The ring compression test as simulation test for the investigation of friction in hot metal forming

Forces and flow of material in metal working decisively depend on friction between workpiece and tool, which can be described by the friction factor m. For investigation of the influence of friction in hot forming of steel and for measurement of the friction factor, the ring compression test is used as simulation test. The influences of material, reduction, strain rate, temperature, and lubricant are discussed.     

基于圆环压缩和挤压–模拟法的Zr-4合金塑性成形摩擦因子测定

采用圆环压缩法和挤压–模拟法测定Zr-4合金有润滑条件下的摩擦因子,讨论了2种方法所测定摩擦因子存在差异的原因。研究结果表明,在模具（砧面）粗糙度Ra = 0.6 μm、实验温度700～800 ℃的条件下,采用圆环压缩法获得的Zr-4合金与模具的摩擦因子为0.18～0.27,摩擦因子随实验温度的升高而增大。挤压温度为750 ℃时,采用挤压–模拟法获得的热挤压平均摩擦因子为0.35。测试结果存在较大差异的原因,是由于挤压过程润滑剂的剪切速率较圆环压缩实验大得多,且挤压过程中润滑剂所受压应力约为圆环压缩实验中的两倍,从而导致润滑剂黏度的增大,表现为摩擦因子较高。圆环压缩法获得的摩擦因子更适合于Zr-4合金的锻造等热加工工况。      

固体润滑助剂对镀铝锌耐指纹钢板表面摩擦特性的影响

在常温及高温条件下,利用表面性能分析仪和拉延设备考察了三种固体润滑助剂（聚乙烯润滑助剂、聚四氟乙烯润滑助剂及聚乙烯/四氟润滑助剂）对热镀铝锌耐指纹钢板表面摩擦特性的影响,并采用三维视频显微镜分析了镀铝锌耐指纹钢板表面划痕状态及深度。结果显示,在常温条件下,固体润滑助剂的加入不仅降低了耐指纹钢板表面的动摩擦因数,而且还提高了耐指纹钢板表面的抗磨损性和加工成型性。在高温条件下,聚乙烯润滑助剂软化导致耐指纹钢板表面润滑性能降低,并影响其加工成型性。聚四氟乙烯及聚乙烯/四氟润滑助剂的软化点较高,在高温下能够保持润滑粒子表面硬度及润滑性能,从而显著提高耐指纹钢板在高温下的表面润滑性能。     

High Temperature Tribological Studies on Surface Engineered Tool Steel and High Strength Boron Steel

The popularity of hot sheet metal forming processes in the recent years has necessitated research efforts to improve tool life and control the friction level during hot forming operations. In this work, the tribological properties of tool steel and ultra high strength boron steel (UHSS) pairs at elevated temperatures have been studied by using a special hot sheet metal forming test rig that closely simulates the conditions prevalent in the real process. This test involves linear unidirectional sliding of a preheated UHSS sheet between two tool steel specimens where new workpiece material is continuously in contact with the tool surface. The study is aimed at investigating different surface treatments/coatings applied on either the tool or sheet surface or on both. The results have shown that it is possible to control the coefficients of friction through surface treatments and coatings of the tool and workpiece materials. The application of a coating onto the sheet material has a greater influence on the friction compared to changing the tool steel surface. After running‐in, the investigated tool steel variants show almost similar frictional behaviour when sliding against the same sheet material. Although coating the UHSS sheet reduces friction, it abrades the tool surface and also results in transfer of the sheet coating material to the tool surface.     

锆合金热挤压用防护润滑剂的试制与性能

为满足锆合金热挤压时的润滑与防护需求,试制了一种锆合金热挤压用防护润滑剂,主要成分包括有机硅树脂、低软化点玻璃粉、氧化铝粉、二硫化钼、石墨粉、滑石粉、云母粉等。实验温度为700～800 ℃时,采用圆环压缩法测得涂覆有润滑剂的Zr-4合金摩擦因子为0.19～0.25,润滑效果良好。将有润滑剂防护的锆合金分别加热至700、800和900 ℃并保温1 h,未发生明显氧化,热防护性能良好。测定了有、无润滑剂条件下Zr-4合金和H13模具钢的界面接触温度随接触时间的变化曲线。当Zr-4合金和H13钢的初始界面温度分别约为700 ℃和350 ℃时,无润滑剂时Zr-4合金表面温度达到稳定的时间为7.7 s,界面换热系数由250 W·m?2·℃?1增大至2700 W·m?2·℃?1;有润滑剂时Zr-4合金表面温度达到稳定的时间延长至12 s,界面换热系数由131 W·m?2·℃?1增大至1900 W·m?2·℃?1。这表明该润滑剂具有较好的高温热障性能。      

Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

混杂短纤维增强无石棉摩擦材料的制备及摩擦磨损性能

采用热压成型结合180℃/8 h固化热处理工艺制备混杂短纤维增强无石棉摩擦材料,通过正交实验优化热压工艺参数。在JF150D定速式摩擦实验机上测试材料的摩擦磨损性能。结果表明:实验制备的混杂短纤维增强无石棉摩擦材料的密度为2.1~2.21 g/cm3,洛氏硬度为62.2~68.0,摩擦过程中摩擦因数在0.37~0.43之间变化,随温度升高先增大后减小,磨损率为0.1~0.54×10 7cm3/(N.m)。制备摩擦材料的最优热压工艺参数是:热压温度为155℃,压力为18 MPa,热压时间为15 min。摩擦材料的摩擦磨损机理以粘着磨损和磨粒磨损为主。     

一体成形电感密度分布不均匀问题的研究与改进

随着信息技术的发展,电感行业蓬勃发展。一体成形电感因其诸多优点,使用尤为广泛。但是其在压制过程中,由于粉体摩擦、压力损失等问题,会出现密度分布不均匀的问题,直接影响了元件的使用性能。而温压成形工艺具有诸多优点,如压坯密度和烧结密度高、压坯强度高、脱模力低、弹性后效小,因此该工艺在近年来备受关注。通过研究温度、压力和润滑剂对电感密度的影响规律,以解决一体成形电感密度分布不均匀的问题,继而通过计算电感整体密度与局部密度差值的方法对成形工艺进行了评价,最后得到了较为理想的温压参数:即在150℃下采用质量分数为0.3%的Apex润滑剂,在800 MPa压力下压制,可以制得密度达到7.23 g/cm~3、局部密度差值为0.02 g/cm~3的一体成形电感。     

7075铝合金板材热冲压成形中的高温摩擦

采用自制的板带高温摩擦试验机模拟实际固溶–冲压–淬火一体化热成形工艺下7075铝合金的高温摩擦过程,分别对上下摩擦头进行冷却和加热以模拟实际热冲压过程对模具和压边圈的冷却和加热,分析了下模加热温度、法向载荷和滑动速度对7075铝合金摩擦行为及磨损机理的影响。结果表明:铝合金摩擦系数随着下模加热温度的升高而增大,磨损机制由300 ℃时的黏着磨损转变为500 ℃时的黏着磨损、氧化磨损和磨粒磨损;施加法向载荷越大,摩擦系数越大,不同载荷下磨损机制均为黏着磨损及轻微的磨粒磨损,且随着载荷增大,黏着磨损程度有所加深;高滑动速度导致了磨损表面局部氧化物的生成,使摩擦系数随着滑动速度增大而减小,滑动速度为30 mm·s?1时,磨损机制主要是氧化磨损、磨粒磨损和黏着磨损。      

Influence of the lubricant on temperature distribution in the forging dies

Experimental and theoretical analysis of the influence of the lubricant on the temperature distribution in the tool during forging is presented. Finite element solution of the general diffusion equation is used to calculate the heat transfer contact between the die and the hot workpiece and during the interpass times. The results of calculations are compared with the results of measurements carried out using thermocouples inserted in the die. Two ways of modelling heat transfer through the contact surface are considered. The first treats a lubricant as a separate layer with the relevant thermal properties. The second assumes a temperature discontinuity at the interface and introduces the heat transfer coefficient. Insulating properties of various lubricants are compared. White-water lubricant shows the best insulating properties. The values of the heat transfer coefficient obtained from the inverse analysis vary from 2600 (without lubrication) to 1500 W/m²K (white lubricant).     

