Manufacturing Engineering — Casting and Welding

Casting and welding are fundamental manufacturing processes tested heavily in GATE ME. Casting questions focus on defect analysis, solidification, and mold design, while welding questions cover process types, heat-affected zone (HAZ), and NDT methods. Expect 3–6 marks from this topic, often with applied numerical or conceptual questions.

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🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Casting Defects — Key Types:

Defect	Cause	Prevention
Shrinkage	Solidification contraction	Add risers, proper feed
Porosity	Gas evolution, trapped air	Proper venting, degassing
Cold shut	Two streams meeting cold	Increase pouring temp
Misrun	Metal solidifies before filling	Increase pouring temp
Hot tear	Stresses during solidification	Design proper modulus

Solidification:

• Solidification time: $t = C \cdot (V/A)^2$ (Chvorinov’s rule)
• Dendrite growth: columnar or equiaxed grains
• Directional solidification: riser at top, sprue at bottom → feed metal to shrinkage

Welding Processes:

Process	Heat Source	Application
SMAW (Arc)	Electric arc	Structural steel, repair
GMAW (MIG)	Continuous wire + gas	Auto, sheet metal
TIG (TIG)	Tungsten electrode	Aerospace, Al, SS
SAW	Flux + wire	Thick plates, pipes
Resistance	Resistance heating	Sheet metal, automotive

Heat-Affected Zone (HAZ): The region adjacent to weld metal where base metal microstructure changes due to thermal cycle. Not the weld metal itself.

⚡ Exam Tip: For casting problems, Chvorinov’s rule is frequently tested: $t = B \cdot (V/A)^n$ where $n \approx 2$.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Casting — Solidification and Mold Design

Solidification of Metals

Metals solidify in two stages:

1. Nucleation: Initial grains form at mold walls (chill crystals)
2. Growth: Dendrites extend into liquid, forming columnar or equiaxed structure

Grain structure zones:

• Chill zone: Fine equiaxed grains at surface (high nucleation rate)
• Columnar zone: Elongated grains growing toward center
• Equiaxed zone: Central region with random orientation

Dendrite Arm Spacing (DAS): Affects mechanical properties — finer DAS (faster cooling) gives better properties.

Chvorinov’s Rule — Solidification Time

$$t = B \cdot \left(\frac{V}{A}\right)^n$$

Where:

• $V$ = volume of casting
• $A$ = surface area in contact with mold
• $B$ = mold constant (depends on material, superheat)
• $n \approx 1.5$ to 2

Practical use: Riser design — riser must solidify after the casting to feed shrinkage. Apply Chvorinov’s rule to both casting and riser.

Riser Design

Riser requirements:

• Volume: Enough liquid metal to compensate for solidification shrinkage
• Solidification time: $t_{riser} > t_{casting}$ (must solidify last)
• Shape: Cylindrical or spherical (minimum $A/V$ ratio)

Riser Efficiency: $\eta = (V/A){riser}/(V/A){casting}$ — higher is better (spherical most efficient).

Module method: Module $M = V/A$. Riser module should be ~20% larger than casting module.

Gating System

Purpose: Deliver liquid metal to mold cavity without turbulence, erosion, or gas entrainment.

Components:

1. Pouring basin → controls flow rate
2. Sprue → vertical channel, should be tapered (wider at top)
3. Runner → horizontal distribution channel
4. Gate → entrance to mold cavity

Gating ratio (for horizontal/vertical flow):

• Against pressure: $A_{gate} < A_{runner} < A_{sprue}$ (prevents backflow)
• For safety against aspiration: $A_{gate} > A_{runner}$

Welding — Processes and Metallurgy

Arc Welding Processes

Shielded Metal Arc Welding (SMAW):

• Consumable electrode (flux coated)
• Manual process
• Good for structural steel, repair work

Gas Metal Arc Welding (GMAW/MIG):

• Continuous wire electrode
• Inert/shielding gas (Ar, He for Al; CO₂ for steel)
• High deposition rate, automated

Gas Tungsten Arc Welding (TIG/GTAW):

• Non-consumable tungsten electrode
• Inert gas shield (Ar, He)
• Highest quality weld, precision applications
• Requires filler rod addition

Submerged Arc Welding (SAW):

• Granular flux covers arc
• High weld quality, fast, deep penetration
• Automatic process

Heat-Affected Zone (HAZ)

HAZ is the base metal that undergoes microstructural changes due to welding thermal cycle:

Grain coarsening zone: Adjacent to weld fusion line. Coarse grains → reduced toughness.

