2Resistance Welding Learning Activities View Slides; Lesson Objectives Read Notes,Listen to lectureDo on-line workbookLesson ObjectivesWhen you finish this lesson you will understand:Keywords
3Aluminized Coated Steel Hot Dipped (continuous or batch)ElectroplatedThermal SprayingPowder Coating with FusionDiffusion CoatingWe will consider theseAluminum coated steel, like the other coated steels, provides a corrosive protection to the steel sheet. There are some additional benefits that we will see below also. The aluminum protective coating is applied in several different ways including hot dipping, electroplating thermal spraying and power coating with subsequent heating to fusion. In addition, a subsequent temperature diffusion treatment can be applied to the coating as well. We will consider only the hot dipped coating and the subsequent diffusion coating as these are the most prevalent.
4Hot Dipped Aluminum Coating Lower Electrical Resistance Type 1: Al - 8% Si (Oxidation Resistance Up to 1200F - 649C)-Reduced FormabilityT1-40 = 0.40 oz/ sq ft (0.001 inch thick)T1-25 = 0.25 oz/ sq ft ( inch thick)Type 2: Commercially Pure Al (Atmospheric Resistance at RT)Lower Electrical Resistance0.75 oz/ sq ft (0.002 inch thick)[Aluminum Coated] Aluminum coated steel is produced by hot-dip coating cold rolled sheet steel on continuous lines. It provides a material with the superior strength of steel and the surface properties of aluminum. There are two types as illustrated here. Type 1 contains up to 8% Si in the coating and finds particular use for elevated temperature oxidation; and type 2 is essentially commercially pure aluminum coating.
5Hot Dipped Aluminum Coating Type 2 This is a cross section of a type 2 (pure aluminum) hot dipped aluminum coating. Note that the pure aluminum is on the top. Some darker spots of aluminum-iron inter-metallic can be seen in this aluminum. The middle region is an iron-Aluminide inter-diffusion phase. And the iron is at the bottom.Iron-AluminideFeASM Handbook , Vol 1, 1990
6Typical Applications of Coated Steels Aluminum CoatedAutomotive exhaust systemsThe aluminum coated steels find a lot of use in the manufacture of automotive exhaust systems.
7Diffusion Aluminizing Pure Aluminum Coating Diffusion Heat Treatment(Applied by Any of the previous processes) (Furnace Heat Treatment)The diffusion coating is produced by post-coating anneal to promote interdiffusion of the aluminum into the steel. No free aluminum remains on the surface, but a complex diffusion zone of iron-aluminum inter-metallic compounds is formed.
8The diffusion aluminized coating can be applied to a range of materials as seen in this table. Note the uses of the diffusion aluminized carbon steels.AWS Welding Handbook
10Welding Aluminum Coated Steels Spot WeldingWelding Current 20-50% higher than uncoated steelElectrode Force 20-50% higher than uncoated SteelWeld Time 40-60% longer than uncoated steelElectrode Dressing somewhat more frequent than Galvanized SteelClass II Truncated Cone Electrodes have been successfulMore electrode indentation with type 2 Aluminized Sheet because it has lower resistance than type 1 Aluminized & generally Thicker CoatingThe table here list some observations about the spot welding of the hot dipped aluminum coated steels. Because of the aluminum, the currents are higher than uncoated steel, the silicon free coatings with the somewhat lower electrical resistance are the highest current; electrode forces are grater and weld time longer. Somewhat more electrode wear is observed than with the zinc coated galvanized steels.
11This table lists some nominal parameters for welding the hot dipped aluminized steels. AWS Welding Handbook
12Welding Aluminum Coated Steels Projection WeldingBenefit of Projection Welding:Reduces need for frequent electrode dressingType 1 coatingRecommend slightly larger projects than for Uncoated steelNormally flat class II Electrodes RecommendedThese are some observations regarding projection welding of the hot dipped aluminized steels.
13Welding Aluminum Coated Steels Seam WeldingAWS Recommendations:% greater welding current than bare steel% greater pressure than bare steelClass II wheels wit Type 1 MaterialPick up of Aluminum on Wheel must be counteredKnurled Drive WheelPower BrushesLarge amount of flood waterMash seam welding is sometimes hampered by the refractory Oxide presentThese are observations for seam welding of the hot dipped aluminized steels.AWS Welding Handbook
14ERW & High-Frequency Seam Welding Welding Aluminum Coated SteelsERW & High-Frequency Seam WeldingBoth 60 Hz and High Frequency Welds can be madeEdges must be square & free of Aluminum to prevent Aluminum Oxide, thus Centerline defectsThese are some observations for the ERW and High Frequency welding of hot dipped aluminized steel.
16Welding Diffusion Aluminized Metals Close to the same as welding uncoated steel with one exception noted belowAluminum Above 1% in the Weld Can Significantly Reduce Ductility and Cause EmbrittlementThe resistance of the diffusion treated aluminized steels is considerably higher than the non-diffusion transformed alloy layer present in the diffusion aluminized steels. Because of this, the welding parameters are close to those of the uncoated steel. There is one exception, aluminum in the solidified weld metal can cause embrittlement.