximum production / minimum production
time
1
TO =
− 1 .TC
𝑛
where,
n = Taylor’s tool life exponent
TC = Tool changing time
II. For minimum cost
𝐶𝑡
1
TO =
− 1 . TC +
𝑛
𝐶𝑚
where,
𝐶𝑡 = Tooling cost (Rs)
𝐶𝑚 = Machining cost (Rs/min)
Cutting speed Vs Time graph
(1) Non prodiction time
(2) Machining time
(3) Tool changing (or regrinding) time
(4) Tol production time
(5) Production rate curve
VO = optimum speed for maximum production
Cutting speed Vs Production rate
From the graphs, it is clear that
(VO )max. production > (VO )min. cost
(VO )max. production
Maximum profit range
(VO)min.cost < (VO)max.profit < (VO)max.prodn
I.
Continuous chips
II. Discontinuous chips
III. Serrated chips
Continuous chips
Without BUE
With BUE
⟹ Work material: Ductile
⟹ Work material: Ductile
⟹ Cutting speed: High
⟹ Cutting speed: Low
⟹ Rake angle: +ve & high
⟹ Rake angle: +ve & high
⟹ Depth of cut: Small
⟹ Depth of cut: Med. To high
⟹ Feed: Low
⟹ Feed: Medium
⟹ Coolant use: Yes
⟹ Coolant use: Absent
Effects for BUE
 Poor surface finish
 High force required
 High power consumption
 It can induce impact & vibration
 Tool geometry is changed
Positive aspect of BUE
 Initial BUE gives strength to cutting edge
Discontinuous chips
⟹ Work material: Brittle
⟹ Cutting speed: Low
⟹ Depth of cut: Medium to High
⟹ Feed: Medium to High
Serrated chips/ Non-homogeneous chip/ Segmented chip
 Temperature ↑ ⟹ Strength ↓ ↓ ↓
 Ex: Titanium
I.
Rake angle (𝜶)[7-30°]
 Rake angle (α) ↑ ⟹ sharpness ↑ ⟹ forces ↓
 α ↑ ⟹ FS ↓ ⟹ shear deformation ↓ ⟹ t2 ↓ ⟹ r ↑
 t2 ↓ ⟹ less force & power consumption ⟹Low cost
 Rake angle ∝ shear angle
 Chip thickness ratio (r) ∝ rake angle
 Increases machinability, surface finish
 Increases tool life
II. Clearance angle (𝛄) [3 – 15°]
 It prevents the tool from rubbing
 It increases tool life
III. Friction angle (𝛃)
 It increases friction force & cutting force
 Increases chip thickness
 β increases shear plane angle (φ)
IV. Cutting edge angle (𝚿𝐬 &𝚿𝐞)
a) 𝚿𝐞 [8-15°]
 Prevents rubbing of end cutting edge with machined surface
 Larger Ψe weakens the tool
 It has no influence on cutting force
 Reduction of Ψe improves surface finish
b) 𝚿s [15-30°]
 Ψs improves surface finish
 Ψs increases chatter tendency
 It increases width of cut
 Wide chip carries more heat, hence less thermal damage
 It reduces chip thichness
 Increases machinability
Special case (Ψs = 0):
 Used when surface of work piece is having distructive
nature
 Less amount of cutting edge is exposed to work piece
V. Lip angle/ Cutting angle/ knife angle/ wedge angle
𝛼 b + 𝛾e + lip angle = 90
 Lip angle ↑ ⟹ strength of tool ↑
 Dull cutting edge increases heat content, cause more damage to
surface.
 Rubbing occurs without chip formation. To produce chips (with
dull tool) higher depth of cut is used.
Case-I: When tool nose radius (R) = 0 (Ideal case)
𝑓
Hmax =
cot Ψe+tan Ψs
Havg =
where, f = feed rate
Hmax
4
Case-II: When tool nose radius (R) ≠ 0
𝑓2
Hmax =
8𝑅
𝑓2
Havg =
18 3.𝑅
 R ↑ ⟹ Hmax ↓ ⟹ surface roughness ↓ ⟹ surface finish ↑
 Joining is a process of making temporary, semi-permanent
& permanent joints between two similar/dissimilar
materials with/without application of pressure, heat & filler
material.
