Modelling of the heat input for face-milling processes

Due to the lack of coolant in dry machining processes, there is an increase of heat input into workpieces during the cutting process. As a result, an inhomogeneous temperature distribution occurs, which can lead to problems concerning compliance with critical tolerances, because of distortions of the workpiece. Using the FEM simulation it is possible to calculate and handle these distortions. This paper contains a mathematical model developed to calculate the surface heat flux as input data for FEM simulation. In comparison to former publications, this paper deals not only with single results, but rather with an overview of the results of the past two years.     

氧化物复合活性焊剂对聚焦光束焊接熔深的影响

研究了复合成份活性焊剂对于低碳钢聚焦光束焊接熔深的影响规律以及焊剂增加熔深的原因.结果表明,复合焊剂的配比是熔深增加量的最显著影响因素,并由此对配比进行了优化.在该配比下,复合焊剂增加熔深的效果优于单一成分活性焊剂.采用焊接热循环曲线检测系统,获得母材热影响区某点在焊接过程中的峰值温度.在母材不熔化状态下,比较涂敷焊剂和不涂焊剂时的峰值温度的变化,可知活性焊剂有效地增加了对光束的吸收率;在母材熔化状态下,比较焊剂涂敷量对测量点的峰值温度和熔深的影响规律发现,活性焊剂有改变熔池流动模式的作用.在聚焦光束活性焊接过程中,焊剂导致熔深增加的原因是焊剂对光束吸收率的提高以及焊剂改变了熔池的流动模式.     

An investigation of workpiece temperature variation in end milling considering flank rubbing effect

Better prediction about the magnitude and distribution of workpiece temperatures has a great significance for improving performance of metal cutting process, especially in the aviation industry. A thermal model is presented to describe the cyclic temperature variation in the workpiece for end milling. Owing to rapid tool wear in the machining of aeronautical components, flank rubbing effect is considered. In the proposed heat source method for milling, both the cutting edge and time history of process are discretized into elements to tackle geometrical and kinematical complexities. Based on this concept, a technique to calculate the workpiece temperature in stable state, which supposes the tool makes reverse movement, is developed. And a practicable solution is provided by constructing a periodic temperature rise function series. This investigation indicates theoretically and experimentally the impact of different machining conditions, flank wear widths and cutter locations on the variation of workpiece temperature. The model results have been compared with the experimental data obtained by machining 300M steel under different flank wear widths and cutting conditions. The comparison indicates a good agreement both in trends and values. With the alternative method, an accurate simulation of workpiece temperature variation can be achieved and computational time of the algorithm is obviously shorter than that of finite element method. This work can be further employed to optimize cutting conditions for controlling the machined surface integrity.     

Analysis of the machining accuracy when dry turning via experiments and finite element simulations

Workpiece and tool are subjected to severe mechanical and thermal loads when turning. These loads cause thermal expansions and mechanically induced deflections of the tool and the workpiece. Such deformations induce deviations from the nominal workpiece geometry. In order to decrease these deviations, the cutting condition needs to be optimized prior to actual machining. In this paper, the accuracy of machining when dry turning aluminum is analyzed via experiments and finite element simulations. For this purpose, seven characteristic values were used: the forces, the deflection of the workpiece, the quantity of heat in the workpiece, the temperature distribution in the workpiece, the temperature of the tool, the temperature of the tool holder, and the actual dimension of the workpiece after turning. These experimentally determined results serve in addition as boundary conditions for a 3D finite element model of the workpiece, which calculates the deformations of the workpiece. The continuous removal of material affecting the temperature distribution in the workpiece is considered. The actual dimensions of the workpiece after turning revealed a remarkable influence of the cutting condition used on the accuracy of machining. Differences of up to 116 μm regarding the deviation from the nominal workpiece diameter of 30 mm were observed. The analysis of the machining accuracy reveals that particularly the use of both high cutting speeds and feeds enhances the accuracy of machining when dry turning aluminum.     

