Coatings

Coatings

Coatings

topofpage

Many formed parts require corrosion protection, achieved through the application of some type of zinc-based or aluminum-based coating.  The primary methods of applying zinc are through a hot dipped galvanizing line, or through an electro-galvanizing process.  Aluminum-based coatings are applied in a hot dipped aluminizing line.

Both aluminized and galvanized coatings provide a barrier layer preventing corrosion of the underlying sheet steel. Zinc-based coatings also provide galvanic protection, where the zinc acts as a sacrificial anode if scratches or impact damage the coating, and therefore corrodes first before the underlying steel.  

Aluminum melts at a higher temperature than zinc, which is why aluminized steels are more frequently used in applications requiring corrosion protection at elevated temperatures such as those found in the engine compartment and in the exhaust system. There are two types of aluminized coatings, known as Type 1 (aluminum with 9% silicon) and Type 2 (pure aluminum).  The aluminum-silicon AlSi coating associated with press hardening steels are these Type 1 aluminized coatings.  There are limited automotive applications for Type 2 pure aluminum coatings.

Zinc-magnesium coatings are relatively new options, offering enhanced cut edge corrosion protection, with lower friction and lower risk of galling and powdering than other common zinc coatings.

Most hot dipped galvanized lines produce surfaces which result in similar coefficients of friction for a given steel type. Different electrogalvanized coating lines may result in different surface morphologies, which can result in significantly different formability characteristics.

Passing a pure zinc-coated steel through a furnace allows the steel and zinc to inter-diffuse and results in an alloyed coating known as galvanneal.  Hot dipped galvannealed coatings have improved joining due to the iron in the coating.  Unlike the uniform coating composition associated with hot dip galvanized and electrogalvanized coatings, GA coatings are composed of different phases with varying composition.  This may lead to different forming, joining, and painting characteristics when comparing products produced on different lines.

Differences in performance due to coating line characteristics is another reason why it is good practice to Identify the intended steel production source early in the die construction and die try-out process.  Tryout material should come from the same source as will supply production.

 

Hot Dipped Galvanized and Galvannealed Coatings

The majority of sheet steel parts on a vehicle require corrosion protection, independent of whether they are made from mild or high strength steel and whether they are intended for exposed or unexposed applications. Hot dipped galvanizing – applying a zinc coating over the steel – is the most common way to achieve corrosion protection. It is an economical solution, since cold rolled steel can be annealed and coated in the same continuous operation.

A typical in-line continuous hot dip galvanizing line such as that shown in Figure 1 uses a full-hard cold rolled steel coil as the feedstock. Welding individual coils together produces a continuous strip.  After cleaning, the strip is processed in a continuous annealing furnace where the microstructure is recrystallized, improving forming characteristics.  Adjustments to the annealing temperature produces the desired microstructure associated with the ordered grade.  Rather than cooling to room temperature, the in-process coil is cooled to just above 460 °C (860 °F), the temperature of the molten zinc bath it enters. The chemistry in the zinc pot is a function of whether a hot dipped galvanized or galvannealed coating is ordered.  Hot rolled steels may be coated with the hot dip galvanizing process, but different processing conditions are used to achieve the targeted properties.

Figure 1:  Schematic of a typical hot dipped galvanizing line with galvanneal capability.

Figure 1:  Schematic of a typical hot dipped galvanizing line with galvanneal capability.

 

There are several types of hot dipped coatings for automotive applications, with unique characteristics that affect their corrosion protection, lubricity for forming, weldability and paintability. One of the primary hot dipped galvanized coatings is a pure zinc coating (abbreviated as GI), sometime referred to as free zinc. The molten zinc bath has small amounts of aluminum which helps to form a thin Fe2Al5 layer at the zinc-steel interface.  This thin barrier layer prevents zinc from diffusing into the base steel, which leaves the coating as essentially pure zinc.

Coils pass through the molten zinc at speeds up to 3 meters per second. Zinc coating weight is controlled by gas knives (typically air or nitrogen) blowing off excess liquid zinc as the coil emerges from the bath. Zinc remaining on the surface solidifies into crystals called spangle.  Molten zinc chemistry and cooling practices used at the galvanizing line control spangle size. In one extreme, a large spangle zinc coating is characteristic of garbage cans and grain silos.  In the other extreme, the zinc crystal structure is sufficiently fine that it is not visible to the unaided eye.  Since spangle can show through on a painted surface, a minimum-spangle or no-spangle option is appropriate for surface-critical applications.

