Hybrid machining processes . Definitions , generation rules and real industrial importance

Some important trends in the development of advanced machining processes with potential applications in Production/Manufacturing 4.0 are presented. In general, both conventional and unconventional machining processes are characterized in terms of potential technological possibilities related to their hybridization allowing the performance of more productive and effective machining processes. This is due to the fact that hybrid processes considerably enhance the advantages of individual processes and minimize potential disadvantages in individual processes. Possible classification systems of hybrid processes including the CIRP terminology are overviewed and some representative examples are provided. In particular, the hybrid machining processes based on the simultaneous and controlled interaction of process mechanisms and/or energy sources leading to the synergic effect (1 + 1 = 3) on the process performance are taken into account. Some conclusions and future trends in the implementation of hybrid processes are outlined.

At present, global expectations towards manufacturing processes boil down to increasing flexibility and efficiency/productivity, and, on the other hand, to maintaining high quality [1].In the case of parts with complex shapes, the manufacturing cycle may include several stages implemented on different devices (fig.1).In such cases, subsequent handling and positioning of workpieces is usually ineffective due to the loss of time and an increased risk of machining errors, which increases the cost of maintaining the required quality (or means getting inferior quality).Not without significance is the larger space needed to accommodate several devices.For these reasons, construction and technological activities have been undertaken for many years to integrate many processes in one hybrid manufacturing platform [2,3].Fig. 1.Scheme of the shortening of process chain resulting from process hybridization [6].CPconventional production process, HPhybrid process Fig. 2. Comparison of conventional optimization with interdisciplinary effect of process hybridization [6] * Prof. dr hab.inż.Wit Grzesik (w.grzesik@po.opole.pl)-Katedra Technologii Maszyn i Automatyzacji Produkcji Politechniki Opolskiej An example of the first such solution can be CNC multifunctional machine tools for integrated (complete) machiningmulti-tasking machine toolswhich allow for a significant reduction of costs and shortening the machining time.At the next stages of integration of manufacturing processes, devices with a laser source supporting conventional processing appeared.
The concept of a hybrid manufacturing process is currently being developed by integrating multi-axis removal and additive manufacturing (AM) in one device [2].The AM machining can be used not only to shape additional elements of the part, but also to repair expensive parts, e.g.turbine blades with traces of cavitation pits or cracks on the surface.Ionized material Plasma jet Plasma AFMabrasive flow machining, AJMabrasive jet machining, AWJMabrasive water jet machining, CHMchemical machining, EBMelectron beam machining, ECMelectrochemical machining, EDMelectrodischarge machining, IBMion beam machining, IJMice jet machining, LBMlaser beam machining, PAMplasma beam machining, USMultrasonic machining, WJMwater jet machining.
Hybridization of manufacturing processes, including material removal process, is an important factor enabling the implementation of the Production/Generation 4.0 strategy, because it promotes innovation.For this reason, hybridization is also an important element in the development of advanced manufacturing processesit gives a wide range of their improvement and optimization.Tab.I summarized advanced material removal processes that may undergo further hybridization (see tab.II).
Fig. 2 shows that after exhausting the possibilities of optimizing the conventional process, it is still possible to significantly improve its performance by eliminating known limitations by introducing additional, external energy sources.In this way, the interaction of new, additional processes supporting the basic conventional process takes place.Another possibility is to analyze the effect-cause type for all components of the technological chain, leading to process improvement by integrating individual processes or by cumulating several processes in one hybrid process.
Tab.I summarized the industrial processes of material removal using various energy sources (mechanical, thermal, chemical and electrochemical), with full physical and technological characteristics.

