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WHAT IS FORGING

Forging

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: cold forging, warm forging, or hot forging. For the latter two, the metal is heated, usually in a forge.

Application

Forging is a very important manufacturing process. Forged part has high physical strength and good tough property. Forging process is used in auto industrial, machine industrial, and construction industrial parts.

Advantages and Disadvantages

Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Additionally, forgings can target a lower total cost when compared to a casting or fabrication. When you consider all the costs that are involved in a product’s lifecycle from procurement to lead time to rework, then factor in the costs of scrap, downtime and further quality issues, the long-term benefits of forgings can outweigh the short-term cost-savings that castings or fabrications might offer.

Some metals may be forged cold, but iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable to precipitation hardening, such as most aluminum alloys and titanium, can be hot forged, followed by hardening.

Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high-temperature furnace is required to heat ingots or billets. Owing to the size of the massive forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.

Forging Type

Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting

Drop hammer forging machine

Drop forging is a forging process where a hammer is raised and then "dropped" onto the workpiece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does.

Advantages /

  • Reduced chance of voids.
  • Better fatigue resistance.
  • Greater strength.
  • Finer grain size.
  • Improved micro structure.
  • Continuous grain flow.

Disadvantages /

  • Not very economical for short runs due to the high cost of die production.
  • Drop forging presents a dangerous working environment.

Press forging machine

Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.

Advantages /

  • Higher productivity than drop forging.
  • Greater accuracy in terms of tolerances.
  • More complicated shapes can be forged.
  • Possibility of process automation through mechanism of blank feeding and forging removal.
  • The operation is completed in a single squeezing action.
  • The press forging, therefore, is suitable for mass production of nuts, bolts, rivets, screws, break levers, bearing races, valves, etc.

Disadvantages /

  • The initial capital cost is higher compared with drop forging.
  • The difficulty of descaling the blank is another short coming of this process.
  • The process is economically suitable only when the equipment is efficiently utilized.

Forging Part Manufacturing Considerations

For forging part manufacture, the designer should consider the following: shape, parting line, draft angle, corner radii, fillet, wall thickness, tolerance and machining allowance.

鍛件的形狀設計原則

  1. 使鍛件能有最少之完工加工面。
  2. 需能適合於鍛造技術。
  3. 盡可能使其有簡單及對稱的形狀。
  4. 避免有陡急的斷面變化,過度的材料堆積,強烈的方向的轉變及稜銳之邊緣。
  5. 連接部分應有充分的圓弧角
  6. 避免有過分凹入或凸起的角,線或窄肋。
  7. 尺寸精度高的部分盡可能集中在上模或下模上。
  8. 盡可能使鍛件廢邊切除容易。

分模線的設計

分模線(Parting line)或稱分離線,分割線,它是上下鍛模的分開線,通常設計鍛件的第一步驟就是決定分模線的位置與形狀。

  1. 分模線的位置應盡量避免在有深陷(Deep cavity)部位,因如在深陷部位有可能使鍛模破裂,或產生欠肉現象,使該部位成形困難。
  2. 分模線的位置應使避免發生側向推力,因側向推力會使鍛造時發生錯模(Mismatch),而降低鍛模的使用壽命,也影響鍛件的精度。
  3. 分模線應盡可能在同一平面,使鍛造作業及模具製造容易。
  4. 分模線的位置應依金屬流動情形並配合經濟原則作雙重考慮,因有時太重視鍛流線分佈,則往往引起成形困難,並增加成本。
  5. 金屬鍛流線以平行於鍛件外形表面較好,並增加成本。
  6. 分模線的位置應盡量利用自然拔模斜度角。
  7. 分模線應盡可能使上下模對稱。
  8. 分模線的形狀應能有助於剪緣去毛邊。

拔模斜度的設計

鍛件分模線決定之後,為使鍛件易於從鍛模取出,還須於鍛件周圍沿鍛造方向傾斜一角度,此角度即為拔模斜度(Draft angle)

  1. 斜模斜度大小的設定並無依訂的規則可循,通常需按鍛造材料,鍛件形狀,鍛造成形設備,鍛造方法等之不同而異,最普通的拔模斜度是五度。
  2. 從材料的節省及縮短切削加工時間來看,拔模斜度是越小越好,但過小將造成脫模困難,故選定時應特別注意。

內外圓角之設計

由於在鍛件中如有銳利的型穴內外隅角,會使材料流動困難,產生流動穴模不足的現象(Flow through)現象,以致形成冷夾層(Cold shut)之缺陷,太小的外圓角(Corner)將使鍛模產生應力集中,同時因熱疲勞而變形,縮短鍛模壽命,有適當的內圓角及外圓角。

鍛件公差

鍛件之公差通常依鍛件性質,形狀等而定,越狹窄的公差則表示鍛件水準越高,但相對的其製造的工程數也隨之增加,因此,非必要最好不要將公差訂的太嚴。很多國家對於鍛件公差皆訂有標準,一般依成型設備而有落錘與機械式壓力鍛造機熱鍛件公差,而鍛件之品及一般有兩級,即普通即與精密級。

訂定公差的先決事項

  1. 鍛件重量:鍛件設計圖的真實重量。
  2. 分模線的類型:分模線有平直,對稱彎曲及不對稱彎曲三種。
  3. 材料異鍛性:材料易鍛性以含碳量之多寡或各種合金成分數量種和為標準。
  4. 鍛件容積比:薄鍛件或多叉之鍛件,因各種以收縮現象,形力及較大的工具耗損,故較簡單而擁擠在一起的鍛件之尺寸更不穩定。

鍛件的加工裕度

由於鍛件於鍛造後,經常需要在進行機械加工,故鍛件設計時須預留加工所需的裕度,於決定加工裕度之前需先對鍛件特定尺寸及形狀特性有所認識與研析。 影響機械加工裕量的因素有:完成鍛造件之形狀,鍛件材料,胚料表面情況,用之加工方法,加工機器,加工工具及鍛件數量等下列益處:

  1. 降低材料浪費。
  2. 提高切削刀具壽命。
  3. 縮短加工時間。
  4. 降低成本。