Grain refined zone: Further from fusion line. Fine grains → improved properties.

Intercritical zone: Partial transformation region.

HAZ softening occurs in work-hardened or precipitation-hardened alloys.

Weld Metal Microstructure

• Fusion zone: Solidifies as columnar dendrites (cast structure)
• HAZ: Experiences peak temperature gradient from $T_m$ to $T_{AC1}$
• Base metal: Unaffected region

Hardness distribution: Maximum hardness at fusion line (for steels), drops as distance increases.

Brazing and Soldering

Brazing

• Joint clearances: 0.025–0.125 mm (capillary action draws filler)
• Filler melts above 450°C but below base metal melting point
• No melting of base metals
• Types: Torch, furnace, induction, dip brazing

Brazing filler metals:

• Copper-zinc (brass brazing)
• Silver alloys (for SS, tool steels)
• Nickel alloys (high temp)
• Aluminum-silicon (for Al)

Soldering

• Process similar to brazing but filler melts below 450°C
• Tin-lead (traditional) — Pb-free now required (Sn-Ag-Cu alloys)
• Applications: Electronics, sheet metal, plumbing
• Fluxes: Rosin (R), rosin activated (RMA), no-clean

⚠️ Key difference: Brazing uses stronger joints and higher temperatures; soldering is for electronics and leak-tight joints.

Non-Destructive Testing (NDT)

Liquid Penetrant Testing (PT)

• Detects surface and near-surface defects
• Process: Clean → Apply penetrant → Remove excess → Apply developer → Inspect
• Limitation: Surface only, requires rough surface to trap penetrant

Magnetic Particle Testing (MT)

• Detects surface and slightly subsurface defects in ferromagnetic materials
• Process: Magnetize → Apply particles → Inspect
• Limitation: Only works on ferromagnetic materials

Ultrasonic Testing (UT)

• Uses high-frequency sound waves (1–10 MHz)
• Detects internal defects, measures thickness
• Pulse-echo technique: $d = vt/2$
• Can locate and size defects

Radiographic Testing (RT)

• X-rays or gamma rays
• Detects internal defects (porosity, cracks, inclusions)
• Film density relates to thickness
• Safety hazard (radiation)

Eddy Current Testing

• Detects surface/subsurface defects in conductors
• Uses electromagnetic induction
• Applications: Tube inspection, surface crack detection

Dye Penetrant vs Magnetic Particle

Feature	PT	MT
Defect detection	Surface, open to surface	Surface, near surface
Material	Any	Ferromagnetic only
Sensitivity	Good for cracks	Excellent for linear defects
Surface prep	Required	Required

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Solidification — Dendrite Growth and Segregation

Dendrite Growth Theory

During solidification, solute partitioning occurs at the solid-liquid interface:

Partition coefficient: $k = C_s/C_l$ (solid/liquid concentration ratio)

• $k < 1$: Solute rejected into liquid (most alloying elements)
• $k > 1$: Solute taken into solid (e.g., carbon in some steels)

Constitutional supercooling: Required for stable planar interface to break down into dendrites.

Microsegregation

Within a dendrite:

• Core: First to solidify, lower solute
• Interdendritic region: Later solidification, higher solute

Homogenization: Diffusion in solid state at high temperature can reduce segregation (requires long times).

Macrosegregation

• Centerline shrinkage: Shrinkage during solidification draws liquid from center → solute-rich liquid replenishes → center becomes solute-rich
• Gravity segregation: Heavy phases settle or float during solidification

Welding — Thermal Analysis

Heat Input

$$Q = \eta \cdot \frac{V \cdot I}{v}$$

Where:

• $V$ = arc voltage (V)
• $I$ = current (A)
• $v$ = travel speed (mm/s)
• $\eta$ = efficiency (0.5–0.9 depending on process)

HAZ Peak Temperature Distribution

At distance $y$ from fusion line: $$T_{peak} - T_0 = \frac{Q}{2\pi k(T_{peak} - T_0)} \cdot \frac{1}{y}$$

Cooling rate at a given temperature: $$\frac{dT}{dt} = 2\pi k \frac{(T - T_0)^3}{Q}$$

HAZ Microstructure in Steels

Grain coarsened zone (adjacent to fusion line):