 Products such as automobiles, AC, washing machine,
machine tool, etc., need to be manufactured in a way to be
able to taken out apart for maintenance or repair
JOINING PROCESS
Mechanical
bonding
Temporary
Semi-permanent
/ Permanent
-Fasteners
-Riveted
-Screw joint
-Shrink fit
-Nut bolt
-Staples
-Stitches
-Seaming
-Crimping
Atomic
bonding
Welding
Adhesive
bonding
WELDING
Solid state
Solid cum
liquid state
Liquid state (fusion)
(Hot)
Cold
welding
Explosive
Diffusion
Friction
welding
Forge
welding
Ultrasonic
welding
Electrical
ARC
Chemical
Resistance
→ SMAW → Spot
→ SAW
→ Seam
→ PAW
→ Projection
→ MIG
→ Electroslag
→ TIG
→ HAW
Thermit
Gas
Soldering
Brazing
Braze
welding
NOTE
 Welding can also be classified as follows,
1) Autogenous welding: when no filler material used
Ex: all resistance & solid state welding except electro-slag welding
2) Homogenous welding: when filler material is same as parent
material
Ex: all arc & gas welding, thermit welding
3) Heterogeneous welding: when filler material is different
from parent material
Ex: soldering, brazing, braze welding
Weldability: ability of being welded into an inseparable joint
having some specified properties like strength, proper structure
etc.
 Weldability depends on
1) Melting temperature ↑ ⟹ weldability ↓
2) Thermal conductivity:- K ↑ ⟹ weldability ↓
3) Co-efficient of expansion:- 𝛼 ↑ ⟹ weldability ↓
4) Surface condition:-
unclean surface condition ⟹ weldability ↓
clean surface condition ⟹ weldability ↑
5) Change in microstructure ↑ ⟹ weldability ↓
Control on weldability:
i.
Proper shielding atmosphere
ii.
Proper filler material
iii. Proper welding process
Ex: Aluminium → SMAW (oxide) ⇒ crack
→ TIG (oxide layer - NO)
iv. Proper cleaning
→ liquid (oil/water) layer is removed by heating
→ machining, wire brushes used to remove contaminating
layers
→ Acid pickling used to remove rusting
v.
Proper heat treatment
IMPORTANT NOTE
 Increasing order of weldability:
Al
<
Co <
Ca <
Ms
(Aluminium < Copper < Cast Iron < Mild steel)
 As carbon content increases weldability decreases
(1) Fillet joint
Lap joint
ii. Corner joint
iii. T-joint
i.
(2) Groove joint
Edge – edge joint
Ex: V-groove, Square groove, Bevel groove, J- groove, Ugroove, Flare-groove
ii. Flare joint
Ex: Curved – flat surface joint, Curved – curved surface joint
i.
(3) Plug & slot joint
 Surface structure
 Corrosion → (oxidation)
Strength ↓
ii. Loss of material ↓
i.
 Non-ferrous material, stainless steel, non-metallic components
are generally having high corrosion resistance.
 Steel & cast iron are having poor corrosion resistance
Condition: both the parts to be joined must be in solid state
Ex: Friction welding
 Solid state welding process includes,
1) Surface deformation
2) Surface film
3) Recrystallization
4) Diffusion
Condition:
Both parts to be joined must come into liquid state.
Both bodies should be soluble with each other
 Metallurgical zones in fusion welding process
i.
Weld pool (liq. State)
ii.
