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Microstructural Effects of Adding Colloidal Graphite to Al-Si-Coated PHS

Optimizing weld morphology and mechanical properties of

laser welded Al-Si coated 22MnB5 by surface application of

colloidal graphite

Researchers at University of Waterloo discovered the microstructural effects of adding colloidal graphite to Al-Si coated 22MnB5 Press Hardened Steel.^K-51 Laser welds were made on 1.5 mm thick Al-Si coated 22MnB5 PHS perpendicular to the rolling direction. Pure colloidal graphite suspended in isopropanol base was applied to the area being welded and the resulting graphite coating after evaporation ranged from 5 µm to 130 μm for testing. Parameters used for the weld are: 4kW power, 6m/min welding speed, beam diameter of 0.3 mm, and laser defocus of 6mm. Samples were then hot stamped by heating for 6 min to 930 ᵒC in a furnace and then water quenched at a cooling rate greater than 30 ᵒC/s.

Al-Si coating is excellent at preventing oxidation and decarburization of high strength steel at elevated temperatures. However, during welding there is diffusion of Al into the fusion zone which stabilizes ferrite at elevated temperature reducing the strength of the welded joint. Colloidal graphite coating decreases the Al content and increases C content of the fusion zone. As shown in Figure 1, The mechanism for reduction in Al content is due the graphite coating acting as an insulator to the Al-Si coating which then causes an ejection of the molten Al-Si coating from the surface. Figure 2 displays a proportional reduction of Al in the fusion zone with increasing graphite coating thickness up to 40 μm where after the reduction in Al is minimal. This is attributed to the initial reduction of Al being caused by the ejection of the molten Al-Si from underneath the graphite coating. Graphite coating greater than 40 µm does not aid in additional ejection of Al-Si and the Al-Si coating already diluted in the weld pool will not be removed.

Figure 1: Al-Si ejection mechanism.^K-51

Figure 2: Al and C content in weld with increasing graphite thickness.^K-51

Summary

Ferrite concentration in the fusion zone was reduced from approx. 40% with no graphite coating to approx. 2% with 130 μm graphite coating thickness (Figure 3). The increase in C content and reduction in Al content resulted in an increase in austenite being stabilized at elevated temperature rather than ferrite and therefore a larger percentage of martensite results after hot stamping. The average fusion zone hardness increased from 320 HV with no coating to 540 HV with 130 μm coating thickness (Figure 4). The weld strength of the sample with no graphite coating was 1249±15MPa whereas the weld strength with a coating of 130 µm was 1561±7MPa which matches the base metal (Figure 5). With an increase in graphite coating thickness there is an increase in weld strength that can eventually match the base metal strength.

Figure 3: Ferrite concentration in weld.^K-51

Figure 4: Fusion Zone Hardness vs. Graphite Thickness.^K-51

Figure 5: Weld strength vs. Graphite Thickness.^K-51

Microstructural Components

Steel grades are engineered to achieve specific properties and characteristics by the manipulation of mill processing parameters to achieve a targeted balance of microstructural components. Among the tools available to the steelmaker are alloy composition, rolling and processing temperatures, and cooling profile.

If steel is slowly cooled, only two components exist at room temperature: ferrite (abbreviated by the Greek letter α) and cementite (iron carbide, Fe₃C). Alternating layers of ferrite and cementite appear under a microscope in a pattern similar to Mother-of-Pearl, leading to the term pearlite.

A steel alloy having approximately 0.80% carbon will contain only pearlite in the microstructure. Lower carbon levels create an alloy that combines ferrite and pearlite. Ferrite-pearlite microstructures form the basis of many C-Mn steels and some of the initial HSLA steels. At a given strength level, pearlite limits sheet formability.

At carbon levels below 0.008% or 80 ppm, only ferrite exists. Ferrite is low strength but very ductile, and is the microstructural phase in ultra-low carbon steels.

Additional phases are formed when the cooling profile can be changed. Some modern annealing furnaces are capable of controlling the cooling rate as well as holding at specific temperatures. This ability is a key facilitator in the production of most Advanced High Strength Steels. In addition to ferrite and pearlite, microstructural phases of bainite, austenite, and martensite can be produced, depending on the chemistry and the thermal cycle profile including quench rate and hold temperature.

Bainite is a phase that is associated with enhanced sheared edge ductility. Accelerated cooling in the hot mill run out table allows for the production of Ferrite-Bainite steels.

Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the use of a combined chemistry and controlled thermal cycle. Deforming retained austenite is responsible for the TRIP effect.

Martensite is a very high strength phase, but has limited toughness. Steels with both ferrite and martensite in the microstructure are known as dual phase steels. A structure of 100% martensite can be produced directly at those sheet mills having equipment capable of achieving a minimum critical cooling rate. The ductility of this product is not sufficiently high for most stamping operations. However, sheet martensite is well suited for properly designed roll forming applications.

Martensite is the microstructural component of processed Press Hardening Steels. An elevated temperature and a more formable microstructure exist at the time of complex forming of these grades. Rapid cooling while the part is under full press load converts the microstructure to the high strength martensite.