Three-dimensional transient cutting tool temperature field model based on periodic heat transfer for high-speed milling of compacted graphite iron

Highlights

• •The transient cutting tool temperature field model was established by using the heat source method and Fluent software.

• •The instantaneous cutting thickness, instantaneous tool-chip contact length, and the uneven spatial distribution of the heat flux on the rake face were all considered.

• •The model was verified by using the thermocouple to measure the specific point temperature of the tool through the high-speed orthogonal turning experiment.

• •Within the range of x ≤ 0.25 mm, y ≤ 0.25 mm and z ≤ 2.00 mm, each point had large temperature gradient at different time.

• •The model provided a theoretical basis for directional and accurate design of special tools for machining CGI.

Abstract

The new material, compacted graphite iron, plays a key role in the reliability of engine and the realization of high emission standard, but it is a difficult-to-machine material. In high-speed milling process of compacted graphite iron, thermal crack induced by thermal stress is the main reason for tool breakage and chipping. In order to reveal the initiation and propagation mechanisms of thermal cracks, and improve tool life by controlling the formation of thermal crack, it is necessary to study the three-dimensional transient cutting tool temperature field under the condition of periodic heat transfer in intermittent cutting. In this paper, the three-dimensional transient FEM cutting tool temperature field model for high-speed milling of compacted graphite iron was established by using the heat source method. The uneven spatial distribution of the heat flux on the rake face of the tool, and the changes of heat flux density due to the transient cutting thickness and tool-chip contact length over time were considered, and the influence of boundary conditions in the cutting stage and the non-cutting stage were also all taken into account. The heat flux action time and the ratio of cutting time and non-cutting time in the model were set according to the experimental conditions of actual milling process. The established three-dimensional transient cutting tool temperature field model was finally verified by measuring the real temperature in high-speed orthogonal turning experiment. The results showed that the predicted value of the model was consistent with the experimental value, and the predicted error was less than 6.0%, indicating that the model has high accuracy for simulation. This model has great theoretical significance for the development of new tool materials for compacted graphite iron processing, studying the mechanism of thermal crack, controlling and preventing tool failure caused by thermal crack.

Introduction

Global warming is directly related to CO₂ in the atmosphere, and on-road vehicles are one of the main sources of CO₂ emission (Lu and Sun, 2021; Pearson et al., 2009; Zhang et al., 2018). In the new regulation, CO₂ emission factor of new registered cars from 2021 would be lower than 95 g/km (Gao et al., 2021). The demand of reducing CO₂ emissions continues to challenge the engine designers and the selected materials, and puts forward higher requirements for the strength, stiffness and heat resistance of engine body materials. At present, the new material compacted graphite iron (CGI), has played a key role in the reliability of engine and the realization of high emission standards. The application of CGI can provide new opportunities for reducing the weight and size of engines, improving power, reducing noise, vibration and improving irregularity. However, according to the cutting performance of CGI (Guo et al., 2012; Reis et al., 2020; Niu et al., 2020; Chen et al., 2018), CGI is a difficult-to-machine material, and its thermal conductivity is about 78% of gray cast iron, and the cutting heat is often accumulated, and the ferrite structure in CGI is easy to bond with the tool during machining, which all aggravated the wear or damage failure of the tool, so that the tool life is low and the machining surface quality is poor. Low tool life and high machining cost limit the application of CGI in engines. Therefore, scholars (Niu et al., 2020; Chen et al., 2018; Xu et al., 2020) have carried out many studies on the processing of CGI in recent decades, coated carbide tool is most used for milling of CGI, but due to the periodic thermal load on the cutting tool during the milling process, the high cutting tool temperature gradient results in high thermal stress, and thermal cracks are easy to occur when milling of CGI. Fig. 1 (a) is the thermal crack generated during high-speed milling of CGI at 680 m/min, which was discovered by Sander et al. (2010). They (Sander et al., 2010) found that the thermal crack led to tool chipping and adhesion wear. Coated carbide tools were used to mill CGI (GJV450) at a higher cutting speed of 800 m/min, thermal cracks perpendicular to the cutting edge as shown in Fig. 1(b) were observed by Su et al. (2018), and the main form of tool failure was chipping. In high-speed milling of CGI, thermal cracks are caused by thermal stress, and the generation of thermal cracks aggravates the tool failure.

