Preform necessity and preform design at hot closed-die forging – a general design approach

. The parts obtained by forging are better than those manufactured by any other metalworking process in many aspects. This determines their widespread use where human safety and reliability are critically important. Industries such as aerospace, defence, automotive and agriculture, construction, mining are some of the largest customers using forged parts. The design of hot closed-die processes very often requires engineers to deal with two important challenges - the necessity of preform (intermediate) steps and the shape of these intermediate steps. A general design approach for determination of necessity of preforms and their shape at hot closed-die forging is presented in this article.


Introduction
Forging is still a viable option today for creating parts that are net-or near net-shaped.A metal workpiece is plastically deformed during the forging process in order to acquire the required shape of the forged component and to obtain a combination of suitable physical and mechanical qualities.The achievement of the aforementioned conditions, as well as a decrease in production costs and a reduction in environmental effect, are ensured by choosing the appropriate types and order of operations during hot closed-die forging.As it directly influences the metal flow in the final impression, influencing the quality of the forged component and die wear, the preform stage plays a critical role.
The challenge of preform design is closely related to another technical issue: the query of the necessity of preform stage.In actual practice, engineers most frequently decide if further procedures are necessary based on manufacturing knowledge, expert opinion, or the approach of trial and error.In handbooks [1,2], where they are considered as connected, such practical advice is given regarding the choice of whether preform phases are necessary and their shape.
Assuming a priori that a preform is required, numerous publications give solutions when considering the issue of the shape of the preform at hot closed-die forging.Decisions are suggested in [3,4] on the basis of model material experiments.A strategy utilizing an upper bound elemental technique is discussed in [5].Numerical metal forming simulations are being used more frequently to support various preform design processes due to the advancement of computers and computational techniques.In general, the development of various techniques and procedures for preform design at hot closed-die forging receives a considerable boost from modelling and simulations of metal forming processes.For preform design, many writers use geometrical techniques, frequently in conjunction with sensitivity analysis, to examine the movement of defining points on the contour of the forgings [6,7,8,9].In [10,11], there are solutions to the problem of forming shapes based on geometrical similarity.Fuzzy logic-based preform design techniques are employed in [12,13].Techniques utilizing approximations with mathematical equations [14], neural networks [15], isothermal surfaces [16], and topological optimization [17] are used to supplement the range of design approaches.
Significantly fewer publications are devoted to determining the necessity of intermediate or preform steps for hot closed-die forging processes.Criterions for necessity of intermediate steps have been proposed in several studies [18,19,20,21].The wide range of methods and procedures, both for preform design and necessity of preform steps at hot closed-die forging, demonstrates the importance of the subjects and the absence of a standardized and universal solution to the problem.Moreover, the design of hot forging processes is highly complicated due to the difficult formalization of the procedures for its realization.

General design approach algorithm for preform necessity and preform design at close-die forging
The presented on the fig. 1 algorithm consider only that part from the whole forging process, witch refers to the neccesuty of intermediate steps and design of their shapes.Аs mentioned above, this is key to achieving flawless forgings.The algorithm consists from three main procedures dealing with necessity of intermediate steps, design of intermediate steps' shapes and forging sequence selection.The last procedure acting like verificator of the choosen forging proces parameters (forging forces, temperatures, rates, lubrication, etc.) also.The input data include forging part drawing and initial billet dimensions.Like interstitial result, the processed data consists a set of alternative forging sequences.
These alternatives can be different as regards of the numbers of intermediate steps and their shape.Later, from the different cases the designers can select appropriate variant for specific forging part according to the other technological requirement.Like an integral part of the process planning of forging sequence, verification by simulation of hot close-die deformation process is included.Shown algorithm allows the intervention from process designer.This is possible in three cases:  Determining criteria for necessity of intermediate step;  Choosing instructions and recommendations for designing of intermediate steps' shapes;  Choosing the rules of forging sequence from the different alternatives.Thus, the engineers have opportunity to influence upon the final results according the specific conditions of the forging process.These three points are the same, which there are not strong generally accepted rules and recommendations.This feature ensures flexibility of the proposed algorithm.The researches of some authors regarding the necessity of preform (intermediate) steps and their shapes at hot closed-die forging fit into the algorithm presented in this work.For example, in [14,22,23].

Example of using the general design approach
In this paper as an example, illustrating proposed approach for closed-die forging process design, forging part with shape shown on fig. 2 is used.As a criterion for necessity of intermediate step proposed in [20] shape complexity factor is used.The criterion for necessity of preform step used in this example is given by condition: If ( 1) is true, then a preform step is necessity.In order to use (1), the values of K1, φH and φA have to be calculated.Volume of forging part V=196847,11 mm 3 (result of calculation with CAD system).Thus, V=VC and calculated HAV=18,63 mm φA = ln (AF/A0) 2 where AF is the area of cross section of forging at parting plane.In this forging parting plane is complex.In such a case, projection of parting plane can be used.In this way, the projection of cross section will be a circle with diameter Dmax.A0 is the cross section of the billet.Assuming the billet with dimensions Ø100x25mm, the condition (1) is 0,795 > 0,296 -a preform step is necessity, too.Applying the general approach algorithm shown above necessity of only one intermediate step between the initial billet and final shape of forging part is needed.
For the intermediate shapes design procedure, according to fig. 1, simple moving average technique have been used.The procedure starts with dividing of the contour of the forging on upper and bottom part, according to parting line.Coordinates of the points describing the shape of each of the two (upper and bottom) parts are collected to sets of data and represented in the form of a functions.The next step requires the application of equation: where: n is number of dataset entries, k is number of subset entries and p1, p2, ... pn are data values, for each of functions.As result of previous operation, a new set of point coordinates are obtained.The newly obtained point coordinates describe the contour of the preform die impression (fig.6).In the present example k = 4.

Conclusion
The results obtained applying the proposed general approach for the determination of the necessity of intermediate steps and design of their shapes at hot close-die forging allow to conclude:  It is suitable approach, which enables to get several alternatives for shape and number of intermediate stages and to select the most appropriate case for specific part. The algorithm is flexible and addition can be done, especially in connection with the criterions for necessity of intermediate steps, the instructions and recommendations for preform design and the rules for choice of eligible variant for close-die forging. Investigations are needed to confirm the opportunity to using this general approach algorithm for forgings with more complex shapes.

Fig. 1 .
Fig. 1.Algorithm for process planning support for intermediate steps designs of axisymmetric hot closedie forging.

Fig. 2 .Fig. 3 .
Fig. 2. Algorithm for process planning support for intermediate steps designs of axisymmetric hot closedie forging.(continuation) /doi.org/10.1051/e3sconf/20234520603030 452 φH = ln(H0/HAV), where H0 is the height of the billet and HAV is average height of forging part.HAV can be calculate from equality of volumes of forging part (V) and imagine cylinder with diameter Dmax equal to biggest diameter of forging part VC (fig.4)

Fig. 5 .
Fig. 5. Calculating of volume of intersection between forging part and billet -VIS.

Fig. 6 .
Fig. 6.Left half of the contours of the upper and bottom dies for investigated forging part.