HAZ (solid)
iii. Initial/original body (solid)
Tapping
⟹
Metallic bridge formed ⟹ Thermionic emission
Step-1: Tapping:
⇒ touch electrode to work piece, current flows
𝐼
current density ↑ =
𝐴↓
Step-2: Withdraw electrode from w.p
⇒ current density ↑
⇒ this bridge starts boiling
Step-3: Thermionic emission:
⇒ both the surfaces emit electrons
⇒ electrons travel in air column, ionised air is formed
⇒ plasma reacts with temperature
⇒ arc is generated
Heat source in fusion welding process
 For Arc welding:
Electric Arc
 For Gas welding:
Chemical flame
 For Thermit welding:
Exothermic reaction
 For Resistance welding: Resistance heating
Chemical reactions
2 C2H2 + 2 O2
→
4 CO + 2 H2 + △H
→
(1)
4 CO + 2 H2 + 3 O2
→
4 CO2 + 2 H2O + △H
→
(2)
(1)
+
(2)
2 C2H2 + 5 O2
→
4 CO2 + 2 H2O + △H
C2H2 + 2.5 O2
→
2 CO2 + H2O + △H
 1 mole of C2H2 requires 2.5 moles of O2
 We can supply C2H2 and O2 in the ratio 1:1 from cylinders.
 Remaining 1.5 mole of O2 is consumed from atmosphere
 Heat source: Exothermic reaction
⇒
3 Fe3O4 + 8 Al +Heat → 9 Fe + 4 Al2O3 + Heat
Thermit mixture
 Oxide are always lighter due to density difference
i.
Resistance heating H = I2RT
where,
I = current (Am)
R =Resistance of interface of mating parts
T = current flow time
𝝅
ii. Volume of nugget (Vn) = (Dn)2Hn
𝟒
iii. mn = 𝝆𝐧Vn
where,
mn = mass of nugget
Vn = volume of nugget
Dn = diameter of nugget
Hn = height of nugget
iv. Heat requirement for weld nugget formation
HR = mncpdT + mnL
where,
cp = specific heat
L = latent heat
v.
Specific resistance
𝜌𝐿
R=
𝐴
vi. Power = V.I
Ω−𝑚−𝑚
𝑚2
J/sec or watt
Imp. NOTE
 If Dn & Hn are not given then,
Dn = 6 𝑡
→ Unwin’s formulae
where,
 From the figure,
2i + Hn = 2t
Hn = 2(t-i)
t = thickness of sheet (mm)
Let,
𝜈 = welding speed
f = feed
d = diameter of electrode
 Material Deposition rate(MDR)
𝜋 2
MDR = f. d → (1)
4
 Area of bead (AB) = B.T
⇒ MDR = B.T × 𝜈 → (2)
∴
𝜋 2
B.T. 𝜈 = f. d
4
 Heat input per unit length
𝐻𝑒𝑎𝑡 𝑖𝑛𝑝𝑢𝑡
𝑃𝑜𝑤𝑒𝑟
𝑉.𝐼
=
=
𝑙𝑒𝑛𝑔𝑡ℎ
𝜈
𝜈
 As
𝜈↑
𝐽/𝑠𝑒𝑐
𝑚/𝑠𝑒𝑐
⇒ Heat input ↓
I ↑
=
𝐽
𝑚
⇒ heat input ↑
⇒ depth of penetration ↓
⇒ depth of penetration ↑
⇒ Heat content ↓
⇒ heat content ↑
⇒ cooling ↑
⇒ cooling rate ↓
⇒ Heat transfer ↑
⇒ Heat transfer ↓
(feed rate = constant)
(feed rate = constant)
i.
Electrode ⟶ Arc generation
𝐻𝑎𝑟𝑐
𝐻𝑎𝑟𝑐
𝜂transfer =
=
𝐻𝑔𝑒𝑛
𝑉.𝐼
ii.