空气介质中电火花表面强化的温度场有限元模拟分析

通过选取合适的热源模型、边界条件,建立了空气介质中电火花表面强化的热传导模型;通过分析其温度场的特点,建立了空气介质中电火花表面强化的二维温度场模型;借助ANSYS有限元分析软件对空气介质中电火花表面强化的温度场进行了模拟,分析了工件在不同时间、不同位置的热流密度矢量和随温度矢量的变化规律,以及电火花强化时温度随深度方向的变化历程。结果显示:电火花表面强化层的放电凹坑主要是由于热量在材料中不同方向的热传导存在差异引起的;电火花表面强化具有骤冷骤热的特点;在相同的加工速度下,节点的温度随着离表面距离的增大而减小。     

Heat flux distribution model by sequential algorithm of inverse heat transfer for determining workpiece temperature in creep feed grinding

The purpose of this study is to determine the heat flux distribution and to estimate the workpiece temperature in creep feed grinding. The sequential algorithm of the inverse heat transfer was used for determining the heat flux distribution. The amount of heat flux to the workpiece, the energy partition and the convective heat transfer coefficients both at the front and at the back of the heat flux were determined. Three heat source models using the determined amount of heat flux were applied to estimate the workpiece temperature. The workpiece temperatures estimated by the heat source models were compared with that measured by the embedded thermocouple. The scalene triangle model correlated best with measured and theoretical temperature profiles obtained for creep feed grinding.     

Analytical modeling and experimental investigation of tool and workpiece temperatures for interrupted cutting 1045 steel by inverse heat conduction method

Cutting temperature is a key factor which directly affects tool wear, workpiece integrity, and machining precision in high speed machining process. The interrupted cutting process consists of several periodical characteristics, such as cutting force and time varying heat source. Induced cutting temperature models with time varying heat flux are developed in this paper to predict temperature distribution at tool inserts and workpiece during interrupted cutting process. A set of interrupted cutting experimental installation is designed to verify the proposed models. The comparison of predicted and measured results for 1045 steel in interrupted cutting processes shows reasonable agreement. The measured temperature of both the tool inserts and workpiece increase firstly and then decrease as the cutting speed increases. The peak temperature of the workpiece appears at 1500 m/min, while the peak tool inserts temperature appears at 1250 m/min approximately. Heat flux is calculated by the inverse heat conduction method. The applicability of Salomon's hypothesis to the temperature of tool inserts and workpiece is discussed during the interrupted cutting process. The dropped temperature at high cutting speed is mainly caused by that heat flux into tool inserts decreases and heat transfer time is not enough after the critical cutting speed.     

高温喷丸强化Ti6Al4V合金的热力耦合数值模拟

发展一种连接喷丸强化与高温加载传热的热力耦合有限元方法,模拟高温喷丸强化Ti6Al4V合金的过程。首先建立一圆盘模型模拟待喷材料的高温加载传热过程。然后将圆盘模型的受热部分取出一小块建立对称胞元喷丸模型,并且将高温加载所致的温度场和热应力场通过解析场的方式导入到对称胞元喷丸模型,模拟高温喷丸强化过程。最后通过回弹计算获得稳定的残余应力场和温度场。创建4种模拟工况：常温喷丸、单独导入温度场的喷丸、单独导入热应力场的喷丸和高温喷丸,探究高温喷丸的残余压应力强化机理。结果表明：在常温环境下,对称胞元喷丸模型模拟的Ti6Al4V表层残余应力与试验结果具有很好的一致性。在高温加载作用下,随着热流密度的增加,受喷材料表层残余压应力有所减小,材料亚表层的残余压应力逐渐增大。影响高温喷丸强化的残余压应力的主要因素是高温加载所致的温度场,热应力场对残余压应力强化起次要作用。     

镍基高温合金车削热通量模型研究

镍基高温合金是多元素组成的合金,车削变形复杂,车削温度极不好掌控,刀具和工件中产生的热通量对工件和刀具磨损有较大影响。建立车削过程仿真模型,提取热通量数据,对数据进行分析处理;建立车削过程神经网络模型。结果表明:将工件热通量、刀具热通量分别作为单一变量进行极差与方差分析,吃刀量是影响热通量的重要因素。将工件与刀具热通量相结合作为双变量,建立神经网络模型,验证了吃刀量是决定性因素;通过神经网络内部的隐藏函数,得到了分析误差与重要性的正态分布图;通过MATLAB求得隐藏函数,该函数与吃刀量拟合度较高。     

An investigation of workpiece temperature variation of helical milling for carbon fiber reinforced plastics (CFRP)

