Materials Substitution Case Study: Conversion of the Ford F-150 body from Steel to Aluminum

Properties and Production ------Christopher R. Owen Purdue University

1 Agenda

1. Customer Requirements 2. Strengthening Mechanisms a) Using the paint bake cycle to achieve final properties 3. Production Flowpath a) Steel b) Aluminum 4. Electrical Resistivity 5. Manufacturability (formability, fewer parts)

2 Customer Requirements for Autobody Panels

• Mechanical Properties – Low yield strength before paint bake process for good formability – High strength after paint bake process for good dent resistance • Good Formability – Stretching and drawing – Hemming (similar to bending) • Surface Appearance – No orange peel or other surface roughening • Good Resistance to Corrosion – Steel • No evidence of rusting – Aluminum • Filiform after painting • Intergranular (IG) corrosion of painted sheet

3 Strengthening Mechanisms

• Applies to both steel and aluminum 1. Grain Size – the finer the grain size, the stronger the metal 2. Work Hardening– cold working increases the dislocation density which increases the yield strength (while simultaneously decreasing the elongation). 3. Precipitation Hardening – formation of a separate phase from a supersaturated solid solution (through thermal treatments). The thermal process that precipitates the phase is referred to as “aging”. • Both steel and aluminum utilize precipitation hardening (during the paint bake process) for structural automotive components.

4 Steel: Types of Low Carbon Steels

210 MPa = typical low carbon steel with ferrite- pearlite microstructure produced by conventional hot rolling with nominal Mn & Si (“Mild”)

Legend • HSS: High Strength Steels • AHSS: Advanced High Strength Steels

• Mild: incumbent low carbon steel product • IF: Interstitial Free • IS: Isotropic Steels • BH: Bake Hardening • CMn: Carbon Mn • HSLA: High Strength Low Alloy • DP: Dual Phase • TRIP: Transformation Induced Plasticity • MART: Martensite

Bake Hardening steels show moderate strength prior to the bake hardening cycle Steel Bake Hardenable Alloys for Autobody Sheet Alloy Composition

Door in 180 BH Door in 260 BH Composition (ArcelorMittal)

6 Bake Hardening Steels for Autobody Sheet

• Steels end up with simple ferritic microstructure but – More complicated chemistry than mild steels • Carbon stays in solution until paint bake cycle • “Paint bake cycle” is Typically 20 min @ 175⁰C • Strengthening Mechanism: – Stamping introduces strain into the part – Causes strengthening due to precipitation hardening, called “Strain aging” – A typical strain aging result is shown on the next slide

7 Strain aged tensile strength

Unaged tensile strength

Stress

Change in total elongation

Pre-strained length Strain Process is similar in Aluminum Alloys Some typical alloys:

• 1000 series, 99% pure Al • 2000 series, alloyed with Cu • 3000 series, alloyed with Mn • 4000 series, alloyed with Si • 5000 series, alloyed with Mg • 6000 series, alloyed with Mg and Si—used in autobody sheet • 7000 series, alloyed with Zn • 8000 series, contain other alloying elements

9 Aluminum: 6xxx Alloys for Autobody Sheet

• Advantages of 6xxx alloys – 6xxx (Al-Mg-Si) developed moderate strength and much better corrosion resistance compared to 2xxx (Al-Cu-Mg) or 7xxx (Al-Zn-Mg alloys. – 6xxx were easier to process and are less expensive than 2xxx or 7xxx. • Alloy 6082 is the more commonly used 6xxx alloy in Europe while alloy 6061 is still predominant in the US. • A variety of other aluminum alloys are used by different manufacturers in autobody and many other applications

10 Aluminum: 6xxx Alloys for Autobody Sheet Alloy development to utilize paint bake cycle

• Initially, the Audi A8 required a separate thermal step to achieve desired properties. • Further refinement of alloy composition in 6xxx alloys allowed the paint bake cycle to provide the enhanced properties needed and thus to eliminate the separate thermal operation. • The paint bake cycle process was used by Audi as early as 2002; in common use today. • The paint bake cycle increases tensile strength by precipitation hardening.

