Views: 0 Author: Site Editor Publish Time: 2026-03-18 Origin: Site
The failure modes of extrusion dies mainly include three normal failure types: wear, cracking and deformation. Additionally, premature failures may occur due to improper operation or nitriding quality issues.
Frictional wear failure is the predominant failure mode of extrusion dies, accounting for over 70% of total die failures. Its essence lies in the gradual wear of die working surface materials under frictional forces.
1.1.1 Microscopic Mechanisms of Wear
During aluminum alloy extrusion, the material contacts the die cavity surface under high-temperature and high-pressure conditions without lubrication, particularly through direct contact with the flat surface of the sizing strip and high-speed sliding, generating significant friction forces. Research indicates that the die working surface exhibits two contact zones: adhesive contact in the entry zone and sliding contact in the exit zone, with an unstable adhesive-sliding transition region existing between them.
The wear of aluminum extrusion dies is primarily caused by thermal wear—friction leads to elevated surface temperatures of the die, material softening, reduced wear resistance and subsequent adhesion with aluminum alloys. Temperature is the critical factor influencing thermal wear; higher temperatures result in more severe thermal wear.
1.1.2 Manifestations of Wear
The specific manifestations of wear failure include:
Blade passivation: The edge of the sizing strip entrance becomes rounded, resulting in dimensional deviation of the profile.
Rounding of edges: Alteration in geometric shape of die protrusions
Planar depression: Concave pits appear on the surface of the working zone
Surface scratch marks: Scratches along the extrusion direction
Adhesive die: Aluminum alloy adheres to the die surface, altering the geometric shape of the die.
1.1.3 Dominant Role of Chemical Wear
Researchers found that the initial stage of wear in extrusion dies is primarily characterized by chemical wear, followed by the detachment of the hard surface layer or pitting corrosion. For dies subjected to nitriding treatment, the compound layer (white-bright layer) first experiences slight abrasion before partial detachment. After the removal of the compound layer, wear pits measuring 20-50μm in depth form at a distance of 0.5-1.5mm from the die entrance. This indicates that the die working zone's entrance area is the most severely worn region.
Cracking failure refers to the phenomenon where cracks propagate in dies during service, ultimately leading to fracture.
1.2.1 Crack Initiation and Propagation
Cracks typically occur in stress-concentrated areas of the die, such as transitional fillets, split-flow edges and cantilever roots. Crack initiation can occur under two scenarios: first, microcracks generated by fatigue after a certain service period gradually propagate; second, pre-existing microcracks during heat treatment or electrical machining that expand during early service stages.
1.2.2 Main Cause of Crack
The design factors contributing to fracture failure primarily include insufficient die strength design and improper selection of fillet radius at transitional areas. Manufacturing factors encompass material defects, excessive surface roughness during processing, and electroforming deformation layers. Operational factors involve inadequate die preheating, excessively high extrusion ratios and overly rapid extrusion speeds.
The research shows that the wire cutting process will form a tensile stress layer on the surface of the die hole. If the sufficient tempering treatment is not carried out, it is easy to produce slagging and spalling, which reduces the life of the die. The electrical discharge machining process will form a metamorphic layer on the machining part due to the heating and cooling effect and the electrochemical effect of the machining fluid, which will produce residual stress and reduce the fatigue strength.
Deformation failure is the phenomenon that the die can not be used because of the change of its geometry.
1.3.1 Typical Manifestations
Bend eccentricity: Deflection of cantilever structure
Depression: Local concavity on the die surface
Eccentricity of the split die tongue: The upper die tongue deviates from the central position
Mold cavity collapse: Compression-induced deformation of the hollow structure
Pore enlargement: Increased die cavity size
Angular collapse: collapse of protruding areas
1.3.2 Deformation Mechanism
The fundamental cause of deformation failure lies in insufficient material strength of the die to withstand extrusion stress, or uneven force distribution leading to localized stress exceeding limits. Specific contributing factors include: improper material selection or incorrect heat treatment processes that fail to fully utilize the strength-toughness properties of die steel; poorly designed split-flow die design resulting in uneven flow velocity across diversion holes and subsequent lateral forces; and inadequate mold machining precision causing irregular metal flow patterns.