Better prediction about the temperature distribution of workpiece has a great significance for improving performance of cutting process, especially relating to the workpiece of carbon fiber reinforced plastics (CFRP). In this paper, a heat transfer model is developed to investigate the temperature distribution of CFRP workpiece in helical milling process. Depending on characteristics of helical milling, two kinds of heat sources have been presented, the geometrical shapes of which are modeled as semicircle arc and line. The complex trajectory of each heat source relative to the stable workpiece has been studied. Based on the analysis, unsteady state three-dimensional governing equation of heat transfer in CFRP workpiece with adiabatic boundary condition is proposed. The solution procedure of this nonhomogeneous heat transfer equation consists of two steps: it is transformed into homogeneous equation according to the heat transfer theory firstly; and then the homogeneous equation is solved using the separation of variables. Basing on the solution of the homogeneous equation, the temperature distribution resulting from the moving semicircle arc heat source and the line heat source has been studied detailedly. In order to calculate the heat generation in the helical milling process, a cutting force model is presented and the heat partition transferring into the CFRP workpiece is solved using the Conjugate Gradient Method. A series of tests of helical milling for CFRP are conducted, and the experiment results agree well with the results calculated by the predicted model. This model can be extended to optimize the cutting condition and restrain the thermal damage of the CFRP workpiece.     

Thermal deformations of cutting tools: measurement and numerical simulation

Challenges for machining include greater and greater material removal rates coupled with an increase in the use of difficult to machine materials, as well as environmental-friendly dry or minimum quantity lubrication machining, small manufacturing batches and frequently changed manufacturing orders. These trends are accompanied by high temperatures in the machining process and large, variable heat flows causing thermo-elastic displacements of the tool, the workpiece and the clamping devices. Although the displacements are small, in the range of a few micrometers, they have assumed more and more importance because of growing requirements for manufacturing accuracy. Thermo-elastic displacements of the tool due to heat flow during machining are investigated and analysed in this paper. Temperatures and displacements are measured on a test bed equipped with measuring instruments. The identification of the thermal boundary and contact conditions is supported by finite element models. Knowledge of the heat flows resulting from the machining process is a prerequisite for control of and compensation for displacements. Since these heat flows either cannot be measured or can only be measured with enormous effort, heat flows are determined by means of numerical simulation of the machining process itself. This strategy has been previously used as a systematic approach for turning in orthogonal cutting conditions. However, further investigations are needed for oblique turning conditions, milling and drilling operations.     

不同磨削参数下高温合金磨削温度场的数值模拟及试验研究

磨削加工产生的温度场会根据自身分布特性对试件造成不同程度烧伤。提出改进的平面磨削热量分配理论模型并结合ABAQUS建立磨削温度场有限元模型,基于热成像法提出一种研究试件表面温度场分布的方法,并对仿真结果进行了验证,利用表面形貌对加工质量进行了分析。结果表明：仿真结果和实际加工过程的温度相对应,且靠近工件中心1 cm范围内都是高温区域;磨削高温所影响的深度大约只存在于温度影响深度的前1/3。     

Effects of Electrolyte Composition on Heat Exchange in Anode Plasma Electrolyte Treatment of Commercial Titanium

The study considers heat exchange in a three-phase system: electrolyte solution—vapour-gas envelope (VGE)—a titanium workpiece. The temperature of a workpiece, current intensity, heat fluxes from the envelope to the electrolyte and the workpiece were measured in an aqueous solution of ammonium chloride with addition of ammonia and glycerol. Addition of ammonia and glycerol proved to decrease current intensity, heating temperature, total energy liberation and heat fluxes to the solution and the sample. Furthermore, it was found that heat shares passing to the electrolyte and the workpiece are not affected by ammonia and glycerol concentrations. The addition of ammonia and glycerol affect heat exchange, increasing the VGE thickness due to intense evaporation of these elements.     

GMAW焊接传热及其对HAZ奥氏体晶粒长大过程的影响

精确分析熔化极气体保护电弧焊（ＧＭＡＷ）焊接传热过程的前提是对施加于工件表面上的焊接热输入分布模式有一个恰当的、合乎实际的描述。本文作者根据电弧物理的基本原理和熔滴与熔池的交互作用,建立了双峰分布的电弧热流密度分布模型,确定了熔滴热焓量在熔池内部的分布区域。以此为基础,建立了ＧＭＡＷ焊接传热的数学模型,给出了焊接热影响区奥氏体晶粒长大的动力学方程。采用数值模拟技术研究了低碳钢和ＨＱ１３０钢ＨＡＺ的     

电火花放电蚀坑的有限元热分析

放电通道等离子体中的热源是工件材料蚀除的最主要动力.通过对电火花放电过程建立数学物理模型,以热流密度和热对流作为最主要热载荷,动态施加于放电点处.用有限元ANSYS软件对所建立的模型进行了温度场分布仿真,讨论了不同峰值电流和脉冲宽度条件下,放电蚀坑深度和半径的变化规律.计算结果和试验结果比较后发现,模型对蚀坑深度和半径的变化有较好的预测精度.     

