In the field of mechanical manufacturing, the selection and dimensioning of forging blanks is an extremely critical step in the entire forging process. It not only directly affects the quality and performance of the final product, but is also closely related to production efficiency and cost. This article will explore in detail the principles for selecting forging blanks and the methods for determining their dimensions, helping readers gain a better understanding of this complex and important process.
The quality of a forging blank forms the foundation for the final product. The precision and properties of the blank directly affect the accuracy of subsequent turning processes, and the quality of turning further determines the accuracy and efficiency of grinding. Therefore, the selection of blanks plays a crucial role in the entire forging process. A high-quality blank not only meets the product's usage requirements, but can also reduce production costs to some extent, making the product more competitive in the market.
The shape features of a forging blank are determined by its usage requirements. Different usage requirements and shape features form corresponding blank forming process requirements. For example, a shaft-type part used in mechanical transmission requires the blank to have a certain strength and wear resistance, while a disc-type part used for decoration may focus more on appearance and dimensional accuracy. Therefore, when selecting a blank, it is necessary to first clarify its usage requirements, including external quality such as shape, dimensions, machining accuracy, surface roughness, and internal quality requirements such as chemical composition, metal structure, mechanical properties, physical properties, and chemical properties.
For forgings with different purposes, the process characteristics of the material, such as forgeability and weldability, must be considered when determining the blank forming method. For instance, some high-strength alloy steels may have poor forgeability but their machinability can be improved through special heat treatment. Therefore, when selecting a blank, both the material properties and process characteristics must be considered to ensure that the blank can meet the requirements of subsequent processing.
When choosing a blank forming method, the machinability of subsequent processing should also be considered. Some complex blank structures are difficult to form using a single method and require considering the possibility of combining various forming processes. For example, for large and complex structural parts, forging may be followed by welding and machining. In such cases, it is necessary to consider not only the feasibility of each forming method, but also whether their combination affects the machinability of the blank.
When selecting a blank forming method, the principles of adaptability and economy should be followed to ensure that the blank meets production requirements while reducing production costs.
The appropriate blank solution should be selected based on the forging's structure, shape, dimensions, and working conditions. For example, for stepped shaft-type parts, when the diameters of the steps differ slightly, bar stock can be used; when the difference is large, forged blanks are preferable. For forgings used under different working conditions, the choice of blank type also varies. For instance, forgings used in high-temperature environments require blanks made of materials with high-temperature resistance.
The economic principle aims to minimize the costs of forging materials, energy consumption, and labor. When selecting the type of forging blank and specific manufacturing method, multiple preselected schemes should be compared economically while meeting part requirements, and the scheme with the lowest comprehensive production cost should be chosen. Generally, for single-piece or small-batch production, free forging, manual arc welding, and sheet metal work can be used. For batch production, machine forming, die forging, automatic submerged arc welding, or other methods may be adopted.
Good forgeability is the basic requirement for producing blanks via forging. Metals with poor forgeability, such as cast iron, aluminum casting alloys, and bearing alloys, can only be produced by casting. Various structural steels, deformed aluminum alloys, and pressure-processed copper alloys can all be used to produce blanks through forging. Therefore, when choosing a blank production method, the forgeability of the material must be considered.
Parts with simple shapes and single-piece production can use free forging to produce blanks. Parts with more complex shapes or larger production volumes can adopt preform forging or die forging methods. For example, small mechanical parts such as bolts and nuts are usually produced by free forging, while large automotive engine crankshafts require die forging.
During forging, differences in final forging temperature, deformation degree, and cooling speed may cause internal defects such as uneven structure, residual stress, and work hardening. This not only reduces the forging quality but also complicates machining and final heat treatment. Therefore, forgings usually require proper heat treatment. Post-forging heat treatment can homogenize the structure, refine grains, and eliminate residual stress. At the same time, by adjusting the hardness of the forging, the machining performance can be improved.
With the development of plastic forming technology, lubrication and protection techniques for forging blanks have also improved. For example, cold forging and cold extrusion involve large deformation and high unit pressure; without proper lubrication, die life will drop sharply. Therefore, for cold forging, cold extrusion, and drawing of low-carbon steel, medium-carbon steel, eutectoid steel, hypereutectoid steel, aluminum alloys (with large deformation during cold extrusion), low-alloy structural steel, and bearing steel, lubrication typically combines phosphate coatings (films) with metallic soaps, fatty acids, graphite, molybdenum disulfide, or organic polymer materials.
The dimensioning of forging blanks is a complex and detailed process, requiring comprehensive consideration of forging type, process, equipment capability, and material characteristics. The following are specific methods and considerations:
Different types of forgings, such as shafts, discs, or rings, have different shape features and dimension requirements. Therefore, when determining blank dimensions, the forging type must first be clarified, and the blank size range preliminarily estimated according to its shape and dimensions. For example, shaft forgings usually determine blank diameter and length based on the maximum or average cross-section, while disc forgings require consideration of diameter, thickness, and metal loss during forging.
Parameters such as temperature, pressure, and speed during forging affect forging dimensions. When determining blank dimensions, the shrinkage during forging must be estimated. Shrinkage depends on the material's thermal and physical properties and forging conditions, usually determined through experiments or experience. Deformation and metal flow patterns must also be considered to ensure blank dimensions meet post-forging requirements.
Different forging equipment has varying tonnage and die sizes, directly determining the feasible forging size range. When determining blank dimensions, the actual equipment capability must be considered, including maximum forging force, die size, and worktable size, to allow reasonable adjustment and optimization in blank design.
Different materials have varying thermal expansion coefficients, thermal conductivity, yield strength, and other physical properties that affect deformation and shrinkage during forging. These characteristics must be fully considered when determining blank dimensions and forging parameters to ensure forging quality and performance.
Machining allowance should be determined based on forging shape, material, and process requirements to ensure finished part accuracy and stability. Generally, it should be as small as possible to reduce material waste and machining costs, but not too small to affect machining precision.
In actual production, blank dimensions undergo forging, machining, and other steps to become the final product, so the influence of each step on dimensions must be considered and adjustments made. For example, forged parts shrink, so the blank size must be appropriately increased.
Computer-aided design tools such as CAD and CAM can more accurately predict deformation and shrinkage, optimizing blank dimension design and improving production efficiency and product quality. Finite element analysis software can simulate metal flow and deformation during forging, helping designers determine blank dimensions and forging parameters.
The selection and dimensioning of forging blanks are key steps in the forging process, directly affecting the final product's quality and production cost. When selecting blanks, usage requirements, material characteristics, and subsequent machinability must be comprehensively considered, following adaptability and economic principles to ensure that blanks meet production needs while reducing costs. When determining blank dimensions, factors including forging type, process, equipment capability, material characteristics, shrinkage estimation, machining allowance, equipment tonnage, die size, and the use of advanced computer-aided design tools must all be considered to ensure accurate and reasonable blank dimensions, thereby improving production efficiency and product quality.
Through this detailed explanation, readers should gain a deeper understanding of the selection and dimensioning of forging blanks. In actual production, this knowledge helps technical personnel better design and manage blanks, improving product quality and market competitiveness.
