Materials and Manufacturing The selection of the material is an important step in the design of a machine element. Fracture Behavior Ductile material – Significant plastic deformation and energy absorption (toughness) before fracture. Characteristic feature of ductile material - necking Brittle material – Little plastic deformation or energy absorption before fracture. Characteristic feature of brittle materials – fracture surface perpendicular to the stress. Steel Before and after fracture In general: •ductile materials are limited by their shear strengths •brittle materials (ductility< 5%) are limited by their tensile strengths The Concept of Stress Uniaxial tensile stress: A force F is applied perpendicular to the area (A). Before the application of the force, the cross section area was AO F Engineering stress or nominal stress: σ = Force divided by the original area. A0 − ll Δl Engineering Strain or Nominal Strain: Change ε = 0 = of length divided by the original length l l00 True stress and strain F Original cross sectional area! Recall: Engineering Stress = σ = Ao If no net volume change (i.e. Ai li = Ao lo) A = instantaneous area F i l = instantaneous length True Stress = σ T = i Ai

li True Strain = εT = ln lo

Notice that past maximum stress point, σ decreases. Does this mean that the material is becoming weaker? Necking leads to smaller cross sectional area!

Only true at the onset of σ T = σ + ε )1( necking T += εε)1ln( Strain Hardening Parameter (n)

n T = KεσT

Strain hardening parameter 0.02

Compression Tests A ductile sample will not fracture under compression. Brittle materials will fracture when compressed. A material that has different tensile and compressive strength are called uneven materials. Torsion Test • Ductile material twist • Brittle material fractures TL Grφ φ = τ MAX = PGI L Stress-Strain Behavior of Ceramics Flexural Strength: the stress at fracture under the bending tests. It’s also called Modulus of rupture, fracture strength, or the bend strength 3-point Bending tests

3 LF σ = f fs 2bd 2 LF σ = f fs πR3 Impact Test (testing fracture characteristics under high strain rates) Notched-bar impact tests are Izod Charpy used to measure the impact energy (energy required to fracture a test piece under impact load), also called notch toughness. It determines the tendency of the material to behave in a brittle manner. Two classes of specimens have been standardized for notched- h impact testing, Charpy (mainly h’ in the US) and Izod (mainly in the UK). Energy ~ h - h’ Ductile-to-brittle transition As temperature decreases a ductile material can become brittle - ductile- to-brittle transition. FCC metals show high impact energy values that do not change appreciably with changes in temperature.

BCC metals, polymers and ceramic materials show a transition temperature, below which the material behaves in a brittle manner. The transition temperature varies over a wide range of temperatures. For metals and polymers is between -130 to 93oC. For ceramics is over 530oC. Charpy Test

High Carbon Steel Stainless Steel DESIGN STRATEGY: STAY ABOVE THE DBTT!

• Pre-WWII: The Titanic • WWII: Liberty ships

Reprinted w/ permission from R.W. Hertzberg, Reprinted w/ permission from R.W. Hertzberg, "Deformation and "Deformation and Fracture Mechanics of Engineering Fracture Mechanics of Engineering Materials", (4th ed.) Fig. Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and 7.1(b), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source: Sons, Inc., 1996. (Orig. source: Dr. Robert D. Ballard, The Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Discovery of the Titanic.) Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.) • Problem: Used a type of steel with a DBTT ~ Room temp.

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Hardness Hardness: a measure of a material’s resistance to localized plastic deformation (eg. Small dent or scratch).

Correlation between Hardness and Tensile Strength

TS (MPa) = 3.45xHB TS (psi) = 500xHB Note: No method of measuring hardness uniquely indicates any other single mechanical property. Some hardness tests seem to be more closely associated with tensile strength, others with ductility, etc. Fatigue :Failure under fluctuating/cyclic stress Under fluctuating / cyclic stresses, failure can occur at loads considerably lower than tensile or yield strengths of material under a static load: Fatigue Estimated to cause 90% of all failures of metallic structures (bridges, aircraft, machine components, etc.). Fatigue failure is brittle-like (relatively little plastic deformation) - even in normally ductile materials. Thus sudden and catastrophic!

Fatigue failure proceeds in three distinct stages: crack initiation in the areas of stress concentration (near stress raisers), incremental crack propagation, final catastrophic failure. Cyclic stresses characterized by maximum, minimum and mean stress, the range of stress, the stress amplitude, and the stress ratio. Fatigue limit occurs for some materials (some Fe and Ti alloys). S—N curve becomes horizontal at large N. Stress amplitude below which the material never fails, no matter how large the number of cycles is. It has values between 0.4 to 0.25 the TS of the material In most alloys (ex. FCC), S decreases continuously with N. Fatigue strength: stress at which fracture occurs after specified number of cycles (e.g. 107). Fatigue life: Number of cycles to fail at specified stress level.