Role of Tool Shoulder Diameter in Friction Stir Welding: An Analysis of the Temperature and Plastic Deformation of AA 2014 Aluminium Alloy

The influence of tool shoulder diameter and its rotational speed on the high temperature plastic deformation of the material during friction stir welding of AA 2014 aluminum alloy is investigated, using the principles of hot working. The soundness of weld and defect formation are analyzed using the Zener–Hollomon parameter ‘Z’ to describe the high temperature plastic deformation behaviour of material, under the simultaneous influence of temperature and strain-rate. The observed hot deformation behaviour is correlated with the deformation processing map for the first time. At a given rotational speed, the volume of shoulder driven flow reduces with increasing shoulder diameter.     

磁性磨粒热压烧结过程的温度场模拟

针对以Fe粉和Al2O3粉为原材料的两相复合磁性磨粒这一研究对象,建立了磁性磨粒热压烧结过程的传热模型,通过计算机模拟方法对模具温度场进行分析,从而优化热压工艺。模拟结果表明:炉体的升温速率对模具的温度场影响较大,同时磨粒在模具中的位置不同其温度场分布也不同。综合磨粒的制备效率和实验设备的要求,本实验条件下应采用在低温区为10～15℃/min、高温区为5～10℃/min的升温速率,并将磨粒置于模具中的z5区域,以降低热压成形过程中磨粒的轴向温差,改善磨粒的物理力学性能。     

热处理工艺对铝硅镀层热成形钢组织性能的影响

为深入理解不同热处理工艺参数对铝硅镀层热成形钢组织性能的影响规律,主要研究了加热温度和保温时间对铝硅镀层热成形钢的硬度、微观组织、镀层厚度和镀层成分的影响。结果表明,当加热温度不大于 900 ℃ 时,铝硅镀层热成形钢的硬度随着保温时间的增加而增加;当加热温度大于 900 ℃ 时,铝硅镀层热成形钢的硬度随着保温时间的增加而下降。当加热温度为850~930 ℃,保温时间为 4、8 min 时铝硅镀层热成形钢的微观组织在模具淬火冷却过程中均转化成为马氏体。在相同加热温度下,铝硅镀层热成形钢合金层的厚度随着保温时间的增加而增大,当加热温度升高至 930 ℃ 时,镀层因氧化而挥发严重,导致镀层变薄,所以铝硅镀层热成形钢的加热温度应控制在 930 ℃ 以下。保温温度升高、保温时间增加导致元素扩散显著,聚集的硅元素含量和面积由于其不断向四周扩散而降低。同时铁元素大量扩散到镀层中,镀层中铁元素含量增加显著。高温下,镀层发生明显的氧化反应,氧化反应促进了微孔洞的形核和长大。     

Effect of Single and Duplex Thin Hard Film Coatings on the Wear Resistance of 1.2343 Tool Steel

In aluminum extrusion process, tool steels used as die materials suffer from mechanical, thermal and tribological stresses causing plastic deformation, wear and heat checking during hot metal flow. Thin hard film coatings like TiN, (Ti,Al)N and CrN are preferred in order to improve the surface properties of the tools. These coatings can reduce the friction force, minimize the adhesive interaction between the die and billet pairs and decrease the plastic deformation of the tool. In this study, effect of single (CrN and AlTiN) and duplex (CrN + AlTiN) thin hard films on the hot wear behavior of DIN 1.2343 tool steel was investigated. Wear tests were performed both at room temperature and elevated temperature to simulate the conditions of aluminum extrusion process. Based on the evaluation of coefficient of friction values, specific wear rates and worn surface examinations, the duplex coating, which had the best performance in the RT wear test showed good resistance to high temperature wear under the simulated aluminum extrusion conditions.     

Densification of porous powder preforms during hot forging. I. Processing lubricant for transverse hot forging

Conclusions Use of lubricants 2, 3, 4 and 5 (Table 1) in transverse hot densification does not prevent buildup formation on the tool. By contrast, the specially developed molybdenum disulfide and graphite base lubricant 1 [13] exhibits excellent friction- and wear-reducing characteristics. With this lubricant it is possible to produce high-density P/M parts of considerable length by transverse hot forging without buildup formation on the tool and with little nonuniformity of deformation in the surface layers of the material.Translated from Poroshkovaya Metallurgiya, No. 1(289), pp. 22–25, January, 1987.     

