Aluminum for Automotive Body Sheet PanelsContent1 Introduction2 Properties of aluminum alloys for automotive body panel sheet3 Design considerations4 Forming aluminum auto body sheet5 Joining considerations in design6 Finishing7 Summary1 IntroductionThe use of aluminum saves weight,40% to 60% compared to steel.Many more aluminum body panels are now being specified for vehicles at the conceptual design stage so that designers can take advantage of the additional weight savings in design of other systems like braking and suspension.Aluminum body sheet has the major advantage that it can be handled,formed,joined,and finished in much the same ways and using largely the same equipment of and systems that are use today for steel sheet panels.The purpose of the publication is to provide specific information on:The mechanical properties,physical properties and formability of the current generation of aluminum alloys.Design considerations,blanking and forming information ,and relevant die design considerations.Joining technology,including spot welding,fusion welding,mechanical fastening,and adhesive bonding technologies.Finishing process,including both conventional phosphating and electro-coating,plus newer alternative methods now becoming available.2 Properties of aluminum alloys for automotive body panel sheet Three types of aluminum alloys:2xxx(Al-Cu) alloys5xxx(Al-Mg) alloys6xxx(Al-Mg-Si) alloyscompositionThe typical mechanical and physical propertiesTypical mechanical propertiesTensile propertyRepresentative properties of aluminum body sheet alloys following a simulated Paint bake cycleFormability comparativeRepresentative fatigue properties3 Design considerations3.1 Dent resistance 3.2 Panel deflection3.3 Bending and torsional stiffness3.4 Flange design3.5 Vibration3.6 Fatigue3.7 Aluminum design manualA weight savings of approximately 40%-50% can be obtained simply by converting an existing steel design to a similar design in aluminum,while a design optimized for aluminum has provided weight savings up to 65% over a comparable steel panel.3.1 Dent resistance The dent resistance of an outer panel is governed by :The yield strength of the materialThickness of the panelThe shapeThe unsupported panel area There are two methods for comparing dent performance of automotive body panels:Threshold velocitythe minimum impact velocity required to initiate a perceptible dentEquivalent depth of dentconsider depth of dent developed in components at a given velocity and mass of object 3.2 Panel deflectionFor exterior panel applications,deflections of the panel in the immediate velocity of the applied load are an important design consideration.Typically, the deflection of a component or subassembly under load must be controlled to avoid contact with other parts.Equally important is the “Feel” ,or deflection of a panel,when a customer applies pressure to it.There are two approaches for minimizing the deflection of an aluminum assembly.Either the outer panel thickness is increased,or the span between inner panel supporting beams is decreased.Equal deflection can be obtained with an aluminum panel with a span 70%the of a steel panel of equal thickness.The second approach is much more efficient.Multi-cone or a teacup design3.3 Bending and torsional stiffness Bending and torsional stiffness is dependent on:panel thicknessthe geometry of the cross-section beamthe beam patternThe multi-clone design 3.4 Flange designTypically,either a down-standing flange or a hemmed flange is used for panels such as hoods and decklids.3.5 VibrationThe natural frequency of vibration for a part in free-free state is proportional to its stiffness(k) and is inversely to its proportional to its mass(m).The stiffness is dependent on both the geometry of the panel and the modulus of elasticity of the material used.The vibration response of aluminum and steel parts with the same geometry and thickness in a free-free state is identical.3.6 FatigueComponents subjected to repeated loads should be carefully checked for the possibility of fatigue failure.Careful design can eliminate concentrations in highly stressed areas ,thereby making the most efficient use of material.Connections,holes or other features that cause of stress concentration are areas most subject to fatigue.3.7 Aluminum design manualThe Aluminum design manual provide a broad reference of design properties,specifications and guidelines for aluminum alloys4 Forming aluminum auto body sheet4.1 Formability characteristics of aluminum alloys4.2 Part,tool,and Process Design for manufacturability4.3 Press setupThe most critical variables during forming are:Formability characteristics of the materialGeometry of the part and tools resulting from the part and tool design Press setup including lubrication and control of the binder pressure4.1 Formability