The other primary hot dipped coating used for corrosion protection is hot dipped galvanneal (abbreviated as GA). Applying this coating to a steel coil involves the same steps as creating a free zinc hot dipped coated steel, but after exiting the zinc pot, the steel strip passes through a galvannealing furnace where the zinc coating is reheated while still molten. 

The molten zinc bath used to produce a GA coating has a lower aluminum content than what is used to produce a GI coating.  Without aluminum to create the barrier layer, the zinc coating and the base steel inter-diffuse freely, creating an iron-zinc alloy with typical average iron content in the 8% to 12% range. The iron content improves weldability, which is a key attribute of the galvanneal coatings. 

The iron content is unevenly distributed throughout the coating, ranging from 5% at the surface (where the sheet metal coating contacts the tool surface during forming) to as much as 25% iron content at the steel/coating interface.  The amount of iron at the surface and distribution within the coating is a function of galvannealing parameters and practices – primarily the bath composition and time spent at the galvannealing temperature.  Coating iron content impacts coating hardness, which affects the interaction with the sheet forming lubricant and tools, and results in changes in friction.  The hard GA coatings have a greater powdering tendency during contact with tooling surfaces, especially during movement through draw beads.  Powdering is minimized by using thinner coatings – where 50 g/m2 to 60 g/m2 (50G to 60G) is a typical EG and GI coating weight, GA coatings are more commonly between 30 g/m2 to 45 g/m2 (30A to 45A)

Options to improve formability on parts made from GA coated steels include use of press-applied lubricants or products that can be applied at the steel mill after galvanizing, like roll-coated phosphate, which have the additional benefit of added lubricity.  The surface morphology of a galvannealed surface (Figure 2) promotes good phosphate adherence, which in turn is favorable for paintability.  

Figure 2:  High magnification photograph of a galvannealed steel surface.  The surface structure results in excellent paint adhesion.

Figure 2:  High magnification photograph of a galvannealed steel surface.  The surface structure results in excellent paint adhesion.

 

Galvannealed coatings provides excellent corrosion protection to the underlying steel, as do GI and EG coatings.  GI and EG coatings are essentially pure zinc.  Zinc acts as a sacrificial anode if scratches or impact damages either coating, and therefore will corrode first before the underlying steel.  The corrosion product of GI and EG is white, and is a combination of zinc carbonate and zinc hydroxide.  A similar mechanism protects GA coated steels, but the presence of iron in the coating may result in a reddish tinge to the corrosion product.  This should not be interpreted as an indication of corrosion of the steel substrate.

Another option is to change the bath composition such that it contains proper amounts of aluminum and magnesium. The results in a zinc-magnesium (ZM) coating, which has excellent cut edge corrosion protection. 

Producing galvanized and galvannealed Advanced High Strength Steels is challenging due to the interactions of the necessary thermal cycles at each step.  As an example, the targeted microstructure of Dual Phase steels can be achieved by varying the temperature and time the steel strip passes through the zinc bath, and can be adjusted to achieve the targeted strength level.  However, not all advanced high strength steels can attain their microstructure with the thermal profile of a conventional hot dipped galvanizing line with limited rapid quenching capabilities.  In addition, many AHSS grades have chemistries that lead to increased surface oxides, preventing good zinc adhesion to the surface. These grades must be produced on a stand-alone Continuous Annealing Line, or CAL, without an in-line zinc pot. Continuous Annealing Lines feature a furnace with variable and rapid quenching operations that enable the thermal processing required to achieve very high strength levels. If corrosion protection is required, these steel grades are coated on an electrogalvanizing line (EG) in a separate operation, after being processed on a CAL line. 

Hot dipped galvanizing lines at different steel companies have similar processes that result in similar surfaces with respect to coefficient of friction. Surface finish and texture (and resultant frictional characteristics) are primarily due to work roll textures, based on the customer specification. Converting from one coating line to another using the same specification is usually not of major significance with respect to coefficient of friction. A more significant change in friction is observed with changes between GI and GA and EG. 

