Bake Hardening Research Articles

Effect of Microstructure on Strain Aging and Hydrogen Embrittlement Behavior of Bake Hardening Steels

The strain aging and hydrogen embrittlement behavior of two bake hardening (BH) steels with different microstructures were investigated in this study. The single-phase BH steel was composed of fully ferrite, while the dual-phase BH steel consisted of ferrite and 3 % martensite. The BH index and aging index (AI) of the two BH steels were measured to compare the effect of microstructure on strain aging behavior. As a small amount of martensite is present in the dual-phase BH steel, the BH index value of the dual-phase BH steel was higher than that of single-phase BH steel, and the AI showed a relatively low value. After the BH steels were pre-strained up to 20% and electrochemically pre-charged with hydrogen, on the other hand, a slow strain rate test (SSRT) was carried out to examine the effect of pre-strain on hydrogen embrittlement behavior. The silver decoration technique and thermal desorption analysis (TDA) were performed to understand the hydrogen embrittlement mechanism from the standpoint of hydrogen trapping. Regardless of pre-strain the dual-phase BH steel had lower hydrogen embrittlement resistance than the single-phase BH steel because martensite formed in the dual-phase BH steel acts as a reversible hydrogen trap site. The hydrogen embrittlement resistance of the two BH steels decreased with increasing amount of pre-strain which increases dislocation density. Based on these results, it can be said that the introduction of martensite and dislocation in BH steels have a negative impact on hydrogen embrittlement resistance.

Effects of semi-solid structure on interface formation of dissimilar aluminum to galvanized steel welds produced by load-controlled Refill Friction Stir Spot Welding

Refill Friction Stir Spot Welding (Refill FSSW) allows welding dissimilar materials providing excellent bonding between both parts. On this work, a multi-scale analysis of load-controlled Refill FSSW was performed to analyze dissimilar AA6016-T4 and Zn-coated DX56D steel joints. These were produced using optimized process parameters and analyzed in both as-welded and bake-hardened conditions. During the process, fusion and subsequent dispersion of Zn favored the formation of a semi-solid structure characterized by an intense microsegregation. Therefore, incipient melting of Zn-rich phase followed by eutectic reaction was observed. The presence of liquid phases along the grain boundaries led to a complex relationship between mechanical properties, microstructure and processing variables. Joints with enhanced mechanical properties were produced by limiting the growth of intermetallic compounds (IMC) at the interface, which coupled with stir zone (SZ) strengthening due to Zn dispersion, led to less pronounceable secondary bending effects. The bake hardening process was also found to have a substantial influence on diffusion-dependent mechanisms and, consequently, on the final performance of the welded joint. The results highlighted a great potential of load-controlled Refill FSSW for producing high-strength dissimilar joints in short cycles, which is desirable for applications in the automotive industry.

Bake hardening and uniaxial tensile behavior in a low carbon steel accompanying inhomogeneous plastic yielding

Bake hardening (BH) is a vital part of application-specific steel manufacturing. Previous studies on BH have focused on steels undergoing homogeneous yielding, which critically fails to observe and characterize various interesting phenomena that occur during inhomogeneous yielding. In this study, we investigated the BH behavior observed upon the application of inhomogeneous pre-strains. Two steels of identical composition that exhibited inhomogeneous plastic yielding during uniaxial tensile testing were prepared. One sample was composed of acicular ferrite while the other was composed of annealed polygonal ferrite. Inhomogeneous plastic yielding during uniaxial tensile testing is inevitably accompanied by the formation of a Lüders band (LB). When the LB propagated partially to the gauge section, no BH response (BHR) was observed in either steel during the re-loading of the pre-strained specimens after baking treatment (BT). However, when the LB propagated completely, BHR was exhibited by both steels. Digital image correlation analysis and macro-indentation testing confirmed that in the former cases, inhomogeneous hardening occurred due to inhomogeneous plastic yielding, whereas in the latter case, the inhomogeneous yielding was resolved, and hardening occurred uniformly throughout the gauge section. This influences not only BHR but also the overall uniaxial tensile properties such as tensile strength and total elongation. Nanoindentation and small-angle neutron scattering tests were performed on the pre-strained specimens before and after BT to determine the origin of the hardening effect. The results confirmed that nano-scale clusters/ε-precipitates can be formed at a low BT temperature if a Cottrell atmosphere is induced, thereby improving not only the BHR but also tensile strength.