characteristics of aluminum alloysGenerally,deep-drawability,stretchability,and bendability are reduced with respect to steel.Test for determining formability fall into two main groups:those which measure material properties and those which attempt to simulate actual press forming operations.Material testsTensile BulgeHardnessPlane strain tensileMarciniak Biaxial stretch testsSimulative testsBend test(bending)Stretch bend test(stretching and bending)Hole expansion test(stretch flanging)Limiting draw ratio test(shrinking flange)Yoshida buckling test(wrinkling tendencies)Draw bead test(friction evaluation)Swift cup test(drawing)Erichsen test(stretching)Limiting dome height(LDH) test (stretching)Fukui test(stretching-draw combination)Marciniak test(stretching)Erichsen test(stretching)4.2 Part,tool,and Process Design for manufacturabilityPunch contactBlank developmentBinder designDraw bead designShape controlRecommend or minimum design radiiDraw bead designHemming 4.3 Process setup Binder control:A proper binder pressure is one of the key parameters of successful aluminum forming.Generally,binder pressure should be high enough only to prevent wrinkling and buckling.Binder pressure for aluminum is generally lower than for steel. Lubrication and surface characteristics:Lubricants are used to provide low friction at the interface to facilitate deformation and to prevent galling.There are numerous water soluble emulsions,light oils,and dry film lubricants available. 5 Joining considerations in design5.1 Spot welding5.2 Ultrasonic welding5.3 weld-bonding5.4 Inert Gas welding process5.5 Stud welding5.6 Laser welding5.7 Adhesive bonding5.8 Mechanical fastening 5.9 Mechanical fastening plus adhesive bonding5.10 Joining dissimilar metals5.1 Spot weldingResistance spot welding(RW)The kiloamperages required for aluminum spot welding are about two-and-one-half times those needed for steel.Aluminum surface oxide is an important factor in electrode tiplife.When the surface protected to retain a low oxide level,tiplife is extended.Generally the less coating substance applied,the better.Shear strength requirementsJoint design guidelines5.2 Ultrasonic weldingUltrasonic welding(USW) now has limited usage in automotive fabrication(example,electrical components).USW equipment has a frequency converter power source that converts 60Hz line power to the kilohertz range,15kHz to 75kHz,but usually 15kHz to 20kHz for the high power units that apply to the aluminum thickness for automotive welding.Compared to RW,USW are typically two-and-one-half times as strong,because the shear areas of the bonds can be maker larger than RW nugget shear areas.USW can usually be made on mill finish sheet without cleaning the sheet.USW machines are limited almost entirely to floor-mounted pedestal and bench-mounted types.5.3 weld-bondingweld-bonding is a process a joint by using a combination of resistance spot welding and adhesive bonding.Following are reasons for combining spot welds and adhesive in a joint:Spot welds tack the joints in fixed position while the adhesive cures.Spot welds provide added resistance to the peel mode of failure.The fatigue life and durability of the joint are improved, compared to spot weld node joints.The adhesive usually acts as a seal against corrosion and gives tighter body construction than spot welding alone.Joint stiffness is greater than in spot welded joints. The common means of combining both joining methods in a joint are:1.RW through an uncured paste adhesive2.Flowing an adhesive by capillary action into the bond area after RW3.By applying a film adhesive which has had holes cut into it and RW through the holes.Method 1 is the most applicable for joining auto body assemblies.Strengths compare Fatigue compare 5.4 Inert Gas welding process The two most commonly used methods for fusion welding aluminum:Gas Tungsten Arc Welding (GTAW or TIG)Gas Metal Arc Welding(GMAW or MIG)Common ground:Employ an inert gas shieldCan be used in all welding positions,although most automotive welding is preferably done in the flat position Are used to weld a variety of aluminum alloys and dissimilar alloy combinationsAre free of flux or slagUse similar priced equipmentGTAWThe 5xxx(Al-Mg) series can nearly all be GTAW without filler ,although filler is usually added.2xxx(Al-Cu) and 6xxx(Al-Mg-Si) series must use filler to avoid weld cracks.GMAW GMAW is faster and more economical for many joining applications than the GTAW.For sustained high volume welding where GMAW can be used it is usually preferred over GTAW because of greater welding speed.GMA spot welds VS RW Advantages:The equipment is considerably less expensiveThe equipment is more portableFully penetrated GMWA spot welds maybe visually inspect. Disadvantages:In some applications,the pressure exerted through the welding gun must being the