2．外文资料原文（与课题相关，至少1万印刷符号以上）： Influence of process parameters on tribological behaviour of AA7075 in hot stamping A.Ghiotti?, E. Simonetto, S. Brusch Abstract The paper presents the results of recent experimental and numerical investigations on the adhesive onset in AA7075 hot stamping, in order to evaluate the influence of the material thermal treatment and the main process parameters. Solid graphite lubrication was investigated for both the solubilized and the T6 material conditions by means of strip drawing tests in the temperature range 200–450℃ varying the normal contact pressure up to 10MPa and the sliding velocity up to 50 mm/s. The results show that the friction coefficient presents an initial decrease as the temperature increases, followed by adhesion phenomena at the highest temperatures with material transfer to the dies. Such behaviour was explained by using a micro-scale FE model showing that the local temperature in proximity of the roughness peaks may locally influence the material strength causing the adhesive wear onset. Keywords: Hot stamping ; Aluminium alloys; AA7075; Graphite; Adhesive wear 1. Introduction The continuous demand of lightness and performances in many sectors of the transport industry, such as aerospace and automotive, is driving the attention of designers and manufacturers towards the use of aluminium alloys, not only limited to bulk components manufactured by casting or forging, but also for structural complex-shaped parts obtained with sheet metalforming processes [1]. Several aluminium series, i.e. the AA5xxx, the AA6xxx and the AA7xxx [2], present me- chanical (high strength-to-mass ratio) and chemical properties (good corrosion resistance and weldability) potentially of great interest for the above-mentioned applications, but at the same time they are often difficult to be shaped with the traditional technologies due to high forming loads, springback and reduced formability [3–5]. Following the evolution in the technological domain of high strength steels stamping [6], the introduction of high temperature process routes in aluminium sheet stamping have demonstrated to be promising in several advanced applications, with a substantial elimination of springback phenomena and noticeable increase of formability [7]. These applications have also demonstrated that friction and wear can become critical aspects due to the tendency that the aluminium has to adhere to the dies since the early steps of the process with consequent dramatic galling [5]. Furthermore, apart for the traditional physical factors, such as the tool geometry and surface conditions, friction and wear at high temperature processes are known to be deeply influenced also by chemical as well as process-dependent aspects, such as the sliding velocity, temperature and contact pressure [7]. A survey of the scientific literature reveals that the tribological investigations on aluminium hot stamping are broadly focused on the effects of the process parameters such as the temperature [8], pressure and sliding velocity [9], as well as the analysis of the combination of different lubricants and lubrication parameters to reduce the frictional phenomena [10–12]. In ball tests carried out on AA2017 up to 450℃ [13], the material transfer was ascribed to the mechanical interaction and compaction of loose aluminium debris on existing surface features, but no concerning about the debris formation was given. Additional oxidation phenomena at high temperature can make the situation even worse, due to the mix of the oxides with aluminium particles, and enhance the visco-plastic flow of the aluminium alloys [14]. Conversely, little attention has been paid to provide a phenomenological evaluation of the phenomena that rule the starting of the adhesion. Further lacks can be found in the analysis of the influence that the material conditions (i.e. the heat treatment, microstructure, etc.) may have on the frictional and wear behaviour of aluminium alloys,Das et al [15] .Investigated the effects of static and dynamic ageing on wear and friction behaviour of AA6082 alloy finding that the metallurgical conditions of the material to-be-formed can have an influence on the wear rate, but the study was limited to relatively cold conditions, with testing temperatures lower than 140℃ that is still far for the condition of hot stamping. With regards to the AA7xxx series, the studies are limited to the aspects related to the material flow stress [16] and mechanical properties of the hot stamped part [17], but no investigations on the tribological aspects can be