Performance of commercial coatings for the hot stamping application
According to the information from the literature data regarding the properties of current coating products for the hot stamping application presented in the last sections, it appears that limits are encountered by all coating materials in terms of hot formability, corrosion protection or subsequent processing, thereby implying a compromise in the performance during the selection of the most appropriate materials for the manufacturing of components. The characteristic challenges and advantages of the main current coatings are summarized in Table 3.
It is clear that the main motivations in the development of optimal coating products are the achievement of good anticorrosive performance, in particular of sacrifical protection, and of good hot formability with the suppression of the susceptibility of the coating materials to LME. As stated earlier, only two commercial products, namely aluminized and galvanized steels have found the interest of OEMs for a wide-scale manufacturing of hot formed components. Both products will be considered as state-of-the-art coatings in the framework of the definition of alternative promising materials and of the study of the performance of new Zn-Mn coating materials compared to commercial products.
Alternative coating systems for press-hardened parts
This part is focused on the definition of alternative coating systems for hot stamping applications with a potential increase of the performance of current products in terms of anticorrosive and hot forming properties. On the basis of the comprehensive literature research reported in the last section, several single and multi layer coating systems were considered to provide interesting properties and are presented in Section 1.6.1. Zn-Mn alloy coatings were selected as the most promising investigation track, due to higher anticorrosive properties and higher melting point compared to pure Zn layers. The target properties and performance of the corresponding deposits are reported in Section 1.6.2. Finally, Section 1.6.3 concerns the deposition of Zn-Mn alloys, in particular by electroplating.
Identification of promising alternative coatings
Two main concepts have been identified in the literature for protective coating for automotive press-hardened parts, in accordance with the coating systems reported in Section 1.5. The first system involves the use of single layer coatings, mostly based on organic or zinc alloy coatings. The second concept involves the use of multilayer coatings in order to combine functional properties of different materials, in particular by using additional protective layers onto current coated products to improve their performance. On the basis of both concepts, various alternative coating systems with promising properties have been proposed in the framework of this study and resulted in patent applications. The various systems are summarized in Table 4.
Coating systems presented in Table 4 a) are based on the use of various Zn alloy layers with presumed advanced properties compared to standard galvanized products. On the basis of high-temperature phase diagrams found in the literature data, the use of Zn alloys such as Zn-Mn and Zn-Co is likely to increase the melting temperature of the coating and thereby to reduce or avoid the susceptibility to LME during direct hot stamping, as reported in Section 1.5.1. In fact, melting temperatures of 567 °C and 740 °C can be obtained with the use of 1 wt.% Co and 23 wt.% Mn respectively, compared to a melting temperature of about 420 °C for pure Zn. In addition, higher anticorrosive properties may be achieved by the use of Zn alloys compared to pure Zn layers, depending on the alloying element [95, 96]. In the case of Zn-Co alloy coatings, more stable corrosion products were found to form compared to pure Zn coatings, which permits to increase anticorrosive performance [97, 98]. However, metallic Co has an electrochemical potential (-0.28 V/SHE) higher than that of both Fe and Zn metals (-0.44 V/SHE and -0.76 V/SHE, respectively), which means that the cathodic corrosion protection of steel is likely to decrease by the use of Co as alloying element for Zn compared to pure Zn coatings. On the other hand, higher anticorrosive properties were reported in the case of Zn-Mn alloy coatings due to the combination of stable corrosion products and to the increase of sacrifical protection of steel due to the lower electrochemical potential of Mn (-1.18 V/SHE) compared to that of Zn and Fe. On the basis of these results, it appears that Zn-Mn coatings show the most promising anticorrosive properties compared to Zn-Co alloys, along with the possibility to increase the melting temperature of the coating compared to pure Zn deposits. Therefore, the present work will be dedicated to the study of single-layer Zn-Mn alloy coatings. The properties of this coating system will be reported in detail in the following part (Section 1.6.2).