In recent years, scholars have studied the cutting temperature of CGI for the problem of high cutting temperature in machining CGI and put forward a variety of cooling methods. Cutting fluid is a common cooling method in metal machining process. However, in CGI machining, the effect of cutting fluid on reducing tool wear is controversial (Su et al., 2016). When the cutting speed is high, it is difficult for the cutting fluid to reach the cutting zone and play a cooling role. Moreover, the graphite contained in CGI is easy to mix with the liquid, which becomes oil sludge, adheres to the tool and causes tool wear. So, wet cutting is generally not used in CGI machining. In order to obtain better cooling effect and minimize the use of cutting fluid, a variety of quasi-dry cooling methods, such as low temperature cooling (such as cryogenic fluid liquid carbon dioxide (CO₂)), oil-on-water (OoW) and minimal quantity lubrication (MQL), have been proposed (Meng et al., 2020; Li et al., 2019; Heep et al., 2019; Ding et al., 2019a; Yao et al., 2018). Cryogenic cooling technology applies cryogenic gases only in the cutting zone to reduce the cutting temperature. However, due to the absence of lubricants, the lubrication efficiency of this technology is low (Debnath. et al., 2014; Pereira et al., 2020). In addition, the pipe used to transport low temperature N₂ and CO₂ must be covered with thick insulation cotton to prevent heat exchange with air. This makes the cooling device somewhat complex and costly.

Therefore, dry cutting is mainly used to machine CGI in actual production. However, when dry cutting CGI, it is easy to produce thermal cracks, resulting in the reduction of tool life. In order to reveal the initiation and propagation mechanism of thermal cracks to better control the formation of thermal cracks and prevent tool breakage and failure, it is necessary to study the three-dimensional transient temperature field and thermal stress field of cutting tool under the condition of periodic heat transfer in intermittent cutting. Although many studies on the machining of CGI have been carried out in recent decades (Niu et al., 2019, 2020; Chen et al., 2018; Xu et al., 2020; Sander et al., 2010; Su et al., 2016, 2018; Meng et al., 2020; Li et al., 2019; Heep et al., 2019; Ding et al., 2019a; Pereira et al., 2020; de Sousa et al., 2017), researches are mainly focused on the selection of various commercial tools, process optimization, cutting temperature reduction by using cooling methods, mechanism and cutting performance analysis. There are few studies on the three-dimensional transient cutting tool temperature field model for high-speed milling CGI.

At present, researches on the cutting tool temperature field model by using the finite element method, finite different method and analytical method are usually performed on the turning process (Ji et al., 2018; Abukhshim et al., 2005; Chen et al., 2017a; Cao et al., 2021). However, in the milling process, the cutting tool is subjected to periodic cutting force and time-varying heat source, the cutting tool temperature depends on the heat flux and the ratio of cutting time and non-cutting time. The transient cutting thickness is constantly changing in the milling process, so the modeling method of cutting tool temperature in the turning process is difficult to directly apply to milling process. In recent years, scholars have explored a simple and accurate method, namely heat source method, to solve the unsteady temperature field of the cutting tool in the milling process. The moving heat source model was used to simulate the temperature of the workpiece in the milling process by Richardson et al. (2006). The dynamic temperature field model of carbide cutting tool in the milling process was established by adopting heat source method (Wei et al., 2016). Huang and Liang (2003) assumed that the heat source of the main shear zone was a uniformly moving oblique heat source, and proposed a model of the obliquely moving heat source. The cutting temperature model of time-varying heat flux was established by Jiang et al. (2013) to predict the temperature distributions of the tool and the workpiece. Periodic heat sources in the milling process were mostly considered in the above studies, but the changes of cutting thickness with time have not been considered, and the spatial distribution of the heat flux between the tool and the chip was ignored, which reduced the accuracy of the calculation to a certain extent. Previous studies have shown that the positive pressure distribution between the tool and the chip would gradually decrease with the increase of the distance from the cutting-edge, resulting in uneven distribution of the normal stress and the shear stress between the tool and the chip (Kato et al., 1972; Usui and Takeyama, 1960), and the slip velocity between the tool and chip was also uneven, both of which resulted in the inhomogeneity of the heat flux between the tool and the chip, and ultimately affected the temperature field.

In this paper, a three-dimensional transient cutting tool temperature field model based on periodic heat transfer for high-speed milling of CGI is established. The uneven spatial distribution of the heat flux on the rake face of the tool, and the changes of heat flux density due to the transient cutting thickness and tool-chip contact length over time are considered, besides, the influence of boundary conditions in the cutting stage and the non-cutting stage are also considered. The changes of boundary conditions, heat flux density and heat flux loading area over time are realized by the secondary development of Fluent.