Effects of Cu additions on the precipitation activation energy and mechanical properties of prestrained Al–Mg–Si alloys

We investigated the effect of different Cu additions (0.1 wt.% and 0.3 wt.%) on the precipitation activation energy and mechanical properties of Al–Mg–Si alloys under different prestraining levels (0%, 3%, and 5%). Tensile tests were performed before and after paint baking (PB). Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) analyses were conducted to explore the clustering and precipitation behavior. Our results show that the precipitation activation energy was reduced by higher Cu addition but increased with prestraining level. Before PB, higher Cu addition significantly improved the elongation and work-hardening behavior, and the improvement was more pronounced in the prestrained alloys than in the nonprestrained alloys. After PB, higher Cu addition slightly increased the bake hardening response (BHR) of the alloy, and this improvement was enhanced in the prestrained alloys. However, only by introducing an appropriate amount of prestraining (3%) could the high-Cu alloy increase the BHR more significantly than the low-Cu alloy. When 5% prestraining was introduced, the high-Cu alloy had no advantage in increasing the BHR. After the introduction of 3% prestraining, a small amount of pre-Q′ phases was additionally formed near the dislocation lines, but the β″ phase was still the main strengthening phase. Higher Cu addition significantly refined the β″ phase and increased its number density, thereby improving the BHR. The formation of pre-Q′ phases was more pronounced in the high-Cu alloy under 5% prestraining, which suppressed the formation of the β″ phase and limited the BHR.

Predicting the effect of strain path on the strain aging behaviour of ultra-low carbon steel

A recently presented model is adapted to predict the effects of strain path change on the strain aging (bake hardening) behaviour of an ultra low carbon (ULC) steel. Samples pre-deformed by rolling were aged and then tested in uniaxial tension at 0, 45 and 90∘ degrees to the prior rolling direction. The results show that aging changes not only the yield strength of the material but also the work hardening rate. The increase in yield strength is interpreted to be dominated by a reduction in mobile dislocation density, this reflecting the classic ideas of Hahn and others. The change in work hardening behaviour is treated as arising from the different role played by solute ‘loaded’ dislocations compared to dislocations generated by straining after aging.

A Materials Science-Based Approach to Finite Element Simulation of Warm-Forming of Al-Mg-Zn Alloys

Warm deep-drawing of pre-aged (under-aged) blanks of 7xxx series aluminum alloys (Al-Zn-Mg) at moderate temperatures of roughly 120–230°C is a promising route for producing parts with considerable geometrical complexity, good paint bake hardening response, and, thus, excellent final mechanical properties. Furthermore, oil-based lubricants can be used, eliminating the need for elaborate cleaning routines. However, finite element (FE) simulation of the process is challenging: time-temperature regimes during coupon testing for material cards should closely follow the real conditions in the press because the material undergoes significant changes at warm-forming temperatures, such as recovery and precipitation/coarsening/reversion of hardening phases. When convective heating is used for Nakajima or tensile testing, heating rates are usually too low to adequately represent real process conditions (where inductive or contact heating may be used). Here we present a method for establishing FE material cards and calibrating the GISSMO damage model using miniaturized tensile specimens for a dilatometer with inductive heating. The simulations are compared with warm deep-drawing experiments of pre-aged 7xxx and good agreement of minimum draw temperature for two alloys is achieved. The findings are discussed with regards to transmission electron microscopy investigations and final mechanical properties published earlier. It was found that warm-forming is suitable to produce complex 7xxx parts with high final strength. Conditions in the press can be represented by using miniaturized tensile specimens and inductive heating for calibration of material cards/damage models.

Improved Implementation of Material Behaviour Change due to Bake Hardening in the Simulation of the Process Chain

Abstract The steadily increasing demands on the crash safety of automobiles with simultaneous weight reduction also mean higher requirements for crash simulation. In addition to manufacturability, the focus in component dimensioning is increasingly on crash properties. In order to improve the predictive accuracy of crash simulation, the individual process steps must be coupled in the simulations. To increase the quality of the predictions, thermal treatments such as curing of the paint, which normally takes place at 170 °C for around 20 minutes, should also be taken into account. In many steels, the so-called bake-hardening effect occurs. This leads to an increase in the yield stress. In this publication, different transformation approaches for the hardening curves will be investigated in order to increase the prediction accuracy in the process chain. An example is used to demonstrate their easy applicability in the simulation of the process chain.