The use of an intermediate Zn alloy layer such as Zn-Mn, Zn-Co or Zn-Mn-Co between a pure Zn layer and steel may permit to reduce or avoid the formation of cracks due to LME during direct hot stamping due to the increased melting temperature of the Zn alloy layer compared to pure Zn. The presence of Zn-based coatings most probably provides good anticorrosive protection to steel, in particular cathodic protection. It can be assumed that the use of an additional protective layer above the Zn-based layers and consisting for instance of oxides possibly permits to reduce or avoid evaporation and oxidation mechanisms of the underlying materials, in particular of Zn. The reduction of these effects may be beneficial for ensuring sufficient sacrificial corrosion protection and good compatibility with subsequent processes such as joining and painting .
Multilayer coatings presented in Table 4 b) consist of metallic Mn or Zn alloy layers such as Zn-Mn or Zn-Co alloys separated from the base material with an intermediary layer. The use of the latter materials is expected to provide cathodic corrosion protection to steel. The intermediary layer can consist of an aluminized coating or a ceramic material and is likely to hamper interdiffusion processes between the Zn-based layers and steel, which would result in the increase of the amount of remaining metallic Zn in the coatings after hot stamping, beneficial for sacrificial corrosion protection.
Table 4: Promising single and multi layer coating systems for hot formed automotive components identified in the framework of the present study.
Steel products investigated
The behavior of current coated products during direct hot stamping with press tools specifically designed for this study was carried out on aluminized (Usibor 1500P – AS150, ArcelorMittal) and galvanized (phs-ultraform, voestalpine GmbH) steels. These coating systems are reported in Section 1.5. Zn-Mn coatings were deposited on bare 22MnB5 (MBW 1500, ThyssenKrupp Steel Europe AG) steel plates. This steel grade was used as substrate for Cyclic Voltammetry (CV) studies. Spectral analyses were carried out on the bare 22MnB5 steel by means of Optical Emission Spectrometry for determining its chemical composition (Table 5). It should be noted that all steel products used in the present work have a blank thickness of 1.5 mm.
Study of the electrodeposition of alternative Zn-Mn coatings
This section is focused on the review of experimental conditions for the electrodeposition of Zn-Mn alloy coatings. On the basis of the literature data, promising electrolytes and additives will be defined for the present work. In addition, electrochemical methods used for the study of the electroplating systems and the definition of optimal deposition parameters will be presented. Finally, the experimental setups and parameters used for depositing Zn-Mn layers on 22MnB5 steel plates with various surfaces will be presented.
Electrodeposition of metallic coatings
Electrodeposition of metallic coatings involves the reduction of metal ions contained in aqueous, organic or fused-salt electrolytes, generally by the use of an external power supply in order to monitor redox reactions . In the present study, Zn-Mn coatings were deposited from aqueous electrolytes. Electrodeposition process and material properties were reported by Sotto et al.  to be based on various variables such as the electrolyte composition, the presence of additives, the properties of the working electrode or the electric parameters applied during deposition, as presented in Figure 18.
Cyclic Voltammetry combined with Electrochemical Quartz Crystal Microbalance studies CV and EQCM methods
Cyclic Voltammetry (CV) permits to study electrochemical oxidation or reduction reactions taking place at the interface between an electrode and a solution in a defined range of potentials. The solution generally consists of a solvent containing the dissolved material investigated and a supporting electrolyte salt to improve electrical conductivity and obtain diffusion-controlled reactions . The evolution of the current during a potential sweep is recorded and permits to obtain a potential-current curve called voltammogram, as illustrated in Figure 19.
This process starts with a scan in one direction, starting at the initial potential and going through a potential domain for which reduction or oxidation of the studied material takes place. The potential scan is then switched at a reverse potential, permitting to visualize the product of the electron transfer reaction of the forward scan a second time . This method is particularly beneficial for studying the composition of electrolytes, the influence of additives on the electrochemical behavior of the electrolytic systems and the interesting potential domains for which reduction or oxidation reactions take place . In this work, the CV studies were carried out in various potential ranges in order to monitor cathodic and anodic signals and improve understanding of the electrochemical system.
Electrochemical Quartz Crystal Microbalance (EQCM) permits to record gravimetric response (mass loss and mass gain) taking place at the electrolyte/electrode during electrochemical processes, generally with the simultaneous use of CV studies. This technique is based on the use of a quartz crystal working electrode, whose resonance frequency varies according to the deposition or dissolution of material on the quartz surface. The plot of the rate of mass change on the quartz crystal surface versus the potential, along with the CV data, permits to differentiate mass changes linked to deposition or dissolution processes accompanied by charge transfer at the electrode surface (Faradaic processes) from processes not associated with charge transfer processes (non-Faradaic processes) .