玻璃润滑剂在钢热挤压工艺中的应用

热挤压成形工艺中,坯料与工模具之间的摩擦是影响模具使用寿命、产品质量的一个重要因素,因此,润滑剂的选择十分重要。介绍了热挤压工艺中玻璃润滑剂的起源,并对玻璃润滑剂的特性、种类及其使用方法进行了详细的阐述。分析了玻璃润滑剂在热挤压工艺中的润滑机理,玻璃润滑膜的厚度对热挤压产品的表面质量会产生较大的影响。玻璃润滑热挤压工艺十分适合不锈钢、耐热钢、镍基合金无缝钢管的生产。     

On the Sensitivity of a Tool–Workpiece Thermocouple to Chemical Composition and Microstructure

Meeting the increasing demands on part quality and profitability of manufacturing processes despite difficult-to-machine materials is only possible with a deep understanding of the process. Herein, knowledge about the process temperature is of critical importance since it affects the material properties, such as hardness or forming behavior, as well as the chemical and physical interactions between the tool, workpiece, and lubricant. A proven thermoelectric method of temperature measurement in machining, forming, and blanking is a tool–workpiece thermocouple. Herein, instantaneous measurement of the temperature development is allowed in this setup during the manufacturing process in situ at the contact area of the tool and workpiece. The accuracy of this method is dependent on the calibration of the thermocouple, for which the Seebeck coefficients of the tool and workpiece material have to be determined. Usually, material samples from different batches are used for this purpose, although the resulting measurement errors due to slight changes in material properties are hardly known. The effects of small changes in the chemical composition and the transformation of the crystal lattice due to hardening on the Seebeck coefficient are investigated for the first time to allow precise quantification of the measurement error resulting from the calibration process.     

Evaluation of deterioration mechanisms and roll life of different roll materials

Evaluation of wear loss of rolls in operation is very difficult since the rolling conditions are hard to keep under control. To solve this problem a high temperature test rig was designed. A conventional roll material was compared to a hot work tool steel. The main wear mechanisms were different. The conventional roll grade was mainly subjected to thermal fatigue as could be expected. The hot work tool steel was mainly subjected to abrasive wear, and no evidence of thermal fatigue was found. The rate of material loss was 6 times higher in the hot work tool steel, but when the deterioration caused by thermal fatigue of the cast iron is taken into consideration, the deterioration rate was of the same order in both materials. The friction between the hot steel and the roll materials were of the same order. The frictional value should be a measure of the wear rate. Since the wear mechanisms are not equal, the coefficient of friction found in the test rig is only qualitatively related to roll wear.     

A Study of Friction in Non‐Lubricated High Temperature Steel Processing

The effect of friction on the cost of steel rolling and the overall quality of the strip has been overlooked in favour of more controllable aspects, such as temperature and microstructure. When friction is considered, predominantly empirical relationships are employed that have been developed in smaller scale experiments and are not necessarily applicable to the industrial scale. An advancement to the adhesion theory of friction has been developed by Straffelini that links friction with material properties, which compares well with pin‐on‐disk experiments at room temperature using metallic tribo‐pairs. For the purposes of creating conditions analogous to hot rolling of steel, a tool steel (100Cr6) dowel and an oxidised steel sample have been used in a reciprocating friction tester to create a relatively controlled environment in which to study friction at high temperature. The simplified adhesive theory was found to agree well with these experimental results, demonstrating that this theory can be applied to a broader range of conditions than previously tested against.     

Vibration-Induced Friction Temperature Field Analysis Under Different Working Condition of Heavy Load Bolt

Based on thermal-stress coupling analysis and material properties studies,the vibration-induced friction process of a special used heavy-load bolt has been simulated.The temperature distribution of the bolt has been calculate with heat radiation and heat convection considered.Also a solid lubricant,polytetrafluoroethylene (PTFE) has bee considered to reduce the friction heat effect.The result shows that,there are very strong heat effect while no lubracant considered,the temperature of the bolt joint increased sharply and up to 700℃,which exceed the allowable temperature range of the steel.When PTFE lubricant has been applied,the temperature of the bolt joint increased to 260 ℃,and the bolt steel can by applied in this satuation.These results provide important guidance for the bolt structure modification and material selection.