Advances in steel and Al alloy materials High strength (HSS) & Advanced high strength steel (AHSS)

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1 Advances in steel and Al alloy materials High strength (HSS) & Advanced high strength steel (AHSS) Low strength steel HSS UHSS Development of new steel grade materials that can withstand more strength during in situ performance Types: Metallurgical designation Low strength steel: Interstitial Free steel (IF), Mild steel (Mild) High strength steels (HSS): Bake Hardenable (BH) steel, High Strength Low Alloy (HSLA), High strength Interstitial Free (IF-HS) Based on Strength values High Strength Steels (HSS) => Yield strengths from MPa and tensile strengths from MPa; Ultra-High-Strength Steels (UHSS) steels =>Yield strengths greater than 550 MPa and tensile strengths greater than 700 MPa. Ultra High Strength Steel (UHSS): Dual Phase (DP), Transformation Induced Plasticity (TRIP) steel, Martensitic steels (MS), Complex Phase (CP) steel

2 The main difference between HSS and AHSS is their microstructure; HSS are single phase ferritic steels, while AHSS are primarily multi-phase steels, which contain ferrite, martensite, bainite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Examples of Steel Grade Properties from ULSAB-AVC

3 Dual Phase (DP) steel Dual phase steel contains two phases => Ferrite matrix & hard martensite second phase (5-15%) Increasing the volume of hard second phases increases the strength as a whole DP steels are produced by (1) controlled cooling from the austenite phase to transform some austenite to ferrite, & (2) a rapid cooling transforms the remaining austenite to martensite. The schematic representation of DP steel microstructure is shown in the figure. This contains soft ferrite & islands of hard martensite. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels undergoes deformation, strain is concentrated in the lower-strength (weaker) ferrite phase. But this is surrounded by islands of martensite (stronger), that resists the straining of ferrite phase creating the unique high workhardening rate exhibited by these steels. Similar to welded blanks deformation behavior, either base material or weld region is soft in this case. Schematic microstructure of DP steel

4 The high work hardening rate plus good elongation give DP steels much higher ultimate tensile strengths than conventional steels of similar yield strength. In the figure shown, the DP steel exhibits higher initial work hardening rate, higher ultimate tensile strength, and lower YS/TS ratio than the similar yield strength HSLA steel. DP Vs HSLA

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6 Transformation-Induced Plasticity (TRIP) Steel TRIP steels contain retained austenite (5-15%) embedded in a primary matrix of ferrite. In addition to this, hard phases such as martensite and bainite are present in varying amounts. TRIP steels are isothermally hold at an intermediate temperature, which produces some bainite. The higher silicon and carbon content of TRIP steels also result in significant volume fractions of retained austenite in the final microstructure. During deformation, the distribution of hard second phases in soft ferrite creates a high work hardening rate, as seen in DP steels. However, in TRIP steels the retained austenite also progressively transforms to martensite with increasing strain, thereby increasing the work hardening rate at higher strain levels.

7 This is shown in the stress-strain graph, where the TRIP steel has a lower initial work hardening rate than the DP steel, but the hardening rate persists at higher strains where work hardening of the DP begins to diminish. Steel grades with same yield strength, but more elongation in TRIP steel TRIP steel (350/600) with a greater total elongation than DP 350/600 and HSLA 350/450 The work hardening rates of TRIP steels are substantially higher than for HSS, providing significant stretch forming. The high work hardening rate persists to higher strains in TRIP steels, providing a slight advantage over DP steels in the most severe stretch forming applications. This basically means that TRIP steel should have better formability when compared to DP steels and HSS steel.