 

Electrogalvanized Coatings

Electrogalvanizing is a zinc deposition process, where the zinc is electrolytically bonded to steel in order to protect against corrosion. The process involves electroplating: running an electrical current through the steel strip as it passes through a saline/zinc solution. 

Electrogalvanizing occurs at room temperature, so the microstructure, mechanical, and physical properties of AHSS products achieved on a continuous anneal line (CAL) are essentially unchanged after the electrogalvanizing (EG) process.  EG lines have multiple plating cells, with each cell capable of being on or off. As a result, chief advantages of electrogalvanizing compared to hot dipped galvanizing include: (1) lower processing temperatures, (2) precise coating weight control, and (3) brighter, more uniform coatings which are easier to convert to Class A exposed quality painted surfaces.

The majority of electrogalvanizing lines can apply only pure (free) zinc coatings, known as EG for electrogalvanized steel.  Selected lines can apply different types of coatings, like EGA (electro-galvanneal) or Zn-Ni (zinc-nickel).

There are no concerns about different alloy phases in the coating as with galvanneal coatings.  The lack of aluminum in the coating results in improved weldability. The biggest concern with electrogalvanizing lines is the coefficient of friction. Electrogalvanized (EG) coatings have a relatively high coefficient of friction – higher than hot dipped galvanized coatings, but lower than galvanneal coatings.  To improve formability of electrogalvanized sheets, some automakers choose to use a steel mill-applied pre-lube rather than a simple mill-applied rust preventive oil.

A representative EG line is shown in Figure 3.  Different EG lines may use different technologies to apply the zinc crystals. Because the zinc crystals are deposited in a different fashion, these different processes may potentially result in different surface morphology and, in turn, a different coefficient of friction.  Dry conditions may result in a higher coefficient of friction, but the “stacked plate-like surface morphology” (Figure 4) allows these coatings to trap and hold lubrication better than the smoother surfaces of hot dipped galvanizing coatings. Auto manufacturers should therefore consult the steel supplier for specific lubricant recommendations based on the forming needs.

Figure 3: Schematic of an electrogalvanizing line. 

Figure 3: Schematic of an electrogalvanizing line.

 

Figure 4:  High magnification photograph of electrogalvanized steel surface showing stacked plate-like structure.

Figure 4:  High magnification photograph of electrogalvanized steel surface showing stacked plate-like structure.

 

Zinc-Magnesium (ZM) Coatings

Galvanized coatings offer excellent corrosion protection to the underlying steel.  Each type of galvanized coating has characteristics that make it suitable for specific applications and environments.  The method by which the galvanization occurs (electrolytically applied vs. hot dipping) changes these characteristics, as does the coating chemistry (pure zinc vs. zinc alloy)

Adding small amounts of magnesium and aluminum affects the coating properties, which influences friction, surface appearance, and corrosion among other parameters. 

Industries like agriculture and construction have used a coating known as ZAM for several decades.  ZAM (Zinc-Aluminum-Magnesium) is primarily zinc, with approximately 6% Al and 3% Mg.  However, the high aluminum content does not lend itself to a continuous galvanizing operation due to increased dross formation.  The coating aluminum also degrades weldability.

A different coating, known as ZM, was commercialized around 2010.  This zinc-rich coating typically has 1% to 3% of both magnesium and aluminum, with some companies using a slightly higher amount of aluminum.  ZM coatings are typically applied using a hot dip approach like GI and GA, but with an appropriate bath chemistry.  Exposed quality surfaces are achievable. 

Even though ZM is a relatively hard coating, it is associated with lower friction and lower risk of galling and powdering than other common zinc coatings.  This allows for parts to be successfully formed using higher blank holder forces, resulting in a wider BHF range between wrinkles and splits.Z-6

ZM coatings have similar joining characteristics and similar performance after phosphating / painting as hot dip galvanized coatings.

Enhanced cut edge corrosion protection relative to EG, GI, and GA coatings occurs due to the formation of a stable passivation layer on the bare steel edge that would be otherwise exposed to the environment.  ZM coating corrosion dynamics are such that similar performance to EG, GI, and GA can be achieved at lower ZM coating weights.

