GATE Metallurgical Engineering Manufacturing Processes Syllabus

Distinguish hot working from cold working in terms of recrystallization temperature.

Hot working occurs above the recrystallization temperature: no strain hardening (dynamic recrystallization), large deformation possible, lower forces, poorer finish/tolerance, oxide scale. Cold working occurs below it: strain hardening, improved strength and finish, better tolerances, higher forces, residual stresses.

For flat rolling, give the formula for projected contact length and draft.

Draft $d = h_0 - h_f$ (reduction in thickness). For roll radius $R$, the projected arc of contact length is $L = \sqrt{R\,d} = \sqrt{R(h_0 - h_f)}$ (small-angle approximation), where $h_0$ and $h_f$ are entry and exit thicknesses.

Define the angle of bite (acceptance angle) in rolling and the condition for the strip to be drawn in.

The bite angle $\alpha$ is the contact angle at entry. For self-feeding without slipping, $\tan\alpha \leq \mu$ (i.e. $\mu \geq \tan\alpha$), where $\mu$ is the friction coefficient. The maximum draft is $d_{max} = \mu^{2} R$.

What is the neutral (no-slip) point in rolling?

The neutral point is the location in the roll gap where the strip and roll surfaces move at the same velocity. Before it (entry side) the strip is slower than the roll (friction acts forward, drawing metal in); after it the strip is faster (friction acts backward). Roll-force pressure peaks at the neutral point ('friction hill').

Give the approximate roll force expression in flat rolling and define the terms.

$$F = \bar{\sigma}\,L\,w$$ where $\bar{\sigma}$ is the average flow stress (plane-strain, $\approx 1.15\sigma_Y$), $L = \sqrt{R\,d}$ is the projected contact length, and $w$ is the strip width. Torque per roll $\approx F \cdot \frac{L}{2}$ and power $P = 2\pi N (FL/2)$ approximately.

What is forging and how do open-die and closed-die (impression-die) forging differ?

Forging shapes metal by localized compressive (hammer/press) forces. Open-die forging uses simple flat/contoured dies that do not fully enclose the workpiece (e.g. upsetting, cogging). Closed-die (impression-die) forging confines metal in shaped die cavities, with excess forming flash; it gives near-net complex shapes.

In open-die upsetting, why does the workpiece barrel, and what role does flash play in closed-die forging?

Barreling occurs because friction at the die-workpiece interface restrains radial flow at the ends while the middle bulges out. In closed-die forging, the thin flash land builds high resistance to outward flow, forcing metal to fill the cavity completely; flash is later trimmed.

Compare direct (forward) and indirect (backward) extrusion.

Direct extrusion: ram and metal flow in the same direction; high friction between billet and container wall, so force is higher and decreases as billet shortens. Indirect extrusion: die mounted on a hollow ram, metal flows opposite to ram motion; no billet-container relative sliding, so lower and nearly constant force, but limited by hollow-ram strength.

Define the extrusion ratio and give the ideal extrusion pressure relation.

Extrusion ratio $R = \dfrac{A_0}{A_f}$ (billet area / extrudate area). True strain $\varepsilon = \ln R$. Ideal (frictionless, homogeneous) extrusion pressure $p = \bar{\sigma}\,\ln R$; with redundant work and friction, an empirical form is $p = \bar{\sigma}(a + b\ln R)$.

What is the difference between hot extrusion and the extrusion of cold material, and which products typically use extrusion?

Hot extrusion (above recrystallization) is used for high-strength alloys and large reductions with lower force; cold extrusion gives better finish, tolerance and strength via work hardening. Typical products: rods, tubes, structural and complex constant-cross-section profiles (e.g. aluminium sections).

What is wire drawing and the formula for percent reduction of area per pass?

Wire drawing pulls wire/rod through a converging die to reduce its cross-section. Percent reduction $r = \dfrac{A_0 - A_f}{A_0}\times 100\%$. True strain $\varepsilon = \ln\!\dfrac{A_0}{A_f}$. It is a cold-working tensile process limited by the strength of the drawn (exit) wire.