8 The strain level at which retained austenite begins to transform to martensite can be designed by adjusting the carbon content. At lower carbon levels, the retained austenite begins to transform almost immediately upon deformation, increasing the work hardening rate and formability during the stamping process. At higher carbon contents, the retained austenite is more stable and begins to transform only at strain levels beyond those produced during forming. At these carbon levels the retained austenite persists into the final part. It transforms to martensite during subsequent deformation, such as a crash event. Steel grades with app. same yield strength, but more elongation in TRIP steel

9 Martensite steel The MS steels are characterized by a martensitic matrix containing small amounts of ferrite and/or bainite. These steels are created by transforming austenite during hot rolling to entirely martensite by quenching MS steels show the highest tensile strength values among the steel seen so for. MS steels provide the highest strengths, up to 1700 MPa ultimate tensile strength. MS steels are often subjected to post-quench tempering to improve ductility, and can provide adequate formability even at extremely high strengths

10 Conventional low and high strength steels Mild steels: Mild steels have an essentially ferritic microstructure. Drawing Quality (DQ) and Aluminium Killed (AKDQ) steels are examples and often serve as a reference because of their widespread application and production volume. Interstitial-Free (IF) steels (Low strength and high strength): IF steels have ultralow carbon levels designed for low yield strengths and high work hardening exponents. These steels are designed to have more formability or stretchability than Mild steels. Some grades of IF steels are strengthened by a combination of elements for solid solution, precipitation of carbides and/or nitrides, and grain refinement. The strength of IF steel (high strength) can be improved by adding phosphorous. The higher strength grades of IF steel type are widely used for both structural and closure applications. High-strength low-alloy (HSLA) steels: This group of steels are strengthened primarily by micro-alloying elements contributing to fine carbide precipitation and grain-size refinement.

11 Bake Hardenable steel Any steel that exhibits a capacity for a significant increase in strength through the combination of work hardening during part formation and strain aging during a subsequent thermal cycle, such as a paint-baking operation can be said as Bake Hardenable steel The forming operation imparts some degree of strain hardening which increases yield strength in bake hardening steels. The paint baking cycle, typically 175 C (350 F) for 20 to 30 minutes, provides another increase in yield strength due to moderate carbon strain aging. Any steel with adequate carbon and/or nitrogen in solution to cause strain-aging may be classified as bake-hardenable. In general, bake-hardenable steels are aluminum-killed steels with an adequate amount of aluminum to combine with the nitrogen as Aluminum Nitride. A combination of relatively low yield strength prior to manufacturing and a high inpart strength after forming and paint baking makes bake-hardenable steels ideal for applications where dent and palm printing resistance is important in applications such as hoods, doors, fenders, and deck lids.

12 When using bake-hardenable steel, the amount of strain introduced during the forming process will largely dictate the final strength of the part. Since automotive parts, have a wide array of designs, there will be a corresponding disparity in the amount of strain introduced in these varying geometries. As a consequence, when using bake-hardenable steel, it is important to design an adequate amount of strain into a part in order to fully utilize this material s dent resistant characteristics Increase in yield strength during forming and baking of bake-hardenable steels R. Ganesh Narayanan, HSS stamping IITG design manual, Auto/steel partnership

13 Chemical compositions of various types of steel

14 Mechanical properties of various types of steel

15 Aluminium alloy materials for automotive and aerospace applications Typical properties for aluminium Property Atomic Number Atomic Weight (g/mol) Valency Crystal Structure Melting Point ( C) Boiling Point ( C) Mean Specific Heat (0-100 C) (cal/g. C) Thermal Conductivity (0-100 C) (cal/cms. C) Co-Efficient of Linear Expansion (0-100 C) (x10-6 / C) Electrical Resistivity at 20 C (µcm) Density (g/cm 3 ) Modulus of Elasticity (GPa) Poissons Ratio Value Face centred cubic

16 Following are several examples of aluminium intensive vehicles with employing aluminium body components: 1. Audi A8 is an aluminium intensive space frame vehicle that reduces the body weight by 40%. The 385 kg aluminium components comprise 125 kg sheet products, 70 kg extrusions, 150 kg castings, and 40 kg other aluminium forms. 2. Ford AIV has a stamped aluminium body structure. The body and exterior panels are 200 kg lighter than the conventional steel model with 145 kg in body structure and 53 kg in closure panels. The total usage of aluminium is 270 kg and the total weight reduction is 320 kg. 3. Honda NSX has also a stamped body structure and exterior panels with a weight of 210 kg of aluminium, about 100 kg of aluminium chassis components and 130 kg of other power train and drive train components Materials Science and Engineering A, 280 (2000), 37 49