Arc generation ⟶ melting
𝐻𝑟𝑒𝑞
𝐻𝑟𝑒𝑞
𝜂melting =
=
𝐻𝑎𝑟𝑐
𝜂transfer×𝑃𝑜𝑤𝑒𝑟
𝐻𝑟𝑒𝑞
= 𝜂melting × 𝜂transfer × 𝑉. 𝐼
iii. Specific power consumption/Specific energy consumption:
𝑃𝑜𝑤𝑒𝑟
S.P.C/S.E.C =
𝑀𝐷𝑅
𝑉.𝐼
Specific energy consumption =
𝐴𝐵.𝜈
𝐽/𝑠𝑒𝑐
𝑚3/𝑠𝑒𝑐
𝜂melting × 𝜂transfer×𝑉.𝐼
Specific energy consumption =
𝐴𝐵.𝜈
=
𝐽
𝑚3
Ohm’s law,
V∝I
V = IR
⟶ Linear
 Ohm’s law is valid only when,
» there exist a conductor to pass the current
» resistance of material, temperature & pressure are constant
 But, Ohm’s law is not valid in welding transformer
 For transformer
1
⟶V ∝
I
where, I = Arc current (amps)
V = Arc voltage (volts)
I.
Constant Current type transformer
• Used in manual arc welding
• Non linear transformer
• Sharp dropping
1
•L∝
I
•L ∝V
1
•I∝
V
II. Constant Voltage type transformer
• Used in Semi-automatic or Automatic
arc welding
• Linear transformer
Important Note
 OCV → Open circuit voltage
→ Voltage required to generate arc at
‘No-load’ condition
→ Also called Supply voltage
 SSC → Short circuit current
→ Current required during arc
generation
 For stable arc
Iarc = Itransformer (for constant current type)
ii. Varc = Vtransformer (for constant voltage type)
i.
iii. V = OCV −
OCV
SCC
.I
 Length of arc,
L ∝V
1
L∝
I
 As L ↑ ⟶ V ↑ & I ↓
⟶ Power increases upto a limit & then
decreases
Structure of Arc
 Cathode spot ⟶ responsible for e-’s
emission
 Cathode space ⟶ gaseous region
⇒ sharp voltage drop
 Arc column ⟶ Visible portion of Arc
⇒ slight voltage drop,
but not sharp
 Anode space ⟶ gaseous region
⇒ sharp voltage drop
 Anode spot ⟶ emits the e-’s
2
2
 Cooling characteristic = +
h
r
1
• Cooling characteristic(c.c) ∝
r
∴ r ↑ ⟶ c.c ↓ ⟶ heat loss ↓ ⟶ hotness ↑ ⟶ 𝜂melting ↑
ρL
R =
A
∴ r ↑ ⟶ R ↓ ⟶ conductivity ↑ ⟶ hotness ↑ ⟶ 𝜂melting ↑
 If r = very high ⇒ bulging arc
r ↑ ⟶ ⟶ 𝜂melting ↑
(upto a certain limit & then decreases)
i.
When electrode (-ve) & work piece (+ve):
 Called
as
Straight
polarity/Direct
current
straight
polarity(DCSP)/Direct current electrode negative(DCEN)
 More heat (2/3rd)at work piece & less heat (1/3rd) at
electrode
 For thick plates
 For lower deposition rate, weld speed & feed
 For higher depth of penetration
ii. When electrode (+ve) & work piece (-ve):
 Called as Reverse polarity/Direct current Reverse polarity(DCRP)/
Direct current electrode positive(DCEP)
 More heat (2/3rd)at work piece & less heat (1/3rd) at electrode
 For thin plates
 For higher deposition rate, weld speed & feed
 For lower depth of penetration
Percentage penetration
𝐴𝑃
𝐴𝑃
% penetration = × 100 =
× 100
𝐴𝑇
𝐴𝑃+𝐴𝑅
where,
𝐴𝑃 = Area of penetration
𝐴𝑅 = Area of reinforcement
I.