Powdering resistance of ZM coatings are similar to that for GI and EG, and better than what is seen on GA coatings.  ZM coatings have superior cyclic, perforating, and stone chip corrosion resistance.

Potential areas of application applications include:

  • Hem flanges of doors, deck lids, and hoods
  • Cut edges in inner hood, door, and deck lid panels
  • Stone chip sensitive parts like hoods, fenders, doors, and body sides.
  • Difficult to form parts that can benefit from the lower friction

 

Back to the Top

 

Forming Modes

Forming Modes

Forming Modes

Stamping and die designers are interested in the forming capabilities of the steels they specify. Complex stampings are created from several different basic forming modes, which are sensitive to different mechanical properties. For this reason, steel formability, especially for AHSS grades, cannot be characterized by a single number, but instead must be done for each basic forming mode.

Forming modes and key points include:

Stretching

  • Stretching is the sheet metal forming process where the punch which creates the part shape forces the sheet metal to thin since lock beads prevent metal flow inward from the flange area.
  • The steel property that improves stretching is the strain hardening exponent, or n-value.
  • Dome testing characterizes stretchability.
  • Higher n-values flatten strain gradients, reduce localized thinning that leads to early failure, and allows the forming of more complex parts.

Bending

  • Tensile testing cannot be used to determine bendability, since these are different failure modes. Different bend tests characterize bendability.
  • The failure strain in the bend is related to the total elongation of conventional steel, but are limited by other issues in AHSS grades.
  • For equal strengths, most AHSS grades have higher total elongations than conventional HSLA steels. However, several AHSS grades have limited local formability based on their microstructure, and may be at risk for cracking during edge expansion. AHSS grades with multiphase microstructures such as DP and TRIP experience shear fracture that severely reduces the bendability before failure occurs.

Drawing

  • Drawing is the sheet metal forming process where the punch that creates the part shape forces the sheet metal to pull in from the flange area.
  • The steel property that improves cup drawing or radial drawing is the normal anisotropy or rm value.
  • The Limiting Draw Ratio (LDR) characterizes cup drawability.
  • Higher r-value increases the LDR, but the absolute value of the LDR also depends on the lubrication, blank holder load, die radius and other system inputs.
Coatings

Tensile Strength

Mechanical Properties

Engineering stress-strain units are based on the starting dimensions of the tensile test sample: Engineering stress is the load divided by the starting cross-sectional area, and engineering strain is the change in length relative to the starting gauge length (2 inches, 50mm, or 80mm for ASTM [ISO I], JIS [ISO III], or DIN [ISO II] tensile test samples, respectively.)

Metals get stronger with deformation through a process known as strain hardening or work hardening. This is represented on the stress strain curve by the parabolic shaped section after yielding.

Concurrent with the strengthening as the tensile test sample elongates is the reduction in the width and thickness of the test sample. This reduction is necessary to maintain consistency of volume of the test sample.

Initially the positive influence of the strengthening from work hardening is greater than the negative influence of the reduced cross-section, so the stress-strain curve has a positive slope. As the influence of the cross-section reduction begins to overpower the strengthening increase, the stress-strain curve slope approaches zero.

When the slope is zero, the maximum is reached on the vertical axis of strength. This point is known as the ultimate tensile strength, or simply the tensile strength. The strain at which this occurs is known as uniform elongation.

Strain concentration after uniform elongation results in the formation of diffuse necks and local necks and ultimately fracture.

Figure 1: Tensile Strength is the Strength at the Apex of the Engineering Stress – Engineering Strain Curve

Figure 1: Tensile Strength is the Strength at the Apex of the Engineering Stress – Engineering Strain Curve.

Coatings

Tensile Testing

Testing and Characterization

topofpage

Tensile testing characterizes the forming and structural behavior of sheet metals. The test involves loading a sample with a well-defined shape along the axis in tension, generally to fracture, and recording the resultant load and displacement to calculate several mechanical properties. Global standardsI-7, A-24, D-19, J-15 prescribe the conditions under which tests must occur.

Sample Size and Shape

Full-size samples for tensile testing of metal sheets have a rectangular section at the edges for gripping by the test machine. Reducing the width in the central area promotes fracture in the monitored region. These geometrical features result in a sample shape which resembles a dogbone, leading to a descriptive term applied to test samples.