Fabrication of Metals • Forming Operations – Forging – Rolling – Extrusion – Drawing • Casting • Powder Metallurgy • Welding Classification of Metal Fabrication Techniques Temperature in Metalworking

• Forming Operations ⎯ are those in which the shape of a metal piece is changed by plastic deformation • Forming processes are commonly classified into hot- working and cold-working operations. Hot Working Hot-working is defined as deformation under conditions of temperature and strain rate such that recrystallization takes place simultaneously with the deformation. Relatively high T – Recrystallization leads to very large deformation – Hot-working processes such as rolling, extrusion, or forging typically are used in the first step of converting a cast ingot into a wrought product – Deformation energy requirements are less than for cold work – Most metals experience some surface oxidation, which results in material loss and a poor final surface finish. Cold Working

The deformation is carried out at low temperatures, where recovery / recrystallization do not take place. Relatively low T • Cold-working operations are usually carried out in several steps, with intermediate annealing operations introduced to soften the cold- worked metal and restore the ductility • A higher quality surface finish and closer dimensional control of the finished piece • Cold-working of a metal results in an increase in strength or hardness and a decrease in ductility. Cold Working Cold working: plastic deformation of a metal or alloy at a temperature where dislocations are created faster than they are annihilated ⎛ 0 − AA d ⎞ %CW = ⎜ ⎟ ×100 ⎝ A0 ⎠

Intermediate Annealing during Cold Working

• When cold-working is excessive, the metal will fracture before reaching the desired size and shape. In order to avoid such difficulties, cold-working operations are usually carried out in several steps, with intermediate annealing operations introduced to soften the cold-worked metal and restore the ductility • This sequence of repeated cold-working and annealing is frequently called the cold-work-anneal cycle Casting • Casting: a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage. • Casting techniques are used when 1. The finished shape is so large or complicated that any other method would be impractical 2. A particular alloy is so low in ductility that forming by either hot or cold working would be difficult 3. In comparison to other fabrication processes, casting is the most economical. Classification of Casting • Sand Casting • Die Casting • Investment Casting (lost-wax casting) • Continuous Casting Sand, Investment, and Lost Foam Casting • Use gravity to fill the mold • Mold is destroyed to remove casting • Metal flow is slow • Walls are much thicker than in die casting • Cycle time is longer than die casting because of inability of mold material to remove heat Aluminum Piston Mold

Aluminum piston for an internal Schematic illustration of the combustion engine. (a) As cast; permanent mold used to produce (b) after machining. aluminum pistons, showing the position of four cooling channels. Investment Casting

• Create Wax Pattern • Assemble Wax Tree • Coat with Ceramic • Melt out wax • Pour in molten metal • Break off ceramic Die Casting • Liquid metal injected into reusable steel mold, or die, very quickly with high pressures • Reusable steel tooling and injection of liquid metal with high pressures differentiates die casting from other metal casting processes Casting of Single Crystal Components

Jet engine turbine blades Forging • Forging is the working of metal into a useful shape by hammering/pressing. Forging is usually carried out hot. • Forged articles have outstanding grain structures and the best combination of mechanical properties. • Wrenches, and automotive crankshafts and piston connecting rods are typical articles formed by forging

Stages in the forging of a crankshaft A macroetched section through a forging indicates that the grain flow follows the contour of the component, which often maximizes strength in the direction of greatest operating stress. Grain (Metallurgy, by B. J. Moniz, American Flow Technical Publishers, Inc., 1994)

Open Die Forging Closed Die Forging Rolling Rolling is the most extensively used metal forming process and its share is roughly 90% The material to be rolled is drawn by means of friction into the two revolving roll gap The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area The geometry of the product depend on the contour of the roll gap.

Extrusion A plastic deformation process in which metal is forced under pressure to flow through a single, or series of dies until the desired shape is produced. (a) Direct (b) indirect (c) hydrostatic (d) impact Drawing Deep Drawing • Blank is allowed to draw into the die, and thickness is normally unchanged. • Limiting Drawing Ratio (LDR)

LDR=d0max/ Dp • Constraint of blank- holder gives improved process control and quality Stamping Failure Diagnosis using Grid Marks Hydroforming

(a) Before (b) Pressure Die Closing Die Open (c) 1st Pressure Stage Die Closing (d) 2nd Pressure Stage Die Closed Automotive Structural Part Stamped Dodge Dakota Hydroformed Dodge Dakota Radiator Enclosure Radiator Enclosure Stamped Radiator Closure Hydroformed Radiator Closure

17 components 8 components ( -9 ) 36.4 Ibs/16.5 kg 25.4 lbs/11.5 kg (-11 Ibs, -30%) Spin forming Powder Metallurgy • A fabrication technique that involves the compaction of powdered metal, followed by a heat treatment to produce a more dense piece. • Competitive with processes such as casting, forging, and machining. • Used when (a) melting point is too high (W, Mo); (b) there is a reaction at melting (Zr); (c) material too hard to machine; (d) very large quantities are required. Near 70% of the P/M part production is for automotive applications. • Good dimensional accuracy. Controllable porosity. • Size range from tiny balls for ball-point pens to parts weighing 100 lb. Most are around 5 lb. pressure

heat area contact densify

point contact densification at low T by diffusion at higher T Examples of typical parts made by powder-metallurgy processes.

Main-bearing powder metal caps for 3.8 and 3.1 liter General Motors automotive engines.

Upper trip lever for a commercial irrigation sprinkler, made by P/M. This part is made of unleaded brass alloy; it replaces a die-cast part, with a 60% savings. P/M Process

RAW OPTIONAL FINISHED MIXING FORMING SINTERING MATERIALS OPERATIONS PRODUCTS

Hot Compaction Optional Manufacuring Steps Isostatic Repressing Extrusion Sintering Coning Elemental or Die Compacting Sizing Alloy Metal Spraying Atmosphere Repressing Powders Pressureless- Vacuum Forging Sintering High Temperature Rerolling Metal Infiltration Finished Mixing Optional Products Finishing Steps Cold Compaction Machining Heat Treating Additives Die Compacting Steam Treating (graphite, die, Isostatic Plastic Impregnation lubricants) Rolling Plating Injection Molding Tumbling Slip Casting Oil Impregnation Shot Peening