17 The main alloying elements are copper, zinc, magnesium, silicon, manganese and lithium. Small additions of chromium, titanium, zirconium, lead, bismuth and nickel are also made and iron is invariably present in small quantities. Designations (US) for alloyed wrought and cast aluminium alloys Major Alloying Element None (99%+ Aluminium) Copper Manganese Silicon Magnesium Magnesium + Silicon Zinc Lithium Unused Wrought 1XXX 2XXX 3XXX 4XXX 5XXX 6XXX 7XXX 8XXX Cast 1XXX0 2XXX0 4XXX0 5XXX0 6XXX0 7XXX0 9XXX0 5XXX, 6XXX are automotive Al alloys; 2XXX, 7XXX are used in aerospace sector

18 Some common aluminium alloys, their characteristics and applications Alloy 1050/ A 3103/ /5052 Characteristics Good formability, weldability and corrosion resistance Heat treatable, High strength, Non-weldable, Poor corrosion resistance Non-heat treatable, Medium strength work hardening alloy. Good weldability, formability and corrosion resistance. Non-heat treatable, Medium strength work hardening alloy. Good weldability, formability and corrosion resistance Application Food and chemical industry Airframes Vehicle panelling, structures exposed to marine atmosphere, mine cages Vehicle panelling, structures exposed to marine atmosphere, mine cages

19 5454* 5083*/ * 6061*/6082* Non-heat treatable, Used at temperatures from C, Good weldability and corrosion resistance. Non-heat treatable, Good weldability and corrosion resistance, Very resistant to sea water, industrial atmospheres. A superior alloy for cryogenic use (in annealed condition) Heat treatable, Medium strength alloy, Good weldability and corrosion resistance, Used for intricate profiles. Heat treatable, Medium strength alloy, Good weldability and corrosion resistance. Pressure vessels and road tankers, Transport of ammonium nitrate, petroleum, Chemical plants. Pressure vessels and road transport applications below 65 C, Ship building structure in general. Architectural extrusions (internal and external), window frames, irrigation pipes Stressed structural members, bridges, cranes, roof trusses, beer barrels. * Most commonly used Al alloys

20 Alloying element Iron Mn Si Cu Mg Zn Cr Ti Pb/Bi Effects of alloying elements Effects Naturally occurs as impurity in aluminium ores; small percentage increases the strength and hardness of some alloys and reduce hot-cracking tendencies in casting Improve castability; improves ductility and impact strength Increases fluidity in casting and welding alloys and reduces solidification and hot cracking tendencies; additions more than 13% make the alloy difficult to machine; improves corrosion resistance Increases strength up to 12%, higher concentration cause brittleness; improves elevated temperature properties and machinability Improves strength by solid solution strengthening and alloys over 3% will precipitation harden; Al-Mg alloys are difficult to cast Low castability; high Zn alloys are prone to hot cracking and high shrinkage; Zn promotes very high strength Acts as grain refiner in small quantities (<0.35%) Occurs as impurity in Al ores; but if intentionally added it will act as grain refiner Improves machinability in some alloys Ref: Engg. materials properties & selection; K.G. Budinski, M.K. Budinski

21 Effects of alloying elements Zr Li Grain refiner in aerospace alloys Added to aerospace alloys to reduce weight; These alloys need a protective atmosphere when being cast Ref: Engg. materials properties & selection; K.G. Budinski, M.K. Budinski

22 Like steel grade materials, Al alloys exhibit formability, weldability & corrosion characteristics AA 6061 A - Al-Cu-Mg; B Al-Mg; C Al-Mn JMPT, 112, 2001, Materials & Design, 28 (2007), Pulsed current tungsten inert gas welding AA 6061

23 Hydroforming of Al tubes High temperature deformation of Al alloy(aa3003) JMPT, , (2007) International Journal of Plasticity 22 (2006) FLC at varied temperatures