2.1.1 Material Property Requirements
Extrusion dies operate under high temperature and high pressure while enduring cyclic loads, which imposes extremely stringent performance requirements on die steel:
Thermal stability: Maintains hardness at high temperatures of 500-600°C
Thermal fatigue: Resistance to repeated heating and cooling cycles
Thermal wear resistance: Wear resistance at high temperatures
Sufficient toughness: Prevent brittle fracture
2.1.2 Advantages of H13 Steel
Currently, 4Cr5MoSiV1 (H13 steel) is widely used in China for manufacturing extrusion dies. H13 steel exhibits excellent hardenability, thermal strength, wear resistance and plasticity, along with high impact toughness and resistance to thermal fatigue. It also demonstrates minimal deformation during heat treatment and superior crack propagation resistance.
Practical data indicate that when manufacturing the same type of die with H13 steel and 3Cr2W8V steel, the former exhibits a service life 3-5 times longer than the latter. H13 steel contains higher concentrations of Cr and Mo elements, which during nitriding treatment form abundant and stable nitrides with dispersed distribution—this is the key factor contributing to its superior performance.
Reasonable die structure design is an important link to prolong service life.
2.2.1 Wall Thickness Difference Treatment
For profiles with unequal wall thicknesses, unequal-length working strips should be designed. The working strip height (h) is determined by the empirical formula h1/h2=b1/b2, where h represents the working strip height and b denotes the profile wall thickness. This approach ensures uniform metal flow and prevents localized overloading.
2.2.2 Avoid Stress Concentration
During design, sharp corners, concave angles, significant wall thickness variations and flat-walled thin-section cross-sections should be avoided to prevent excessive stress concentration. The selection of fillet radius is critical—excessively small fillets lead to stress concentration, while overly large fillets may compromise die strength. Proper adjustment of fillet radius can ensure more uniform metal flow.
2.2.3 Die Hole Dimensions Determination
The determination of die hole dimensions requires comprehensive consideration of the profile properties and the shrinkage rate of die materials. For 6063 aluminum alloy and H13 steel, the design shrinkage rate of die holes should be set at 1.01%-1.09% (appropriately selected based on die hole size).
2.2.4 Die Cavities Position Arrangement
Die cavities should not be positioned too close to the die edge, as this may reduce die strength and lead to metal flow into dead zones, thereby compromising product surface quality. The extrusion coefficient (elongation coefficient) should be controlled within the range of 10-50.
The quality of heat treatment directly affects the service life of the die.
2.3.1 Recommended Heat Treatment Process for H13 Steel
Production Processes | Temperature Range | Soaking Time | Remarks |
Preheat | 600-630℃ → 830-850℃ | 1.5-2.0h | Reasonable adjustment of micro defects |
Quenching | 1040-1080℃ | 2-2.5h | Quench the oil after heating, then remove and air-cool at approximately 130°C |
Single Tempering | 380-400℃ → 580-600℃ | 1h → 2h | Gradually increase the temperature to prevent cracking |
Secondary Tempering | 560-580℃ | 2h | Air cooling after coming out of the furnace |
2.3.2 Technical Points
H13 steel is highly sensitive to quenching temperature and exhibits excellent quenching performance at high temperatures, necessitating high-temperature quenching. Significant internal stresses remain in the die after quenching, requiring tempering within 1-2 hours to eliminate quenching stresses. Secondary tempering ensures microstructural stability and complete removal of residual austenite.
2.4.1 Nitrogen Treatment
Nitriding treatment is currently the most commonly used surface strengthening method for extrusion dies, accounting for approximately 90-95%. Nitriding can significantly increase surface hardness while maintaining sufficient toughness in dies, thereby reducing thermal wear.
Key points of nitriding treatment:
Multiple nitriding: Perform 3-4 repeated nitriding treatments during the mold service life.