Fusion welding
1) Arc welding
a) Shielded metal arc welding (SMAW)
 Weld pool:- amount of liq. metal b/w two
mating parts
 Weld bead:- amount of material added
b/w two mating parts
 Root gap:- min. gap b/w two mating parts
 Reinforcement:- projected part of weld bead from surface
of body
 Penetration:- depth of weld bead
 Fillet weld:-
Throat ⟶ weakest section of weld bead (min. material)
Toe ⟶ junction of weld bead & workpiece
Root ⟶ deepest point of fillet
Purpose of flux coating
1. To develop gaseous shield & protect weld pool from atmosphere
2. To provide slag, which controls viscosity of liquid metal
3. It reduces heat transfer rate of weld bead by creating slag layer
4. Acts as deoxidiser
5. Acts as carrier of alloying elements (↑ strength) for weld pool
6. It stabilises arc
Different elements used in flux coating
a) Slag formation:
-Iron oxide
-Titanium oxide
-Silicon dioxide
-Silica flour
-Calcium fluoride
b) Gas forming agents:
-Cellulose
-Calcium carbonate
c) Alloying elements:
-Chromium ⟶ corrosion resistance
-Nickel ⟶ corrosion resistance & strength
-Cobalt ⟶ hardness
d) Deoxidising elements:
-Graphite
-Alumina
-Ferro silicon
-Ferro manganese
e) Arc stabilizers:
-Calcium oxide
-Sodium oxide
-Potassium oxide
i.
Flat welding
• Electrode moves in horizontal surface
• Condition: both work pieces are lying in side by side
• Symbol ⟶ ‘F’
ii. Horizontal welding
• Condition: both work pieces should be up & down
• Symbol ⟶ ‘H’
iii. Vertical welding
• Symbol ⟶ in upward direction: ‘V’
⟶ in downward direction: ‘D’
iv. Overhead welding
• Condition: when welding is done on upper surfaces like bridges,
roof etc.
• Symbol ⟶ ‘O’
 Coating factor (C.F)
𝐷
C.F = > 1
𝑑
Case-I ⟶ when d = constant
C.F ∝ D
∴ D ↑ ⟶ C.F ↑ ⟶ coating thickness ↑
Case-II ⟶ when D = constant
1
C.F ∝
𝑑
∴ d ↓ ⟶ C.F ↑ ⟶ coating thickness ↑
Imp. Note
 C.F ↑ ⟶ coating thickness ↑ ⟶ Quality of weld ↑
 Quality of weld:
» Coating (manual arc welding)
» Current
» Voltage
» Weld speed
Types of electrode
i.
Consumable
• Act as filler material
• When consumable electrode is used, the welding process is
called Metal arc welding
Ex: steel, CI, copper, brass, bronze, Al
• May or maynot be coated.
• Coating is different for AC (ex: potassium silicate) & DC
(sodium silicate) welding
• Coating ⟶ Iron powder
» Penetration will be high
» Deposition rate will increase
ii. Non consumable
• Autogenous welding
• Electrode doesn’t act as filler material. but will be provided
from outside, if needed
Ex:
carbon, graphite ⟶ for AC
tungsten ⟶ for both AC & DC
• When non-consumable electrode used welding process are
termed on the basis of electrode material
Ex: TIG, carbon arc welding
 Selection of electrode length:
a) Welder convenience
b) Current carrying ability
⟶ electrode length ↑ ⟹ resistance ↑
⟹ heating of electrode ↑
⟹ coating will start decomposing
 Selection of electrode diameter:
a) Thickness of plate ⟶ thickness ↑ ⟹ diameter ↑
b) Weld position
NOTE
 Coating is electrically insulating, it enables deep grove joints or
narrow groove joints
Electrode specifications
i.
As per Indian standards (IS)
BIS: Bureau of Indian standards
E
XXX
[XXX]
P
A
B
C
D
A. Prefix
⟶ ‘E’- extruded/solid extrusion
⟶ ‘R’- reinforced extruded
B.
X
X
X
↓
↓
↓
Type of coating
weld position
type of current
X
X
X
↓
↓
↓
Yield strength
% elongation
C.
Tensile strength
D. Suffix
⟶ ‘P’ – deep penetration
⟶ ‘H’ – hydrogen controlled electrode
⟶ ‘J, K,L’– content of iron powder
ii.