11 Aluminum: 6xxx Alloys for Autobody Sheet Aging time-temperature response of 6022 alloy: No pre-strain vs. 2% pre-strain

Similar results as for steel; note 2% strain hardens faster and works well for strain aging during the paint bake cycle

Production Process for Sheet Products Comparing Aluminum and Steel • Aluminum • Steel – Refining – Steelmaking – Cast Ingot – Cast Ingot – Homogenize – Reheat – Hot Roll – Hot Roll – Anneal – Pickle – Cold Roll – Cold Roll – Solution Heat Treat – Heat Treat – Trim to Size – Trim to size – Ship to Customer – Ship to Customer

13 Solution Heat Treat Affect on Grain Size Careful control of temperature needed to control surface texture

Deformed grains in Typical cold-rolled 6xxx microstructure

Recrystallized grains after heat treatment

These pictures are of aluminum, but the same phenomenon occurs with steel. Recrystallized grain size is important because too large a grain size can be seen by the eye and make the surface seem rough—called “Orange Peer” effect.

14 Material Properties in Autobody Panel Production

• The sheet is delivered and forming is done in the as-received temper condition. – It is important that the sheet have good ductility and is not too strong at this point. • After forming, the panels are assembled, painted and then given a baking treatment to cure the paint. • Bake cycle allows precipitation hardening to occur to bring the part to the desired strength. – It is important that the sheet ages quickly (within 20 minutes) to a high strength level during the bake.

15 Production Summary

• Both steel and aluminum – Utilize precipitation hardening to provide strength – Have develop compositions that provide adequate strength at a lower cost – Have similar flowpaths – Utilize the paint bake cycle as the thermal process as the driving force for precipitation hardening

16 What about Manufacturability: The Ability to Manufacture and to Properly Form a Product?

What is the difference between Aluminum and Steel in the manufacturing process? • Principal property difference is in modulus of elasticity o Steel 200 GPa o Aluminum 74 GPa

17 Manufacturability - Formability

• Elastic modulus of Steels is 3x that of Aluminum • This means that aluminum parts will require ~3x more strain to reach a permanent plastic deformation. When released, the part will exhibit ~3x elastic “springback” • The difference in modulus of elasticity can be designed into dies, but manufacturers will notice a considerable difference in behavior between the two metals.

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Manufacturability - Formability

• The second and third tier suppliers have been producing parts from steel for years. • Now they are being asked to produce parts from aluminum. • Body panels are stamped to form parts (hoods, deck lids, doors, roofs, quarter panels)

Source http://european-aluminium.eu/media/1528/aam-manufacturing-3- 19 forming.pdf

Manufacturability - Formability

• Drawing is another typical forming method where the desired shape is stamped into a sheet by means of dies that allow the material to flow into the part. • Left side of diagram shows start—see flat part in brown. • Right side shows result of application of pressure.

20 Manufacturability –Formability Many other methods are used in autobody manufacturing

• Bending methods • Post-bending and – Press bending reshaping – Compression bending – Punching – Roll bending – Embossing – Rotary drawing – Extrusion – Rotary stretching – Machining and deburring Details available at http://european-aluminium.eu /media/1528/aam- manufacturing-3-forming.pdf

21 Manufacturability - formability Surface Quality Issues After Forming

• Orange Peel – Seen when grain size becomes large enough to be seen by eye (see photo on slide 14) • Directional Roughening (Paint Brush Lines or Roping) – Surface roughening that occurs upon deformation due to clustering of grains of similar orientation (or size). In severe cases visible peaks or valleys run parallel to the rolling direction and may become visible through paint. • Necking – The reduction of the cross-sectional area of a material in a localized area by uniaxial tension of by stretching. Paint Brush Lines

Orange Peel

Steel Part Aluminum Part 22 Manufacturability: Fewer Parts Audi’s Experience

• As Audi developed its aluminum cars over the years, it found that fewer parts were needed: Fewer parts, lower weight, fewer manufacturing steps. Data is for models D2 (1994), D3 (2002), D4 (2009)

23 Manufacturability Summary

• Surface appearance is critical. Both steel and aluminum can have issues with surface roughening following forming (stamping) operation such as ‘orange peel’. • Aluminum is generally considered less formable than steel. This is not only due to springback but other behaviors as well (i.e. more susceptible to scratching, minimum bending radii, lubrication requirements, etc). • A benefit of aluminum is fewer parts which reduces manufacturing costs.

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25 Materials Substitution Case Study: Conversion of the Ford F-150 body from Steel to Aluminum

• This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE #1400619

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