Nitriding layer thickness: Generally required to reach 0.15-0.20 mm
Nitriding timing: Perform repeated nitriding during the initial use of the mold to achieve optimal surface properties.
2.4.2 Rigid Thin Film Coating
Physical vapor deposition (PVD) and chemical vapor deposition (CVD) coating technologies are being progressively applied. Studies have demonstrated that CVD TiC+TiN coatings exhibit superior wear resistance compared to nitriding treatments. Thermal diffusion treatment combined with the presence of V and Nb carbides/nitrides significantly enhances wear resistance.
For extrusion dies of miniature porous aluminum tubes, applying a hard film coating can effectively resolve adhesion issues between aluminum or lubricants on the die surface layer, thereby reducing product scrap rates.
2.5.1 Extrusion Speed
Extrusion speed directly affects die temperature and metal flow uniformity. Excessive extrusion speed significantly increases die stress, thereby accelerating wear and leading to uneven metal flow and elevated die temperature. If the residual heat generated during deformation is not promptly removed, the die may fail due to localized overheating. The recommended extrusion speed is generally controlled below 25 mm/s.
2.5.2 Die Preheating
Dies must be thoroughly preheated before use, typically reaching 440-460°C and maintained at this temperature for over 2 hours to ensure uniform temperature distribution both internally and externally. Insufficient preheating can lead to excessive temperature difference between the die surface and core, resulting in thermal stress and accelerated formation of thermal fatigue cracks.
2.5.3 Stepwise Utilization Intensity
The die should adopt a low-high-low usage intensity strategy throughout its service cycle:
Early stage of use: The microstructure properties of the die are still in a fluctuating phase. A low-strength operation protocol is adopted to facilitate the transition of the die to a stable state.
Mid-term usage: The die exhibits optimal comprehensive performance, with appropriate enhancement of operational strength.
Later usage: Internal structural deterioration occurs, leading to reduced thermal fatigue strength. The usage intensity should be appropriately decreased to prevent deformation and cracking.
Optimize die structure: Utilize computer-aided design (CAD) and finite element analysis (FEA) to ensure uniform stress distribution and avoid stress concentration.
Rational material selection: Prioritize H13 steel to ensure material quality and conduct pre-inspection of material properties.
Control processing quality:
Thorough tempering after wire cutting improves the surface tensile stress state;
Prevent the formation of the degradation layer during discharge machining, and remove the degradation layer when necessary;
Within the permissible range of drawing design, the larger the diameter of the wire cutting wire, the better;
Standardized design: Facilitates die interchangeability, storage and maintenance.
Strictly implement heat treatment processes: control heating rate, quenching temperature, quenching rate and tempering temperature.
Multiple tempering: Perform at least two tempering cycles to ensure microstructural stability
Preparatory repeated nitriding: Perform 3-4 nitriding treatments prior to use to achieve a nitride layer thickness of 0.15-0.20 mm.
Consider advanced coatings: For precision dies, CVD/PVD coatings may be attempted.
Scientific use system:
Strictly implement the die preheating system;
Control the extrusion speed ≤25 mm/s;
Adopt a low-high-low stepwise usage intensity;
Avoid extreme temperature fluctuations and alternating loads.
Regular nitriding maintenance: Perform nitriding again after a certain service cycle to restore surface hardness.
Timely die repair: Address minor wear or deformation promptly to prevent defect propagation.
Operating Condition Optimization:
Water-cooled or nitrogen-cooled dies were employed;
Ensure adequate lubrication to reduce friction;
Optimize extrusion temperature and extrusion ratio to reduce die load.
Establish die archives: Record materials, heat treatment processes, usage frequency and repair history for each set of dies.
Prediction: Based on the Archard Wear Model and Finite Element Analysis, predict the remaining of die life
Scrap criteria: Establish clear die scrap criteria, including dimensional tolerance and crack depth.
Failure Analysis: Systematic analysis of early failure of dies to find out the cause and improve.