found yet. The present research work aims at investigating the influence that the material heat treatment may have on the frictional and adhesive wear behaviour in the case of AA7075 hot stamping. In particular, the focus is on the comparison between the common T6 heat treatment condition and the solubilized condition when deformed at high temperature with different process parameters, namely the normal pressure, the sliding velocity and the temperature. The paper is organized into three parts: after a brief introduction of the industrial reference industrial case, the materials object of the investigation are described. The second part deals with the description of the laboratory tests and experimental plans. Finally, the results of the experiments are discussed with the support of the numerical simulation of the tests to estimate the thermal and mechanical phenomena at the micro-scale. 2. Reference industrial case The reference industrial process is the hot stamping of the AA7075 aluminium alloy for aerospace and automotive applications. Different examples of hot stamping cycles can be found in literature [18], where the main difference is the time the blank is kept at high temperature (over 480℃ in an electric or gas furnace) to obtain the solubilization of the alloying elements (up to 600s). Then, after transferring to the cold dies [7,18], the stamping step is performed using a ram speed that can range from 5 mm/s up to 50 mm/s, in order to keep the blank temperature as closed as possible to the temperature of 350–450℃. Finally, the part is held for 15s to ensure the sheet quenching. Typical applied pressures are in the range of 10–15MPa with sliding velocities in the range of 5–30 mm/s [19]. Finally, the part is age hardened to obtain the T6 state at 120℃for 24h [20]. In each stamping cycle, the sheet is lubricated with specific lubricants for hot forming processes [21], with the aim at reducing the sliding resistance and possible defects on the part due to the dies wear. 3. Materials 3.1 Sheet metal The commercial aluminium alloy AA7075 was provided in sheets with a thickness of 2.0( ± 0.1) mm in the T6 state. Table 1 shows the nominal chemical composition of the alloy and the mechanical characteristics in the as-delivered conditions. The surface roughness Sa of metal sheets in the as-delivered condition was measured by means of a 3D surface profilometer Sensofar? Plu Neox equal to 0.60( ± 0.10) μm. Fig. 1 shows the topography of the test specimen measured in the unlubricated (area z1) and lubricated conditions (area z2). Table 1 Nominal chemical composition (wt%) and mechanical characteristic of the AA7075. Fig. 1. Specimen surface topography when unlubricated (area z1) and lubricated (area z2) 3.2 Tool steel The tool steel grade is the EN X38CrMoV5-1 alloyed steel whose nominal chemical position is reported in Table 2. The tool steel was quenched and tempered in order to increase its surface hardness to a final value of 51( ± 1) HRC, consistent with the characteristic of the tools used in the hot stamping processes of high strength aluminium alloys. The surface of the tool was manufactured to obtain a final surface roughness Sa equal to 0.019( ± 0.005) μm, verified with the 3D profilometer. Fig. 2(a) shows the die shape and the topography of the sliding surface, while Fig. 2(b) shows the mean roughness along the x and y directions. The direction of the surface texture was realized according to the reference industrial conditions. Table 2 Nominal chemical composition of the EN X38CrMoV5-1 steel (wt%). Fig. 2. (a) Surface topography of the EN X38CrMoV5-1 die, (b) surface profile along the main axes. 3.3 Lubricant The lubricant, selected according to the industrial practice, is the commercially available Bonderite? L-GP Aquadag, which consists of powder graphite dispersed in water available in the form of a water- based thixotropic gel.The 90% of the graphite particles has a maximum size lower than 1 μm. When applied to the metal sheet at temperatures ranging from 93℃ up to 177℃ it allows the formation of a lubricant thin film that perfectly adheres to the surface. For the optimal usage, the lubricant is diluted in demineralized water in a percentage of 15% and is typically applied by spraying technique [18]. Fig. 1 shows both the lubricated samples and the topography of the lubricated area (area z2 of the specimen). The application of the lubricant does not modify the peaks height of metal sheet sample since the sprayed liquid tends to fill the surface valleys. 