Dimensions of the dogbone samples are associated with tensile test standard from which they apply. ISO I, II, and III (described in Citation I-7) corresponds to the ASTMA-24, DIND-19, and JISJ-15 shapes, respectively. Figure 1 shows the dogbone shapes, highlighting the critical dimensions of width and gauge length. Refer to the Test Standards for other dimensions, tolerances, and other requirements.

Figure 1: Full-size tensile sample shapes for ISO I (ASTM), ISO II (DIN), and ISO III (JIS) standards.

Figure 1: Full-size tensile sample shapes for ISO I (ASTM), ISO II (DIN), and ISO III (JIS) standards.I-7, A-24, D-19, J-15

 

Significant differences exist in the width and gauge length of these tensile bar shapes. Although the ASTM and JIS bars have similar gauge length, the width of the JIS bar is twice that the ASTM bar. The ASTM and DIN bars have a 4:1 ratio of gauge length to width, where the JIS bar has a 2:1 ratio.

These shape differences mean that the calculated elongation changes depending on the test-sample standard used, even when testing identical material. With the combination of the shortest gauge length and widest sample, elongation from JIS bars typically are higher than what would be generated from the other shapes.

Yield strength and tensile strength are not a function of the shape of the tensile bar. Strength is defined as the load divided by the cross-sectional area. Even though each of the bars specify a different sample width (and therefore different cross-section), the load is normalized by this value, which negates differences from sample shape.

Shearing or punching during sample preparation may work-harden the edges of the tensile bar, which may lead to generating an inaccurate representation of the mechanical properties of the sheet metal. Test Standards require subsequent machining or other methods to remove edge damage created during sample preparation. Milling or grinding the dogbone samples minimizes the effects sample preparation may have on the results.

 

Tensile Test Procedure

The gauge length is the reference length used in the elongation calculations. Depending on the test standard, the gauge length is either 2 inches, 80 mm, or 50 mm. Multiplying the width and thickness within the gauge length determines the initial cross-sectional area before testing.

Grips tightly clamp the edges of the sample at opposite ends. As the test progresses, the grips move away from each other at a prescribed rate or in response to the restraining load. A load cell within the grips or load frame monitors force. An extensometer tracks displacement within the gauge length. Samples are typically tested until fracture.

During the tensile test, the sample width and thickness shrink as the length of the test sample increases. However, these dimensional changes are not considered in determining the engineering stress, which is determined by dividing the load at any time during the test by the starting cross-sectional area. Engineering strain is the increase in length within the gauge length relative to the starting gauge length. (Incorporating the dimensional changes occurring during testing requires calculating true stress and strain. The differences between engineering and true stress/strain are covered elsewhere (hyperlink to 2.3.2.1-Engineering/True)

A graph showing stress on the vertical axis and strain on the horizontal axis is the familiar engineering stress-strain curve, Figure 2. From the stress-strain curve, numerous parameters important for sheet metal forming appear, including:

Figure 2: Engineering stress-strain curve from which mechanical properties are derived.

Figure 2: Engineering stress-strain curve from which mechanical properties are derived.

 

Influence of Test Speed

Conventional tensile testing is done at strain rates slow enough to be called “quasi-static.” These rates are several orders of magnitude slower than the deformation rates during stamping, which itself is several orders of magnitude slower than what is experienced during a crash event.

Stress-strain curves change with test speed, typically getting stronger as the speed increases. The magnitude of these changes varies with grade. Significant challenges exist when attempting to characterize the tensile response at higher strain rates. Improved equipment and data collection capabilities are among the required upgrades.

 

Influence of Tensile Test Equipment

Advanced High-Strength Steels (AHSS) may challenge older test equipment. The load and displacement response must reflect only the contributions of the sheet metal, and not be influenced by the load frame and other testing equipment. In much the same way that insufficiently stiff press crowns deflect when stamping AHSS parts, tensile test load frames may similarly deflect, resulting in inaccuracies in the load-displacement measurements.