• Austenite grain growth
• Coarse prior austenite grains
• Can lead to HAZ softening in some alloys

Intercritical zone (500–727°C for hypoeutectoid):

• Partial austenitization
• Hard and soft regions coexist
• Often weakest zone in low-alloy steels

Fine grained zone (near AC1):

• Unaffected or grain-refined austenite
• Good properties

Weld Metal Properties

• Strength: Often equals or exceeds base metal
• Toughness: Lower (cast dendrite structure)
• Ductility: Lower than base metal
• Corrosion resistance: May be lower (if shielding is inadequate)

Casting Defects — Detailed Analysis

Shrinkage Cavities

• Microporosity: Fine distributed pores due to interdendritic shrinkage
• Macroporosity: Gross shrinkage defects in isolated regions

Prevention:

• Proper riser design (size, placement)
• Directional solidification (chill bars, insulation)
• Pressurized feeding (in die casting)

Gas Porosity

• Dissolved gases: H₂, N₂, O₂ in liquid metal
• Reaction gases: CO, CO₂, H₂O vapor from mold materials

Prevention:

• Degassing (N₂ purge, vacuum)
• Low-moisture raw materials
• Proper mold coating
• Controlled cooling rate

Hot Tears (Hot Cracking)

Mechanism: Stresses develop during solidification when metal is weak (mushy state). If stresses exceed strength at solidus temperature, cracking occurs.

Susceptible conditions:

• Complex geometry with varying section thickness
• High restraint during cooling
• Low melting point impurities at grain boundaries

Prevention:

• Design changes (uniform sections)
• Add support (chills, risers)
• Modify composition (add Mn, reduce S)

Mold-Metal Interactions

• Penetration: Metal enters mold grain boundaries → surface roughness
• Reaction: Chemical reaction between metal and mold → non-metallic inclusions
• Burn-on: Severe metal-mold reaction → metal fusion with mold surface

Fusion Welding Processes — Advanced

Friction Stir Welding (FSW)

• Solid-state process (no melting)
• Rotating tool with pin and shoulder
• Shoulder provides heat, pin stirs material
• Applications: Al alloys (aerospace, marine, rail)
• Advantage: No distortion, excellent mechanical properties

Laser Beam Welding (LBW)

• High energy density (10⁶–10⁸ W/cm²)
• Deep penetration, narrow weld
• High speed, minimal HAZ
• Applications: Automotive, electronics, thin sheet

Electron Beam Welding (EBW)

• Vacuum process
• Very high energy density
• Deep penetration, minimal distortion
• Applications: Aerospace, nuclear (vacuum required)

NDT — Advanced Methods

Acoustic Emission Testing

• Passive method (detects stress waves from growing defects)
• Online monitoring during pressure testing
• Detects active cracking

Thermography

• Infrared imaging detects surface temperature variations
• Subsurface defects show as temperature anomalies
• Active (外部热源) or passive

Shearography

• Laser interferometry detects surface strain changes
• Sensitive to subsurface defects
• Used for composite inspection

Example Problem

GATE 2022 (ME) Style: A cylindrical riser of 10 cm diameter and 15 cm height is used for a steel casting. Determine if the riser will feed the casting properly if the casting volume is 0.01 m³ and surface area is 0.5 m². Solidification time ratio should be at least 1.2.

Solution approach:

1. Riser volume: $V_r = \pi \times (0.05)^2 \times 0.15 = 1.18 \times 10^{-3}$ m³
2. Riser surface area: $A_r = 2\pi r h + 2\pi r^2 = 2\pi(0.05)(0.15) + 2\pi(0.05)^2 = 0.047 + 0.016 = 0.063$ m²
3. Module: $M_r = V_r/A_r = 1.18 \times 10^{-3}/0.063 = 0.0187$ m
4. Casting module: $M_c = V_c/A_c = 0.01/0.5 = 0.02$ m
5. $M_r/M_c = 0.935 < 1.2$ → Riser solidifies before casting — INADEQUATE
6. Increase riser size or use spherical riser

⚡ GATE Tip: Always compare riser module to casting module. Riser module must be larger to ensure it solidifies last and provides feed metal throughout.

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Previous Year GATE Pattern

Year	Topic Focus	Marks
2023	Solidification, riser design	3
2022	HAZ, welding processes	5
2021	Casting defects, NDT	2
2020	Chvorinov’s rule, brazing	3

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