Enhancing yield stress and uniform elongation in an ultrathin packaging steel via controlling dislocation density

Ultrathin sheet steels used in the global packaging industry for lightweight packaging applications have been achieved by an extra cold processing (double reduction) to increase yield stress, but usually at a severe loss of uniform elongation and formability. From an industrial point of view, achieving the synergy of strength and uniform tensile ductility in ultrathin sheets is difficult, owing to the stringent industry demands for low alloying and simple processing routes that are often neglected on a laboratory scale. In this work, we demonstrate that high strength and large uniform elongation of ultrathin sheet steels can be simultaneously achieved at low costs by simply using the standard industrial procedures. An optimized double reduction process, which exploits the beneficiary effects of dislocation density control and bake hardening, was designed. This optimized process leads to an ultrathin sheet steel that shows a significantly improved uniform elongation without sacrificing yield stress, thus offering an efficient way to improve the formability of ultrathin sheets with high yield stress. This enhanced uniform elongation is due to the initially unsaturated dislocation density that allows for the further multiplication of dislocations during plastic straining. The results highlight the important role of precisely controlling dislocations in producing the low-cost ultrathin sheet steels with high strength and high uniform tensile ductility, which are highly demanded for large-scale industrial production.

Microstructure evolution and electrochemical corrosion behavior of Al–Zn–Mg aluminum alloy

Electrochemical corrosion behavior of Sn-containing Al–Zn–Mg aluminum alloy has been studied in detail. The localized corrosion behaviors were studied by electrochemical impedance spectroscopy (EIS) analysis, and the potentiodynamic polarization measurements. The grain structure, grain-boundary microstructure, grain-boundary microchemistry, pitting and intergranular corrosion morphology were characterized and observed using SEM, EDS, TEM, SAED and HRTEM analyses. Based on these tests, the effects of grain-boundary on the corrosion resistance in our Sn-containing Al–Zn–Mg alloys before/after bake hardening were analyzed systematically. Finally, the relationship between chemical composition, microstructure evolution and corrosion behaviour was revealed. The results indicate that the bake hardening process improves the corrosion resistance compared to the pre-aging state. The grain size has little effect on the electrochemical corrosion bahavior.

Bake hardening of 7Mn–3Al-0.2C steel: Influence of intercritical annealing temperature

Bake-hardening behavior in a medium Mn steel was investigated after intercritical annealing at either 680 °C or 760 °C. After 2% prestraining and baking at 180 °C for 20 min, the 760 °C -annealed steel exhibited bake hardening response (BHR) equal to 156 ± 3 MPa, while the BHR of 680°C-annealed counterpart was 60 ± 1 MPa. The BHR difference was attributed to martensite in the 760°C-annealed steel, which generated dislocations near adjacent austenite and ferrite on martensite transformation during quenching and prestraining. Baking enabled pinning of more dislocations by carbon and precipitation of fine carbides by martensite tempering, which could effectively increase the strength. Moreover, baking changed the state of strain incompatibility between the phases and stabilized austenite in the 760°C-annealed steel. As a result, strain hardening rate stayed at a high level, which enhanced ductility.

Effect of silicon content on nucleation and growth of multiscale precipitates in Al–Mg–Si–Cu–Zn alloys for different aging paths

Controlling the precipitation evolution is an efficient strategy for optimization of those properties in Al–Mg–Si–Cu-(Zn) alloys. In this work, we have studied the precipitation evolution and mechanical behavior in Al–Mg–Si–Cu–Zn alloys with different aging routes and Si contents (0.55–1.08 wt%). The results reveal that multiscale precipitates can be formed under a certain aging route in Al–Mg–Si–Cu–Zn alloys, which can simultaneously improve the strength and elongation. Accordingly, the nucleation mechanism of multiscale precipitates in Al–Mg–Si–Cu–Zn alloys was put forward, which provides important guidance on designing the novel Al–Mg–Si–Cu–Zn alloys with high formability and rapid bake hardening response for automotive applications.

Effect of grain size on the static strain aging of a ULC-bake hardening steel

Abstract Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66–15 µm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2–5%) and aging temperatures (50–170°C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.