4.Experimental 4.1 Experimental procedure In order to investigate the effects of both the material heat treatment and the process parameters on the onset of adhesive wear in AA7075 hot stamping, different material and tribological tests were carried out. Hot Hardness Tests (HHT) were first carried out on aluminium alloy specimens in the T6 hardening state (as delivered conditions) and just after the solubilization heat treatment to check the variation of material properties and to assess the uniformity of the mechanical properties over the metal sheets. Then, High Temperature Tensile Tests (HTTT) were used to obtain the flow stress curves under different heat treatments and for the calibration of the constitutive parameters of the Finite Element (FE) numerical models used to investigate the contacts between the material and the dies at the microscale. Finally, Hot Flat Strip Drawing Tests (HFSDT) were performed in the two different heat treatment conditions to investigate the influence of the main process parameters, namely the temperature, the pressure and the sliding velocity. The details of each experimental set up and experimental plan are explained in the following sections. 4.2 Hot Hardness Tests (HHT) Rockwell surface hardness test at high temperature were performed using an Instron-Wolpert Rockwell 2000 hot hardness tester, equipped with an electrical furnace that allow performing tests up to 750℃. The tests were carried out on 10.0×10.0×2.0 ( ± 0.1) mm samples both in the T6 and solubilized conditions. HR 15W tests were performed using a 1/8′′ steel ball with a total load of 15 kgf in five temperature conditions, namely room temperature, 200( ± 2)℃, 250( ± 2)℃, 300( ± 2)℃ and 350( ± 2)℃; tests at higher temperatures were not possible due to the dramatic decrease of the material strength. The loading cycle consisted in the application of an initial load F0 for 5s, to recover the elastic deformation of the indenter, followed by the application on an additional load F1 for 10s. After every test, the indenter was cleaned using diethyl ether. Table 3 shows the details of the testing parameters. Table 3 Experimental plan for the HHT. 4.3 Hot Tensile Tests (HTT) Tensile tests at high temperature were performed on dog bone samples [22], having a gauge length of 50.0( ± 0.1) mm, a width of 12.5( ± 0.1) mm and a thickness of 2.0( ± 0.1) mm, laser-cut from the as delivered sheets. The specimens were tested using a MTS-322 50kN dynamometer, equipped with a 30 KW high frequency generator and a frontal inductor to heat up specimens. The temperature was controlled in closed-loop during the overall tests thanks to a type-K thermocouple spot welded on the central part of the specimen gauge length. The thermo-mechanical cycle consisted of a heating step with a heating rate of 30( ± 1)℃/s up to the testing temperatures, respectively 200( ± 5)℃, 250( ± 5)℃, 300( ± 5)℃, 350( ± 5)℃, 400( ± 5)℃ and 450( ± 5)℃, followed by an holding time of 10s to allow a proper temperature homogenization and the final application of the tensile deformation till rupture with a strain rate equal to 0.1 s?1. Table 4 shows the details of the testing parameters. Table 4 Experimental plan for the HTT. 4.4 Hot Flat Strip Drawing Tests (HFSDT) Flat Strip Drawing tests at high temperature were carried out on the AA7075 specimens shown in Fig. 1, both in the as-delivered and solubilized conditions. Each specimen was lubricated by means of a specifically developed spraying set-up to make the application reliable and repeatable for all the specimens [25]. Before the deposition each specimen is heated up to 110( ± 3)℃ by using short wave infrared lamps, so, once the lubricant is sprayed, the water component evaporates, and the graphite element adheres to the surface. The topography of the coated specimen in the area identified as z2, where a quantity of lubricant equal to 1.5 ( ± 0.2) g/m2 was deposited. Fig. 3. (a) HFSDT apparatus; (b) scheme of the load and thermal cycles applied during the test. Fig. 3(a) shows the hot strip drawing tester, whose description was already presented in [23,24]. The specimen is clamped on a hot table and made to slide against the tool with a controlled normal force FN measured by means of a load cell. The contact surface of the tool was designed equal to 336 mm2 with a peripheral fitting radius of 2.0(±0.1) mm (see Fig.2). Being the contact only on one side of the specimen, the friction coefficient is calculated according to: μ=FT/FN (1) where FN and FT are respectively the normal force applied by the die and tangential force measured during the specimen
工艺参数对AA7075热冲压摩擦性能的影响外文文献翻译、中英文翻译