Give the ideal drawing stress relation and the maximum reduction limit per pass.

Ideal (homogeneous, frictionless) draw stress $\sigma_d = \bar{\sigma}\ln\!\frac{A_0}{A_f} = \bar{\sigma}\,\varepsilon$. The draw stress must stay below the exit yield strength, so the maximum ideal reduction per pass is $\varepsilon_{max} = 1$ (i.e. $A_0/A_f = e \approx 2.72$, about 63% reduction). In practice friction limits it to ~20-35% per pass.

Why does wire drawing use a conical die with a small half-angle, and what are the die zones?

A small die half-angle reduces redundant deformation and friction work, optimizing draw force (an optimum angle balances friction, which falls with larger angle, against redundant work, which rises). Die zones: entrance (bell), approach (cone) where reduction occurs, bearing (land) sizing the wire, and back relief.

Classify sheet metal forming into its three basic operations.

(1) Shearing operations: blanking, punching/piercing, trimming (separation by shear). (2) Bending: V-bending, edge bending, plastic deformation along a straight axis. (3) Drawing/stretching: deep drawing of cups and stretch forming. Spinning and embossing are additional forming methods.

Define springback in bending and how it is compensated.

Springback is the elastic recovery after bending that reduces the bend angle/increases radius once load is released. The springback ratio $K_s = \dfrac{R_i}{R_f}$ (or angle ratio) decreases with higher yield strength and lower modulus. Compensation: overbending, bottoming/coining, or stretch bending.

Distinguish blanking from punching (piercing) in sheet shearing.

Both are shearing operations cutting a closed contour. In blanking the slug removed IS the desired part (the part is the piece punched out). In punching/piercing the punched-out slug is scrap and the remaining sheet with the hole is the desired part.

Give the formula for the punch force in shearing sheet metal.

$$F = \tau \cdot t \cdot L$$ where $\tau$ is the shear strength of the material, $t$ is the sheet thickness, and $L$ is the total length of the cut (perimeter). Often approximated as $F \approx 0.7\,\sigma_{UTS}\,t\,L$. Angular (shear/bevel) on the punch reduces peak force.

What is the limiting draw ratio (LDR) in deep drawing?

LDR is the maximum ratio of blank diameter to punch (cup) diameter that can be drawn in a single operation without tearing: $\text{LDR} = \dfrac{D_{blank,\,max}}{d_{punch}}$. Practically about $1.8$-$2.2$; it increases with the normal anisotropy ratio (plastic strain ratio $r$).

List common defects in deep drawing and sheet forming.

Wrinkling (in flange or wall from compressive instability, insufficient blank-holder force), tearing/fracture (excess tensile thinning, too-small die radius), earing (from planar anisotropy producing uneven cup rim), orange peel (coarse grain surface roughening), and stretcher strains/Luders bands (from yield-point elongation).

Define earing in deep drawing and its cause.

Earing is the formation of a wavy (eared) top edge on a drawn cup due to planar anisotropy of the sheet, quantified by $\Delta r$ (variation of the plastic strain ratio $r$ with orientation). High $|\Delta r|$ produces ears; ideal deep-drawing sheet has high mean $\bar{r}$ and low $|\Delta r|$.

Explain the plastic strain ratio (Lankford coefficient) $r$ and its significance in formability.

$r = \dfrac{\varepsilon_w}{\varepsilon_t}$ = ratio of true width strain to true thickness strain in a tensile test. High $\bar{r}$ (normal anisotropy) resists thinning, improving deep drawability (higher LDR). $\bar{r} = \frac{r_0 + 2r_{45} + r_{90}}{4}$ and $\Delta r = \frac{r_0 - 2r_{45} + r_{90}}{2}$ measures planar anisotropy (earing).

What is the principal difference between soldering, brazing, and welding by joining temperature and base-metal melting?

Welding melts the base metals (with/without filler), forming a fusion bond. Brazing and soldering join without melting the base metal, using a lower-melting filler drawn by capillary action. The distinction between them is filler melting temperature: brazing filler melts above $450^{\circ}\text{C}$, soldering filler melts below $450^{\circ}\text{C}$.

