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The "rough forging" and "finishing forging" during the hot forging process
Forging in the rough and forging in the fine sense are not simply a process of "first rough and then fine". Instead, they represent a complete material modification process chain from "forming" to "quality determination".
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| Dimensions: | Rough Forging (Coarse Casting) | Precision Forging (Fine Casting) |
| Core objectives | Forming: Achieve close shape quickly and distribute volume | Qualification: Control dimensions, streamline, and surface quality |
| Processing allowance | Large (usually 1.5-3mm on one side) | Minimal (can be controlled within 0.5mm, even net forming |
| Mold characteristics | Simple structure, high strength, long service life. | Complex structure (often with ejection mechanism), high precision. |
| Microstructure | The initial crushed cast structure contains a work-hardening layer. | The structure is uniform and dense, with clear flow lines. |
| Economy | The material utilization rate is relatively low, but the mold cost is low. | The material utilization rate is extremely high, but the investment in molds and equipment is large. |
II. Why must "roughing first and then precisioning" be done?
Extrusion forging is not a high-precision processing directly carried out on the billet; instead, it is a "finishing" process for the rough forgings. This two-step method is designed to address the physical contradictions:
Protecting the mold: The rough forging bears over 80% of the deformation force, quickly shaping the large-section billet. If the precision forging mold were to directly withstand such huge impacts and deformation forces, the expensive precision molds would quickly wear out or crack.
Relieving stress: The heat treatment (such as normalizing) after rough forging can eliminate the internal stress generated during rough processing, providing a stable microstructure base for precision forging and preventing the parts from deforming after precision forging.
Controlled flow lines: Rough forging determines the general direction of the metal fibers, while fine forging further refines these lines to ensure that the flow lines perfectly conform to the contours of the parts (such as the tooth shapes of gears), avoiding any interruptions in the flow lines.
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III. Where does the "precision" in precision forging lie?
Precision forging is not merely about achieving "more accurate dimensions"; its technical threshold lies in the extreme control of the material state:
Dimension accuracy: It can reach IT8-IT9 level. For some complex parts (such as mating gears), net shaping (Net-Shape) can be achieved, eliminating the need for subsequent machining.
Surface quality: With the use of high-precision molds and lubrication, the surface roughness can reach Ra 3.2 - 6.3 μm, which is much better than the Ra 12.5 - 25 μm of rough forging.
Organizational performance: During cold or warm precision forging, the material undergoes further processing and hardening, resulting in a compressive stress layer on the surface, which directly enhances the fatigue strength and wear resistance of the part.
IV. Practical Reference for Process Chain
1. Materials and Cutting
Selection: The preferred materials are 6082 (high strength, suitable for sports cars) or 6061 (mature process, low cost).
Cutting: Cold shearing is recommended. It should be noted that aluminum alloy is prone to shearing deformation. It is recommended to use a high-speed precision band saw to control the weight tolerance within ±0.5%.
2. Rough Casting (Hot Die Casting)
Heating: The billet is heated to 480-520¡æ (6082) or 450-500¡æ (6061). It is strictly prohibited to exceed 550¡æ, otherwise the grain size will rapidly become coarse.
Lubrication: Water-based graphite lubricant must be used, which serves both lubrication and cooling purposes.
Process: Pre-forming → Final forging. The key lies in one-time forming without repeated heating. After rough forging, leave a residual allowance of 0.8 - 1.2mm on one side.
3. Precision Forging (Cold Finishing)
Process: Conducted at room temperature. By leveraging the work-hardening property of aluminum alloy, the surface hardness is directly enhanced.
Key areas: Installation holes for the straightening direction column, handle clamping groove, and edge of the weight reduction holes. Ensure that the streamline completely covers the threads and edges, preventing stress concentration and cracking.
Accuracy: The dimensions can reach IT9 level, the surface roughness is below 3.2 μm, and it can be directly anodized.
4. Heat treatment (T6 state)
Process: Solution treatment (530°C ± 5°C) → Water quenching (transfer time < 15 seconds) → Artificial aging (175°C / 8-10 hours).