Basics of hybrid processingprinciples of creating machining processes
The evolution of conventional and unconventional machining processes after World War II consisted in combining processes and using various active energy sources or implementing several methods of machining, or even several subsequent stages of the technological process in one production unit with the aim to achieve synergy.This means that as a result of the hybrid process, using a hybrid machine tool or a production device, an effect exceeding the sum of the effects of the component processes, carried out separately.
In a natural way, the definition of hybrid processing has evolved due to the development of techniques and methods of machining that are unfortunately now called technologies.The first definition of the 1970s and later definitions included combinations of two or more processes that can be used simultaneously to remove material (e.g. a combination of abrasion with EDM or ECM), or one of them is only supporting, contributing to a favorable change in the process conditions, e.g. in extreme cases by laser heating of the material or its cryogenic cooling.
CIRP defines hybrid manufacturing processes that include manufacturing/machining processes [7]: Hybrid manufacturing processes are based on simultaneous and controlled interaction of process mechanisms and/or energy sources/tools having a significant effect on the process.
The term "simultaneous and controlled interaction" means that energy processes/sources must interactmore or lessin the same zone of the hybrid process and at the same time.The amplification of the total effect of process hybridization is represented by the "1 + 1 = 3" rule, which indicates an increase in the efficiency of the machining process, e.g. by thermal softening with the laser of the material being processed.
There are distinguished (fig.3) processes based on combining different energy sources or different tools (different methods and ways of shaping), included in Group I, and processes using controlled mechanisms of various processes that are implemented in conventional component processes (group II).The first group is distinguished by assisted processes (subgroup I.A) and mixed processes (mixed/joint processessubgroup I.B).
In the case of conventional and unconventional machining processes, the most important is the design of hybrid processes according to the principle of IA (assisted by various vibration energy, thermal laser and liquid and gas media, grinding, polishing, EDM, ECM) and IB (e.g.joining grinding and EDM; joining grinding and ECM, ECM and EDM).Other hybrid processes, also related to plastic deformation, are described in [7].
In group II, one can give examples of combining kinematic features of two cutting methods, e.g. in the form of turn-milling and turn-broaching, surface hardening during grinding (grind-hardening), surface hardening by intensive burnishing a part cooled with frozen CO 2 (cryogenic deep rolling) or combining machining with burnishing.Fig. 3. Classification of hybrid manufacturing/machining processes according to CIRP [2,7] Fig. 4 presents the time relations between the constituent processes of different varieties of hybrid processes classified in fig. 3.In variants I and II, the basic and support processes can be implemented simultaneously (case A) or sequentially (case B), while combining two basic processes 1 and 2 takes place sequentially (case C 1 ), a good illustration of which is the combination of rough cutting with finish burnishing, while case C 2 can be referred to the ECDG process, in which EDM and ECM are abrasive aids, which means that the hybrid process can be assigned to time of the process or to the machining zone [7,9].Fig. 4. Time relations in hybrid machining processes [9].Asimultaneous interaction of the basic and supporting process, Bsequential interaction of the basic and supporting process, C 1sequential interaction of two basic processes, C 2sequential interaction of two basic processes and the supporting process Tab.II presents the combinations of hybrid processes, based on the classification of material removal processes proposed in tab.I.In the group of classic cutting methods consisting in the mechanical interaction of a tool with defined cutting edge geometry (T) for the material being machined, process support belongs to the principle I.A.For this reason, the cutting process is assisted by laser (TLB), plasma (PLB) and ultrasonic vibrations (UST).In turn, grinding support (A) concerns the principle of I.B and that is why it is possible to assist in erosion (AEDM), chemical (MCP), electrochemical (AECH) and ultrasonic vibrations (ultrasound).In contrast, unconventional processes (ED, CH, EC) can be combined, e.g.ECDM, ECAM, and assisted, e.g.EDUSM, USECM, ECML.
A better explanation of the principles of hybridization of machining processes can be made using triple graphs (fig.5), showing the type of interaction of process mechanisms depending on the working medium and energy carriers used.Fig. 5a and fig.5b give examples of assisting abrasive machining with electric discharges (erosion-abrasive machining) and spark erosion machiningwith ultrasonic vibrations.