As per American welding society (AWS)
a) EXXXXP
E
XX
↓
strength
X
↓
X
↓
P
weld position Type of current
b) EXXXXXP
E
XXX
X
X
↓
↓
↓
strength
weld position Type of current
P
Welding current
⟶ upper limit depends on heating
» I↑↑ ⟶ heating ↑ ⟶ coating starts decomposing
⟶ lower limit depends on arc stability
» I ↓ ↓ ⟶ ionization process gets disturbed ⟶ extinguish the arc
Case -I ⟶ when current is more than optimum
a) Wide & flat weld bead
b) Deep crater & penetration
c) Resistance heat ↑
d) Excessive spatter lose
Case -II ⟶ when current is less than optimum
a) Poor penetration
b) Narrow bead
c) Metal will pile up
d) Bead profile is poor
e) Poor control on slag
 Selection of welding current depends on
i. Thickness of plate
⟶ for thin sheet – DCEP
⟶ moderate thickness sheet – AC
⟶ for thick sheet – DCEN
ii. Length of cable⟶ l ↑ ⟶ resistance ↑ ⟶ AC(less voltage drop)
⟶ l ↓ ⟶ DC(more voltage drop)
iii. Kind of arc initiation
⟶ for easy & stable arc – DC
⟶ for lower value of current -DC
iv. Odd welding positions ⟶ say overhead, vertical etc.
v.
Arc blow:
Deflection of arc by the magnetic field created due
to the flow of welding current
⟶ Predominant in DC welding
⟶ Predominant while welding of strong magnetic
material like nickel alloys
⟶ More in bare electrode
 Reduction of arc blow
⟶ AC used (change in polarity nullifies magnetic field)
⟶ Lower value of current, arc length & weld speed
Welding Voltage
Case-I ⟶ when voltage is more than optimum
a) Arc wander
b) Spatter loss
c) Deposit irregularities
Case-II ⟶ when voltage is less than optimum
a) Poor penetration, unstable arc(may even extinguish arc)
b) Pile up ⟶ poor weld bead
Welding speed
Case-I ⟶ when speed is more than optimum
a) Heat input is less
b) Poor penetration
c) Narrow weld bead
d) Under cuts
Case-II ⟶ when speed is less than optimum
a) Heat input is less
b) Penetration is more
c) Heat factor
⟶ melt defects
⟶ energy loss
 For obtaining a sound weld ⟶ weld current, voltage & speed
should be optimum
 If current is drawn from power source for longer time, it produces heat.
 This heat can cause damage to power source, cables, windings, coils
etc.
Total cycle time = welding time + rest time
𝑊𝑒𝑙𝑑 𝑡𝑖𝑚𝑒
Duty cycle =
× 100
𝑡𝑜𝑡𝑎𝑙 𝑡𝑖𝑚𝑒
(or)
𝐴𝑟𝑐𝑖𝑛𝑔 𝑡𝑖𝑚𝑒
Duty cycle =
× 100
𝐴𝑟𝑐𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 +𝑅𝑒𝑠𝑡 𝑡𝑖𝑚𝑒
 Duty cycle refers to % of welding time of the total weld cycle time
NOTE
• As per AWS ⟶ Standard weld cycle time = 10 mins
• As per IS ⟶ Standard weld cycle time = 5 mins
Duty cycle vs Current
Let 60% duty cycle,
As per IS
Tarc
⟹ 0.6 =
5 mins
∴ Tarc = 3 mins;
Trest = 2 mins ⟹ high current is drawn from P.S
Similarly, for 100% duty cycle,
As per IS,
∴ Tarc = 5 mins;
Trest = 0 mins ⟹ low current is drawn from P.S
For numerical purpose
IR2 × DR = I1002 × D100
where,
IR = Required current
DR = Required duty cycle
I100 = Current at D100
D100 = 100% duty cycle
Modes of metal transfer
#Consumable electrodes
 Forces acting on drop
A. Gravitational force
• If down hand position (flat)
• If up hand position (overhead)
B. Surface tension force (prime factor)
• It will always retaining force
• Surface tension (S.T) ↑ ⟹ size of drop ↑
• Surface tension depends on temperature & quality of metal
 Quality of metal,
⟶ If pure metal ⟹ surface tension ↑ ⟹ difficult to detach
⟶ If impure metal ⟹ surface tension ↓ ⟹ easily detachable
 Coating composition
⟶ eliminates all impurities ⟹ S.T ↑ ⟹ difficult to detach