Effect of Dissolution of η′ Precipitates on Mechanical Properties of A7075-T6 Alloy

The effects of dissolution of the η′ phase by solution treatment on the mechanical properties of A7075-T6 alloy were investigated. Immediately after solution treatment of the T6 sheet at 450 oC or higher, elongation significantly increased and dissolution of the η′ phase occurred. η′ is the main hardening phase. After natural-aging, GPI, which is coherent with the aluminum matrix, was formed and strength increased. When bake hardening after natural-aging was performed, the yield strength slightly increased due to partial dissolution of the GPI and re-precipitation of the η′ phase. In contrast, after solution treatment at 400 oC, there was less elongation increase due to the precipitation of the coarse η phase at grain boundaries and low dissolution of the η′ phase. In addition, when bake hardening after natural-aging was performed, the yield strength decreased due to insufficient GPI, which is the nucleation site of the η′ phase. To promote reprecipitation of the η′ phase, the solution treatment temperature was set to a level that would increase solubility. As a result, the yield strength was significantly increased through re-precipitation of a large number of fine and uniform η′ phase. In addition, to increase the effect of dissolution, a pre-aging treatment was introduced and the bake hardenability can be improved after dissolution.

Giant bake hardening response of multi-scale precipitation strengthened Al-Mg-Si-Cu-Zn alloy via pre-aging treatments

A high-performance Al-1.20Mg-1.60Si-0.20Cu-3.00Zn (wt%) alloy was designed based on the concept of high degree alloying with the main element content exceeding the range found for the 6xxx aluminum alloys. Using the optimized pre-aging (160 °C-6 min) treatment, compared to the 6016 and 6111 alloys, both the plasticity (T4P) and strength (BH) of the alloy were improved. In particular, the bake hardening response was as high as 180.2 MPa, which was more than 2 times higher than those of the 6016 and 6111 alloys. The enhanced performance of the alloy was attributed to micron/nanoscale multi-particle strengthening effects and optimized deformation heat treatments. The micro-scale phases were Mg2Si and Al(Fe,Mn)Si, and the nano-scale phases were GP zones and β"phases. Precipitation kinetics calculations, microstructure quantitative statistics, and strengthening contribution ratio analysis showed that the optimized pre-aging treatment effectively suppresses the negative natural aging effect, reduces the β "phase activation energy, and increases the mean diameter and number density of the GP zones (β"-nuclei) prior to paint baking and the β" phase number density after paint baking, thus improving the effectiveness of the paint baking treatment.

Enhanced strength-ductility synergy achieved through twin boundary pinning in a bake-hardened Mg–2Zn-0.5Ca alloy

Bake-hardening (BH) processing is an efficient way to rapidly increase the strength of solution-treated Mg alloys, which is attributed to GP-zone strengthening or dislocation pinning caused by solute segregation. Here we report a new hardening mechanism of BH in Mg–2Zn-0.5Ca (wt. %) alloy, i.e., twin boundary pinning, which could obtain enhanced strength-ductility synergy, with a flow stress of ∼200 MPa (increased from ∼141 MPa) and an elongation to failure of ∼17% (increased from ∼12%). Microstructure analyses show that the co-segregation of Zn and Ca atoms at 101¯2 twin boundaries can impede twin boundary migration, which effectively suppresses crack propagation and therefore improves the ductility. Meanwhile, the twin boundary pinning obviously increases strength in conjunction with the pinning effects of co-segregated Zn and Ca atoms at dislocations. Such a twin boundary pinning mechanism suggests that BH technique can be generally applied to develop high-performance Mg alloys with enhanced strength and ductility.

Formability study of bake hardening steel and its correlation with microstructure

In the present study, the bake hardening (BH 240) steel sheet’s formability behavior is studied experimentally based on the Nakajima test method. A forming limit diagram is successfully constructed using universal sheet metal forming machine by offline measuring the strains of deformed specimens. In-process strain measurement based on a digital image correlation technique is also performed for a few selected samples to compare the results with manual strain measurement techniques. All the deformed specimens are also characterized to correlate the sample geometries with hardness values and microstructure. It is observed that the hardness value gradually decreases with an increase in sample width for all the deformed specimens up to the width of 150 mm, except for full-width sample of 200 mm. Microstructural analysis reveals that the morphology of ferrite grains changes with the sample geometry in all the deformed specimens. Microstructural characterization at the top surface of the specimen demonstrates that the aspect ratio of grains is maximum for a 25 mm width sample due to uni-axial stretching. It is also noted that grain’s aspect ratio is close to one (minimum) for a 200 mm width sample due to almost equi-biaxial elongation in grains. It is also observed that the aspect ratio of grains obtained from specimen cross-section indicates that the ratio increases gradually from 25 to 200 mm width samples.