State the principle of soldering, common solder, and the role of flux.

Soldering joins metals using a filler (solder) melting below $450^{\circ}\text{C}$ that wets the surfaces and is drawn into the joint by capillary action, bonding without melting the base metal. Traditional solder is Pb-Sn (e.g. $60\%\,\ce{Sn}$-$40\%\,\ce{Pb}$); flux (e.g. rosin, zinc chloride) removes oxides and promotes wetting. Bond strength is relatively low.

State the principle of brazing and key requirements for a sound brazed joint.

Brazing joins metals with a filler melting above $450^{\circ}\text{C}$ but below the base-metal solidus; molten filler is drawn into a close-fitting joint by capillary action. Requirements: good wetting, small controlled clearance (typically $0.025$-$0.2\,\text{mm}$) for capillarity, clean oxide-free surfaces (flux or inert/vacuum atmosphere), and proper joint design (lap joints preferred).

How does wettability (contact angle) govern soldering/brazing capillary flow?

Good wetting requires a small contact angle $\theta$ (ideally $\theta \to 0^{\circ}$, $\theta < 90^{\circ}$ for wetting). Wetting is governed by Young's equation $\gamma_{SV} = \gamma_{SL} + \gamma_{LV}\cos\theta$. Flux lowers $\gamma_{SL}$ and removes oxides, improving spreading and capillary filling of the joint gap.

Classify welding processes into the two broad categories with examples.

(1) Fusion welding: base metal is melted, e.g. arc welding (SMAW, GMAW/MIG, GTAW/TIG, SAW), gas welding (oxy-acetylene), resistance welding (spot, seam), and high-energy beam (laser, electron-beam). (2) Solid-state welding: no melting, e.g. friction (FSW), forge, diffusion, ultrasonic, and explosive welding.

Define the weld zones: fusion zone, heat-affected zone (HAZ), and base metal.

Fusion zone (FZ): region that melted and resolidified, having a cast/dendritic structure. Heat-affected zone (HAZ): unmelted base metal whose microstructure/properties changed due to the weld thermal cycle (grain growth, phase transformations). Base metal: unaffected parent material beyond the HAZ. The fusion boundary separates FZ from HAZ.

What governs the weld thermal cycle and why is cooling rate critical in steel welding metallurgy?

Heat input per unit length $H = \dfrac{\eta\,V\,I}{v}$ (efficiency $\eta$, arc voltage $V$, current $I$, travel speed $v$) controls peak temperature and cooling rate. In steels, fast cooling in the HAZ can transform austenite to hard, brittle martensite, raising the risk of cracking; preheat and controlled heat input slow cooling to avoid it.

What causes hydrogen-induced (cold) cracking in welded steels and how is it prevented?

Cold cracking needs three factors together: (1) diffusible hydrogen (from moisture/contaminants), (2) a susceptible hard microstructure (martensite in the HAZ), and (3) tensile residual stress, at temperatures below ~$200^{\circ}\text{C}$. Prevention: low-hydrogen electrodes (baked/dry), preheat and post-heat, controlled heat input, and reducing restraint. Higher carbon equivalent increases susceptibility.

Define carbon equivalent (CE) and its role in steel weldability.

Carbon equivalent expresses the combined hardenability effect of alloying elements, e.g. $$CE = \%C + \frac{\%Mn}{6} + \frac{\%Cr+\%Mo+\%V}{5} + \frac{\%Ni+\%Cu}{15}.$$ Higher CE means greater hardenability and higher cold-cracking risk. $CE > 0.4$-$0.45$ generally requires preheat and low-hydrogen practice for good weldability.

List common defects in welded joints (general).

Porosity (gas entrapment), slag inclusions, incomplete fusion (lack of fusion), incomplete/inadequate penetration, undercut, overlap, cracks (hot/solidification and cold/hydrogen), spatter, distortion, and residual stresses. Each links to specific causes such as moisture, contamination, low heat input, or poor technique.

What is the main weldability problem of aluminium alloys and its causes?