Electroplating cells and experimental parameters
The influence of the electrolyte composition and the applied cathodic potentials on the Mn content of deposits was studied in a classical three-electrode cell using a Radiometer PGZ 100 potentiostat/galvanostat interfaced with a computer. These experiments on small-scale samples permitted to study the electrochemical system and define the optimal electrolyte composition and electric parameters. 22MnB5 steel plates with various active areas comprised between 2 and 17 cm² were used for deposition. The deposition duration was ruled by an electric charge of 44.56 Q cm-2 corresponding to a coating thickness of about 20 μm.
In order to investigate the behavior of the coating materials during heat treatment and hot forming and to evaluate the influence of heat treatment on anticorrosive properties of the alloys, electroplating of Zn-Mn alloys on specimens with larger dimensions was required. A geometry of 200 x 200 mm and a blank thickness of 1.5 mm were defined on the basis of the requirements for the construction of press tools for hot forming. An overview of the electroplating setup is presented in Figure 20. The VersaSTAT 3 potentiostat/galvanostat coupled with a current booster (Princeton Applied Research) were used for deposition. The electroplating cell which was designed for this study consists of four modules assembled with a threaded rod. All modules are separated with seals in order to prevent electrolyte leakage. A more detailed view of the electroplating cell during service is shown in Figure 21. The electroplating cell contains an electrolyte volume of about 2000 ml, in which the working electrodes is vertically immersed. The electric contact is provided with a steel plier insulated with a PVC layer.
Scanning Electron Microscopy SEM
In Scanning Electron Microscopy, a finely focused electron beam irradiates a surface or a microvolume with primary electrons, which leads to the emission of secondary electrons, backscattered electrons, auger electrons, x-rays and other photons of various energies. These signals can be used for studying various material properties such as topography, crystallography or composition. Secondary electrons enhance topographic contrast, contrary to material contrast for backscattered electrons. The analysis of the x-radiations permits to provide qualitative identification and accurate quantitative elemental information about volumes of about 1 μm³. This technique is called Energy Dispersive X-ray Spectroscopy (EDS) and is generally coupled to SEM for chemical analyses. A considerable advantage of this technique for the present work is the possibility of obtaining elemental distributions on surface maps, in particular for cross-sectional studies of Zn-Mn coatings post to heat treatment [151, 152].
The morphology and the composition of the surface of the coatings prior to and post to heat treatment were investigated by means of SEM (VEGA3, Tescan) and coupled to an EDS analyser (Nano, Bruker) for chemical composition analyses. The chemical compositions were determined on the basis of sets of at least 3 measurements. An accelerating voltage of 20-30 kV was used.
The study of the microstructure and the composition of Zn-Mn coatings on cross-sections post to heat treatment was carried out on a Field Emission Gun Scanning Electron Microscope (FEG-SEM) (MIRA3, Tescan) coupled to an EDS analyser (Nano, Bruker). The mean values of the chemical composition of Zn-Mn coatings in cross-sections were determined on the basis of sets of at least 6 values. An accelerating voltage of 15 kV was used. Semi-quantitative values were obtained after Phi-Rho-Z correction for chemical analyses.