Basics of hybrid processingsupported processes
Fig. 8 presents the combinations of basic and assisted processes used in various manufacturing techniques.The most frequently used assisted energy sources are: vibrations with the frequency of 0.1÷80 kHz and amplitude 1÷200 μm, liquid and gaseous media (CCS under pressure, liquid nitrogen LN 2 , cooled CO2) and laser [7,13].For this reason, the three most widespread groups of assisted processes (Group IA in fig. 3) are supported by vibrations (vibration/US-assisted machining), thermally-assisted machining and media in various states and under different pressures (media-assisted machining).
Previous applications of assisted processes and the state of advancement of research are shown in fig.9. ` Fig. 8. Combinations of assisted hybrid processes [7] Fig. 9. Advances in assisted machining processes [7]

Basics of hybrid processingcombined processes
As mentioned, the combination of two or more subtractive machining processes isas definedconditioned by their simultaneous influence to a greater or lesser extent on the material removal mechanism (group I.B in fig.3).So far, the widest application has been found in the processes combining the abrasive grinding mechanism and the thermal effect of electrical discharges, i.e.AEDG (EDG) or electrochemical dissolution, i.e.AECG (ECG), and the combination of EDM and ECM, i.e.ECDM.In the latter case, a third abrasive mechanism may occur, i.e. in the ECDG hybrid process.During the electrochemical grinding process, not only grinding (ECG) is performed, but also honing (ECH) and superfinishing (ECS).The principles of combining component processes in hybrid processes are presented in fig.10.Fig. 10.Rules for integration of machining processes based on EDM and ECM [6] Basics of hybrid processingprocesses in micro and nanoscale Fig. 11 shows differences in the structure of hybrid processes in micro (fig.11a) and nanoscale (fig.11b).It is clearly visible that the largest share in the micromachining (about 64%) are subtractive and additive processes.In turn, in nano-machining, the additive process is still important (despite the significant share of cutting) (layer depositionshare of approx.33%).In micromachining, 55.8% are assisted subtractive processes carried out simultaneously, and 47.7%processes carried out sequentially.In nano-machining, 30.7% of processes have the C2 structure, i.e. the third supporting process occurs, and 69.2% of the total hybrid processes are carried out sequentially [9].

Conclusions
 Implementation of the Production/Generation 4.0 strategy is closely related to the hybridization of manufacturing processes, including the material removal process, due to the large role in achieving a high level of manufacturing innovation.Hybridization applies not only to normal machining processes, but also to processes in the micro and nanoscale. Hybrid machining processes contribute, due to the synergy effect, to the resulting effect exceeding the sum of the effects of component processes performed separately.Therefore, there are additional possibilities to optimize the process. In practice, hybrid processes based on supporting an additional source of energy, combining various energy sources and/or tools, and controlling various mechanisms of component processes (removal, metal forming, heat treatment, additive machining) can be used. In the group of conventional cutting methods (turning, drilling, milling) the most important is the support of vibration energy US, laser and technological media (liquid under high pressure, liquid nitrogen). In the group of conventional abrasive methods (grinding, honing, polishing, lapping) the most important is the strengthening of the abrasive effect by electro-erosive and electrochemical interactions and magnetic forces.However, the development of this group of hybrid processes limits ecological restrictions. Due to ecological reasons and under the influence of requirements on the functionality of the surface of machine elements, hybrid processes are developed in which the mechanisms of constituent processes are controlled, e.g.controlled heat flow in grind-hardening, intensive deformation of the surface layer combined with cryogenic cooling and phase change of the material. Rapid development of hybrid processes and devices that combine additive shaping and CNC machining is also observed.
Some important trends in the development of advanced machining processes with potential applications inProduction/Manufacturing 4.0 are presented.In general, both conventional and unconventional machining processes are characterized in terms of potential technological possibilities related to their hybridization allowing the performance of more productive and effective machining processes.This is due to the fact that hybrid processes considerably enhance the advantages of individual processes and minimize potential disadvantages in individual processes.Possible classification systems of hybrid processes including the CIRP terminology are overviewed and some representative examples are provided.In particular, the hybrid machining processes based on the simultaneous and controlled interaction of process mechanisms and/or energy sources leading to the synergic effect (1 + 1 = 3) on the process performance are taken into account.Some conclusions and future trends in the implementation of hybrid processes are outlined.KEYWORDS: hybrid machining, assisted machining,