mating material surfaces together, which may result in too little pressure for good contract.It is slower than resistance spot welding.It produces a nugget on the weld side which may be objectionable.Filler alloys for Inert gas welding5.5 Stud weldingStud welding is a specialized type of welding in which an aluminum stud is attached endwise to an aluminum surface.The stud can be a bolt,screw,rivet,rod,pin or similar device.The larger studs d8mmThe smaller studs d6mm5.6 Laser welding Advantages:The laser beam,unlike other fusion processes,can be projected in air with no loss in power,and no vacuum chamber is needed.The ease of beam transmission and directional control permits multi-station operation.Nearly 100% duty cycle is possible by switching the beam station to stationThe high energy density beam enables rapid welding speeds,narrow welds and narrow heat affected zones,and therefore goof retention of metal properties and relatively low distortion of the work piece. Disadvantages:Most of the multi-kilowatt laser systems operate at only 10 to 20% efficiency for converting electrical power into a focused infrared laser beamPrecise fit up(a fit up tolerance of 15% of material thickness is desirable)is necessary on butt and lap joints for good weld quality.The high cost of laser beam and equipment ranges from approximately $30,000 to $1,000,000.This large capital investment would require high volume production or critical weldment applications to justify the expenditure.Three joint designs appear best suited for laser welding:Butt jointsLap jointsFlange joints5.7 Adhesive bonding Advantages:More evenly load distribution ; stress concentration are minimized; Have longer fatigue lives than spot welds or mechanical fasteners.Tended to dampen vibrations.Are stiffer than spot welded or mechanical fastened joints.Join dissimilar materialsMetals of different thickness can be easily assembled at low,room or elevated temperaturesJoined large areas easily Electrically conductive or insulative joints Only method to join metals and nonmetalsGood looking5.8 Mechanical fasteningRivetingSelf-piercing rivetsClinchingSelf-drilling self-tapping screwsSelf-piercing rivetsClinchingClinching VS Spot weld5.9 Mechanical fastening plus adhesive bondingHave improved properties over those resulting from the use of mechanical fasteners alone or adhesive aloneSeal5.10 Joining dissimilar metalsCrevices are usually the most critical sites for galvanic corrosion.The aluminum/steel transition sheet is typically made by rolling sheets of the two metals together under high roll force.6 Finishing6.1 Cleaning6.2 Phosphating6.3 Electrocoating6.4 Performance requirements6.5 Alternative methodsThe major stepsAlkali cleanRinsePhosphate conditionerPhosphatingChemical rinseRinseCathodic electrocoatBakePrimer/surfacerTopcoatsBake6.1 CleaningThe principal function of the cleaning process is to remove all contaminants.Principal components of a cleaner include silicates,carbonates,and phosphates.6.2 PhosphatingMost phosphating systems currently in use employ a tricationic systems,namely zinc,nickel,and manganese.6.3 ElectrocoatingThe incorporation of aluminum components into mixed metal vehicles does not present any major problems compared to steel or galvanized steel in terms of the currently used cathodic electrocoat primers.6.4 Performance requirementsThe most common tests in North America are a stone chipping test to check on paint adhesion and resistance to spalling,and a SCAB test to evaluate corrosion resistance.6.5 Alternative methodsThe cost of finishing a vehicle to todays standards for corrosion and paint performance is a major component of overall automobile costs,both in terms of capital cost for finishing line and operation cost.Because of the superior corrosion resistance of aluminum compared to steel,it may not be necessary to phosphate and electrocoat an all-aluminum BIW. methods to make savings:Spray primerPreprimed aluminum Prefinished aluminum7 Summary These steps to successful application of aluminum alloy sheet may b summarized as follows:Select alloys for strength,formability,and corrosion resistance required in the application.Design effectively,considering the unique properties of aluminum.including its low modulus of elasticity.Select part dimensions utilizing the opportunities to form and join efficiently.Take advantage of the variety of forming processes applicable to aluminum and the forming process parameters outlined herein.Select the appropriate joining processes,including the option to combine mechanical,adhesive bonding,and welding where most effective for performance and durability.Provide the appropriate level of protection,taking full advantage of their natural corrosion resistance of aluminum,and supplementing that for the specific requirements of the application in question.