Table of contents :
1. State of the art about coating systems for press-hardened parts
1.1 Hot stamping in the automotive industry
1.1.1 Automotive bodies
1.1.2 Hot stamped body parts
1.2 Press-hardening process
1.3 Direct and indirect hot stamping processes
1.4 Challenges in corrosion protection of press-hardened steel parts
1.4.1 Corrosion of car bodies
1.4.2 Corrosion protection of hot formed components
1.4.3 Assessment criteria and anticorrosion performance of current coatings for hot formed components
1.5 Current coating products for hot stamped body parts
1.5.1 Galvanized coatings
1.5.2 Aluminized coatings
1.5.3 Further protective coating systems
1.5.4 Performance of commercial coatings for the hot stamping application
1.6 Alternative coating systems for press-hardened parts
1.6.1 Identification of promising alternative coatings
1.6.2 Properties of alternative Zn-Mn alloy coatings
1.6.3 Deposition process of Zn-Mn coatings
2 Aim and experimental approach of the present work [EN + FR]
3 Materials and experimental methods
3.1 Steel products investigated
3.2 Study of the electrodeposition of alternative Zn-Mn coatings
3.2.1 Electrodeposition of metallic coatings
3.2.2 Cyclic Voltammetry combined with Electrochemical Quartz Crystal Microbalance studies
3.2.3 Electroplating cells and experimental parameters
3.3 Materials characterization
3.3.1 Global procedure
3.3.2 Metallographic characterization
3.3.3 Crystallographic characterization
3.3.4 Scanning Electron Microscopy SEM
3.3.5 Atomic Absorption Spectroscopy AAS
3.3.6 Focused Ion Beam FIB
3.3.7 Transmission Electron Microscopy TEM
3.3.8 Electron Backscatter Diffraction EBSD
3.3.9 Glow Discharge Optical Emission Spectroscopy GDOES
3.3.10 Raman Spectroscopy
3.3.11 Anticorrosive properties
3.4 High temperature processes
3.4.1 Heat treatment in air
3.4.2 Heat treatment under protective atmosphere
3.4.3 Experimental conditions during hot stamping
4 Deposition and characterization of as-deposited Zn-Mn coatings
4.1 Study of the Zn-Mn electroplating system
4.1.1 CV and EQCM studies for Zn-Mn deposition
4.1.2 Definition of optimal electroplating solutions and electric parameters
4.2 Deposition of Zn-Mn coatings on large-scale plates
4.2.1 Homogeneity of Zn-Mn coatings deposited on large-scale plates
4.2.2 Deposition of Zn-Mn alloy coatings on large-scale plates
4.3 Materials characterization of as-deposited Zn-Mn coatings
4.3.1 Chemical composition of Zn-Mn coatings
4.3.2 Microstructural properties of Zn-Mn coatings
4.3.3 Crystallographic properties of Zn-Mn coatings
4.3.4 Anticorrosive properties of Zn-Mn coatings
4.3.5 Overview of the materials properties of electrodeposited Zn-Mn alloys [EN + FR]
5 Characterization of Zn-Mn coatings post to heat treatment
5.1 Methodical and experimental procedure
5.2 Characterization of Zn-Mn coatings post to heat treatment in air
5.2.1 Heat treatment to 700 °C in air
5.2.2 Heat treatment to 900 °C in air
5.3 Characterization of Zn-Mn coatings post to heat treatment in a protective atmosphere
5.3.1 Crystallographic properties of Zn-Mn coatings post to heat treatment in a protective atmosphere
5.3.2 Surface morphology of Zn-Mn coatings post to heat treatment in a protective atmosphere
5.3.3 Microstructural and anticorrosive properties of Zn-Mn coatings post to heat treatment in a protective atmosphere
5.4 Suitability of Zn-Mn coatings for high temperature processes in air and in a protective atmosphere [EN + FR]
5.4.1 lnfluence of the austenitizing atmosphere on the coating behavior
5.4.2 Behavior of Zn-Mn coatings during austenitizing in a protective atmosphere
6 Hot stamping experiments on commercial and alternative Zn-Mn-coated steels
6.1 Design and startup of press tools with current coated products
6.1.1 Simulation of hot stamping and design of press tools
6.1.2 Startup of the press tools with real specimens
6.1.3 Hot forming behavior of aluminized coatings
6.1.4 Hot forming behavior of galvanized coatings
6.2 Direct hot stamping experiments with Zn-based coatings
6.2.1 Hot stamping of galvanized steel
6.2.2 Hot stamping of Zn-Mn-coated steel
6.3 Suitability of Zn-Mn coatings for direct hot stamping [EN + FR]
7 Summary and outlook
8 Résumé et perspectives
Appendix 1: Further protective coating systems
Appendix 2: Preliminary heat treatment studies of Zn-Mn coatings in air
Appendix 3: Zn-Mn-Fe phase diagrams
Appendix 4: Ellingham diagram
Appendix 5: Comparative AAS and SEM-EDS studies