Aging Behavior of Intercritically Quenched Ductile Iron

Although extensive aging and strain aging (bake hardening, BH) studies have been carried out on dual-phase steels, the aging behavior of the dual matrix structure (DMS) ductile iron (DI), as a potential way to improve its mechanical properties, has not been addressed until now. This research was designed to study the aging behavior of DI with a ferrite-martensite matrix structure. DMS-DI with a martensite volume fraction of 30% was produced by intercritical austenitizing at 785 °C followed by quenching in water to room temperature. Aging treatments were carried out without pre-straining at aging temperatures of 140, 170, and 220 °C for 2–10,000 min. DMS-DI was investigated by light optical microscopy (LOM) for unaged samples and scanning electron microscopy (SEM) for selected samples after aging treatments. The effect of aging conditions on the mechanical properties were investigated. Microhardness measurements for ferrite and martensite were also examined as a function of aging conditions. The increase in yield strength due to aging was determined. The results indicate that the aging conditions have a small effect on the ultimate tensile strength UTS. It is shown that the yield strength increased to a maximum value of 45 MPa (~11% increase) after aging for particular time, which is found to be dependent on the aging temperature. The peak aging response is followed by a decrease in yield strength, which is observed to be attributed to martensite tempering as confirmed by microhardness measurements.

Reversion of natural ageing and restoration of quick bake-hardening response in Al-Zn-Mg-Cu alloy

The high-strength Al-Zn-Mg-Cu alloy provides much better dent resistance and weight-reduction potential compared to the conventional Al alloys used for the automobile body panels. However, natural ageing (NA) significantly reduces the formability of Al-Zn-Mg-Cu alloy. The reversion of natural ageing has been systematically investigated by hardness test, tensile test, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Substantial reversion of natural ageing and thus hardness decrease occurs immediately upon thermal treatment at 120–210 °C in an Al-Zn-Mg-Cu alloy. Although the hardness of the most reverted state decreases with increasing temperature, the lowest hardness is still higher than the as-quenched state by 30HV. As revealed by the complementary DSC and TEM observations, this is ascribed to the synchronization of the dissolution and the re-precipitation of the solutes in the NA clusters during reversion ageing. Reversion at 180–210 °C for less than 30 s leads to a hardness decline of 40HV. The hardening kinetics during NA after reversion is slower than that during first-time NA due to the reduced vacancy concentration. Artificial ageing at 180 °C for 30 min after secondary NA of less than 24 h induces intensive precipitation of plate-like pre-η phases and a giant strength increase of 188–204 MPa. Potential use of high strength Al-Zn-Mg-Cu alloy in automobile body panels could be realized by appropriate reversion treatment improving the formability and the quick bake hardening response.

Quantitatively Investigate the Effects of Natural Aging on Mechanical Properties and Bake Hardening Behaviors of Al-Mg-Si Alloys

The effect of natural aging on mechanical properties and bake hardening behaviors of Al-Mg-Si alloys was quantitatively investigated by a series of tensile experiments along the rolling direction. The natural aging period is from three days to three months after heat treatment. As the results, within three months, along the rolling direction, 0.2% offset yield strength and ultimate tensile strength respectively increased from 97 MPa to 145 MPa, 210 MPa to 248 MPa. The strain hardening exponent n-value and the increment of yield strength after bake hardening respectively decreased from 0.2804 to 0.2186, 127 MPa to 89 MPa. The percentage elongation after fracture varies from 22% to 24% during natural aging and varies from 13% to 16% after bake hardening. A large amount of detailed data has been given, which quantitatively describes the change in mechanical properties and bake hardening behaviors of Al-Mg-Si alloys during natural aging.

New insights into the properties of high-manganese steel

In the Collaborative Research Centre 761’s “Steel ab initio - quantum mechanics guided design of new Fe based materials,” scientists and engineers from RWTH Aachen University and the Max Planck Institute for Iron Research conducted research on mechanism-controlled material development with a particular focus on high-manganese alloyed steels. From 2007 to 2019, a total of 55 partial projects and four transfer projects with industrial participation (some running until 2021) have studied material and process design as well as material characterization. The basic idea of the Collaborative Research Centre was to develop a new methodological approach to the design of structural materials. This paper focuses on selected results with respect to the mechanical properties of high-manganese steels, their underlying physical phenomena, and the specific characterization and modeling tools used for this new class of materials. These steels have microstructures that require characterization by the use of modern methods at the nm-scale. Along the process routes, the generation of segregations must be taken into account. Finally, the mechanical properties show a characteristic temperature dependence and peculiarities in their fracture behavior. The mechanical properties and especially bake hardening are affected by short-range ordering phenomena. The strain hardening can be adjusted in a never-before-possible range, which makes these steels attractive for demanding sheet-steel applications.