Treatment of Folding Defects in Forged Aluminum Alloys

During the forging of aluminum alloys, a common issue that can arise is a defect called "folding", which occurs due to oxidation during the deformation process. Folding happens when oxide scale builds up on the surface and causes the metal layers to merge together. This defect typically appears on the surface of the metal. If not addressed in time, folding can negatively affect both the mechanical properties and the appearance of aluminum alloy parts. In this article, we'll explore the causes, types, and solutions for folding defects in forged aluminum alloys, with the aim of improving product quality.

What are Folding Defects?

Folding defects occur when oxide scale accumulates on the metal's surface during deformation. As the scale builds up, it merges with the surface metal, creating a layered defect. This defect is usually visible to the naked eye after removing the oxide scale. If the folding is shallow, it generally means more polishing is needed. However, if the folding is deep, it could lead to the entire part being scrapped.

For defects that are harder to see with the naked eye, non-destructive testing methods can be used. Techniques like Magnetic Particle Inspection (MPI) or Penetrant Testing (PT) can detect hidden defects, preventing undetected folding from affecting further processing and use.

Oil Seal Folding Defects

Oil seal folding is most common in crankshafts with oil seal flanges during testing. The defect appears in two forms: poor closure or incomplete closure. This issue is typically linked to axial movement during the forming process. Early on, part of the shape is formed, but as the process continues, excessive axial displacement occurs. This causes the already-formed part to shift axially, which leads to folding defects when the material is replenished.

Treatment Methods

Pre-forging Process: Adjust the blank's shape during pre-forging to reduce step height. This helps improve the efficiency of forming the rear material and reduces folding defects.

Increase Blank Length: If pre-forging isn't used, increasing the length of the blank at the rear can prevent axial displacement.

Blank Positioning: When mass producing parts, be extra careful with positioning the blank. This will avoid folding defects that occur when the blank shifts to one side.

Connecting Rod Neck Folding Defects

Connecting rod neck folding is a common issue in crankshaft forging. It typically happens at the upper part of the mold's split face. The defect appears as either a transverse "I" or "8" shape, or a longitudinal "I" shape, or as misaligned grooves. This defect is often seen when there is no pre-forging step, especially if the mold design is poor.

Cause Analysis: Without pre-forging, during the initial deformation of the crankshaft, the mold's protruding part forms first, creating a pit in the blank. Later, the pit shifts sideways, and the metal flows back to fill the unfilled pit, causing a folding defect.

Treatment Methods

Adjust the Mold: Control the impact force during different forming stages. Increase the round corners of the mold's protruding parts to reduce uneven metal flow.

Increase the Web Thickness: Increase the web thickness at the opening to improve metal flow, preventing pits and reducing folding.

Optimize Pre-forging Design: By fine-tuning the pre-forging process and parameters, folding defects in the connecting rod neck can be avoided.

In crankshafts produced by forging presses, a pre-forging step is usually used to prevent these defects. However, if the material storage and distribution structure during pre-forging isn't designed well, grooves or folds may still appear.

Folding Defects from Edge Burrs

During forging, high longitudinal burrs created during edge trimming can be pressed into the forged part during later correction, causing folding. This is especially common with crankshafts that have twisted shapes. After edge trimming, burrs may be transferred to the mold groove during the correction process. In hot correction, long burrs are pressed into the forged part, eventually causing folding.

Treatment Methods

Replace Edge Trim Dies: Regularly replace or repair worn edge trim dies to prevent the creation of excessive longitudinal burrs.

Adjust the Edge Trimming Process: Optimize the edge trimming process to reduce burr formation and prevent folding defects.

Folding Defects in Incompletely Formed Parts

During forging, some parts, especially at the balancing block split face, may have deep folding defects, even after final forming. This often happens when the blank size is too small, causing incomplete forming.

Treatment Methods.

Increase Blank Size: Increase the size of the blank to ensure that it fully fills the mold during the forming process, avoiding underforming in certain areas.

Optimize Mold Design: For crankshafts that are easy to form, even with smaller blanks, use mold adjustments like round corners and proper material distribution to reduce folding defects.

Use Semi-Closed Process: When pre-forging is not used, a semi-closed process can help prevent folding defects.

Conclusion and Recommendations

Folding defects in forged aluminum alloys are common and can be complex to deal with. Their causes include oxide accumulation, mold design issues, the forming process, and blank size. To effectively reduce folding defects, the following measures should be taken.

Optimize the Forming Process: Use proper pre-forging design, increase blank size, and adjust mold parameters to avoid folding defects.

Improve Mold Design: Adjust the mold structure, such as increasing round corners and web thickness, to optimize metal flow and prevent folding.

Regular Equipment Maintenance: Regularly check and replace worn edge trim dies and correction molds to avoid defects caused by equipment issues.

Quality Control: Carefully control blank positioning, material storage, and distribution processes to ensure quality at every step and prevent folding defects.

By applying good process design, strict quality control, and regular maintenance, the occurrence of folding defects in forged aluminum alloys can be significantly reduced, leading to better product yield and quality.