Dependence of heat transfer coefficient at quenching on diameter of cylindrical workpieces

Abstract

For computer simulation of a quenching process, the fundamental prerequisite is to have the relevant heat transfer coefficient (HTC) calculated as a function of the workpiece’s surface temperature and time respectively. In order to calculate the HTC experimental measurement of the temperature–time history (cooling curve) near the workpiece surface is necessary. In this investigation, cylindrical probes with diameters of 20, 50 and 80 mm are used. The cooling curve was always measured 1 mm below the surface of the probe. Special care has been taken to keep all other factors (e.g. design of the probes, temperature measurement, quenching conditions and calculation procedure), which can influence the calculated HTC, constant, in order to ensure that the only variable is the diameter of the probe. Assuming a radially symmetrical heat flow at the half length of the probe, the HTC was calculated using one-dimensional inverse heat conduction method. The unexpected striking result of this investigation is the fact that for the probe diameter (80 mm) the calculated HTC as a function of surface temperature does not show the film boiling phase. A plausible explanation for this effect is given, based on the critical heat flux density. The possibility of establishing a simple fixed relation (a correction factor) between the HTC and the diameter of cylinders is discussed.     

Three-dimensional temperature predictions in machining processes using finite difference method

The purpose of this study is to determine the three-dimensional temperature fields on the chip, tool and workpiece during machining, which is one of the most important characteristic of machining processes; since the fields can affect other properties such as residual stresses and tool wear, and thus tool life and fatigue life of finished parts. The finite difference method (FDM)-based model proposed in this paper offers very rapid and reasonably accurate solutions. Finite difference-based simulation results are validated with infrared thermal measurements which are determined from the machining of AISI 1050 and AISI H13 materials under various cutting conditions.     

Heat partitioning in dry milling of steel

A predictive model for heat induced shape deviations would facilitate the optimization of dry milling strategies. Results from milling experiments aiming at a physically based regression model for the heat flux distribution along the contact arc are presented. The contact arc was discretised by varying the width of cut on four levels for each combination of cutting speed, feed speed, and depth of cut. Heat fluxes to the workpiece were iteratively determined in an inverse procedure. Heat partitioning not only depends on the thermal number as in orthogonal cutting but also on the feed speed and the depth of cut.     

基于分段热边界的杆件连续淬火过程的数值模拟

提出了一种针对连续淬火过程数值模拟的分段热边界方法,并得出了模型分段判据。以30CrNi3MoV钢多孔杆件为例,采用MARC软件对其进行了水淬模拟,得到了温度场、应力场、组织分布比例与硬度场。模拟结果表明,分段热边界法较整体热边界淬火模拟更符合实际工况,且工件内部温度随入水深度的增加而降低,工件应力由工件前端向后端衰减;淬火后的表面硬度为51.9~53.7 HRC,心部硬度为36.8 HRC,其与工件实际淬火后的硬度差最大不超过10%;孔偏向中心的畸变量为0.18~0.20 mm,工件长度的畸变量为1.85 mm,孔和工件的畸变量相差在5%以内。该结果验证了分段热边界方法的准确性。     

焊接热输入对HQ130钢焊接热影响区组织硬度的影响

ＭＩＧ／ＭＡＧ焊接热输入包括焊接电弧热流和熔滴带入熔池的热焓量两部分,本文以作者提出的焊接热输入分布模型为基础,建立了熔流场和温度场的数值分析模型,采用数值模拟技术研究了焊接热输入对ＨＱ１３０钢焊接热影响区（ＨＡＺ）组织和硬度的影响规律,给出了不同焊接热输入时ＨＡＺ不同部位奥氏体晶粒尺寸及组织和硬度的计算结果,实验表明,ＨＱ１３０钢焊接热循环及ＨＡＺ组织,硬度的计算值和值吻合良好。