Grip strength also becomes critical when testing AHSS samples. The high strength of the metal sheets requires more grip pressure to prevent sample slippage through the grips. Pneumatic grips and even some mechanical grips may not generate the necessary pressure. Hydraulically actuated grips may be necessary as the strength increases.

 

Back to the Top

Coatings

Friction and Friction Testing

Friction, Testing and Characterization

Friction is a restraining force that limits metal flow resulting from contact with another surface during sheet forming. Friction is influenced by the complex interaction between the sheet steel, lubricant, and tooling material, as well as many parameters of the forming system, some of which are shown in Figure 1.

Figure 1: Friction is influenced by the complex interaction between the sheet steel, lubricant, and tooling material, as well as many parameters of the forming system. (image modified from Citation K-13)

Figure 1: Friction is influenced by the complex interaction between the sheet steel, lubricant, and tooling material, as well as many parameters of the forming system. (image modified from Citation K-13)

 

Friction changes throughout the stamping process as well as across the part, since there are changes in contact pressure, contact temperature, geometry, the sheet steel strength (work hardening), and even the sheet metal surface due to flattening of the peaks after flowing over beads and radii. There is no one friction value that applies to all geometries and forming scenarios that can be encountered.

Characterizing friction in a simulative test is similarly challenging, since there is no one simulative test that approximates all regions of an engineered stamping. For many years, tooling, design, and simulation engineers used one friction value for each galvanized coating / lubricant combination, and applied that value across every part stamped from that metal with that lubricant. The industry has evolved to using different tests to generate friction values appropriate for different forming conditions.

Simulative tests to characterize friction include test to reflect bending under tension, pulling through draw beads, and movement over flat surfaces. Depending on the test, it may be possible to evaluate the effect of tooling material, temperature, radii, and speed in order to better simulate production conditions. Figure 2 indicates which of these simulative tests apply to the regions of an in-process stamping. The value obtained in one test is a function of only those specific test conditions and should not be compared against values obtained in other tests.

  • Bending Under Tension test: In the Bend Under Tension (BUT) test, a metal strip is drawn over a fixed cylindrical pin with a pair of independently controlled hydraulic actuators offset by 90 degrees. Two load cells measure the pulling force and back tension force. (Figure 3)
  • Draw Bead test: Two sets of tests are done using a Draw Bead Simulator (DBS). In the first test, the force to pull sheet steel through a set of fixed draw beads is measured and reflects both the bending and unbending forces as well as the friction forces. Another sample is pulled through frictionless roller beads in a similar arrangement. Here, only the bending/unbending forces are active. Subtracting the results allows for determination of the forces due solely to friction. (Figure 4)
  • Strip Draw: In the Strip Draw test, friction is determined by pulling sheet steel through opposing flat platens. The restraining condition is controlled by the force applied on the flat platens. The influence of bending is not considered in this test. (Figure 5)

Other tests are also used to provide some assessment of formability. Dome Testing simulates stretch forming conditions and can provide a relative comparison between different materials and lubricants. The Twist Compression Test (TCT) is used to compare the performance of different lubricants under controlled, repeatable conditions. Lubricant breakdown, which results in adhesion between the tool and the sheet steel, can be evaluated in the TCT.

Figure 2: Pertinent Simulative Tests During Stamping (image modified from Citation S-42)

Figure 2: Pertinent Simulative Tests During Stamping (image modified from Citation S-42)

 

Figure 3: The Bending Under Tension Test (image modified from Citation S-42)

Figure 3: The Bending Under Tension Test (image modified from Citation S-42)

 

Figure 4: Draw Bead Simulator Testing determines the effect of friction from the differences in pulling force in two test conditions. a) The strip pulled through fixed draw beads experiences restraining forces due to bending/unbending as well as forces due to friction. b) The strip pulled through frictionless roller beads experiences restraining forces due only to bending/unbending.

Figure 4: Draw Bead Simulator Testing determines the effect of friction from the differences in pulling force in two test conditions.
a) The strip pulled through fixed draw beads experiences restraining forces due to bending/unbending as well as forces due to friction.
b) The strip pulled through frictionless roller beads experiences restraining forces due only to bending/unbending.

 

Figure 5: The Strip Draw Test (image modified from U-6)

Figure 5: The Strip Draw Test (image modified from U-6)