⟶ produces lots of contamination ⟹ S.T ↓ ⟹ easily detachable
⟶ shielding gas ⟶ lightly ⟹ S.T ↓ ⟹ easily detachable
⟶ heavily ⟹ S.T ↑ ⟹ difficult to detach
C. Electromagnetic force (Lorentz force)
• Cross-section of conduction ↑ ⟹ current & magnetic force are in
the same direction
• Cross-section of conduction ↓ ⟹ current & magnetic force are in
the opposite direction
D. Hydrodynamic action of plasma
• Detaching force
• Modes of transfer:
» Short circuit transfer or dip transfer
» Globular transfer
» Spray transfer
» Pulsed spray transfer
» Rotating spray transfer
Imp. NOTE
 Current magnitude affects mode of transfer
Ex:
lower value of current ⟹ short circuit transfer
very high value of current ⟹ rotating spray transfer
 Magnitude of current affects rate of deposition
short circuit < globular < pulsed spray < spray < rotating spray
Increasing order of current
Short circuit transfer
⟹ used in odd position like overhead
⟹ penetration is lower
⟹ no spatter loss
⟹ for thin sheets
Globular transfer or droplet transfer
⟹ current is slightly higher than short circuit
⟹ dia. of drop > dia. of electrode
⟹ Initially, surface tension force is dominating
⟹ Size of droplet is more
⟹ Finally gravity will be dominating
⟹ detachment takes place
 Two possibilities are there
1. Drop size is much higher ⟹ short circuit transfer
2. Free flight transfer
⟹ there is spatter loss, not suitable for overhead welding
⟹Occurs when CO2 used as shielding gas in GMAW
Spray transfer
⟹ when current is more than that of
globular transfer
⟹ current increases ⟹ magnetic force
is dominating
⟹ It is projected mode of metal transfer
⟹ Most reliable mode of metal transfer
⟹ Can be used in any position (flat,
overhead, vertical)
 Transition current
⟹ increases with increases in dia. of
electrode
⟹ decreases with electrode extension
⟹ at high current, spray drops become finer
⟹ occurs only when shielding gas
contains 90% argon
⟹ deep penetration is possible
⟹ most common mode of metal transfer
⟹ not suitable for thin plates/sections
i.
3D Heat flow
• Ex: Spot welding ⟶ (x, y, z)
ii.
2D Heat flow
• Ex: Arc welding ⟶ (x, y)
• In Z-direction, heat transfer is very less.
Hence considered as 2D
Heat flow of 2D source
1
𝜗𝑤
Q = 8.k.𝜃𝑚.t +
5
4𝛼
where,
watts
k = thermal conductivity
𝜃𝑚 = temperature difference = Tm – Ta
t = thickness of plate
𝜗 = welding speed
w = maximum width of weld bead
𝑘
𝛼 = thermal diffusivity =
𝜌𝑐
Note
 Weld bead is strongest portion, because of the alloying elements
𝜎weld bead > 𝜎parent > 𝜎HAZ
 Prevention: preheating
Weld decay
⟹ internal granular corrosion
⟹ takes place in corrosion resistive material like stainless steel
⟹ prevention: Niobium(Nb) or Titanium(Ti)
⇓
strong carbide former
Temperature variation diagram
A. Aim of preheating
∆T = Tm – Ta = 1170
• To reduce heat losses from weld bead
• To prevent cracking
• To prevent plate distortion
• To reduce contraction rate
• It provide sufficient time for hydrogen to escape from weld
zone
• It prevents brittle fracture
B. Aim of post heating
• Heat treatment process (annealing) → stress relieving process
• Chances of crack formation will be less
b) Submerged arc welding (SAW)
• Generally used for thick plates (10-
50mm)
• Automatic welding process
• Continuous welding process
• Used only in flat position
• Not used for odd positions like
vertical & overhead
• Depth of penetration is high
• Deposition rate is high
Applications
→ ship building, pressure vessels, bridge
construction, container of big sizes