Aluminium alloys are prone to (1) gas (hydrogen) porosity and (2) hot (solidification) cracking. Hydrogen solubility drops sharply on solidification, releasing porosity; the tenacious refractory $\ce{Al2O3}$ oxide (melting ~$2050^{\circ}\text{C}$) hinders fusion; high thermal conductivity and expansion plus wide freezing range promote hot tearing and distortion.

Why does hydrogen cause porosity in aluminium welds but cold cracking in steel welds?

In aluminium, hydrogen has high solubility in the liquid but very low solubility in the solid, so on solidification it is rejected to form rounded gas porosity (no hard phase to crack). In steel, atomic hydrogen diffuses and concentrates at the hard martensitic HAZ, embrittling it and causing delayed cold (hydrogen) cracking under residual stress.

What is hot (solidification) cracking in welds and how does composition affect it?

Hot cracking occurs near the solidus when low-melting eutectic films (e.g. from S, P, or in Al certain Mg/Si/Cu levels) remain liquid in the interdendritic regions while solidification shrinkage and thermal stress pull the grains apart. It is worsened by wide freezing range and high restraint; controlled by filler composition, lower heat input, and crater fill.

Why is filler-metal selection important when welding aluminium alloys?

Some Al alloys are 'crack-sensitive' at compositions giving wide freezing range and low-melting eutectics. Choosing a filler (e.g. 4xxx Al-Si or 5xxx Al-Mg) shifts weld-metal composition away from the crack-sensitive range and reduces hot-cracking susceptibility, while also improving fluidity and matching strength/corrosion requirements.

List the principal methods of producing metal powders for powder metallurgy.

Atomization (water/gas/centrifugal) of molten metal into droplets, chemical reduction of oxides (e.g. sponge iron), electrolytic deposition (high-purity powders), mechanical comminution (milling/crushing, mechanical alloying), and thermal decomposition of carbonyls (e.g. Ni, Fe carbonyl). Atomization is the most widely used industrial route.

How does atomization produce powder and how do gas vs water atomization differ in particle shape?

Atomization breaks a molten metal stream into droplets using a high-pressure jet of gas or water, which solidify into powder. Water atomization: faster cooling, irregular particle shapes, slightly oxidized surfaces, lower cost. Gas atomization: spherical particles, cleaner surfaces, better flow, used for high-quality/reactive powders.

Define key powder characteristics important to compaction and sintering.

Particle size and distribution, particle shape (affects packing and interlocking), apparent density and tap density, flowability (Hall flow), compressibility, green strength, and specific surface area. These govern die filling, packing density, the green density achievable, and final sintered properties.

What is compaction in powder metallurgy and what is 'green strength'?

Compaction presses loose powder in a die under high pressure (typically $100$-$900\,\text{MPa}$) to form a 'green compact' of the required shape with enough handling strength. Green strength is the mechanical strength of the as-pressed (unsintered) compact, derived from mechanical interlocking and cold welding of particles.

Why does density vary within a single-action die-pressed compact, and how is it reduced?

Wall friction and inter-particle friction cause pressure (and thus density) to fall with distance from the moving punch, giving a density gradient (lowest density far from the punch). It is reduced by double-action pressing (punches from both ends), lubricants, lower height-to-diameter ratios, and isostatic pressing for uniform density.

Compare cold isostatic pressing (CIP) and hot isostatic pressing (HIP) for powder compaction.

CIP applies uniform fluid pressure to a powder in a flexible mould at room temperature, giving uniform green density and complex shapes, then sintered separately. HIP applies high pressure via an inert gas at elevated temperature simultaneously, achieving near-full density (eliminating porosity) in one step for high-performance parts; HIP is costlier and slower.

What is sintering and the primary driving force and bonding mechanism?

Sintering heats the green compact below the melting point (typically $0.7$-$0.9\,T_m$) so particles bond by atomic diffusion, forming necks that grow, reducing porosity and increasing strength/density. The driving force is the reduction of total surface (interfacial) free energy; mass transport occurs mainly by surface, grain-boundary, and volume diffusion.