Introduction
Integrated die-casting technology offers advantages like high production efficiency and low manufacturing costs. Currently in a phase of rapid development, it holds potential for producing multiple large components, simplifying body structures, and revolutionizing body manufacturing processes [1]. The integrated die-cast rear floor consolidates over 70 original parts into a single component, significantly reducing vehicle weight and improving production efficiency. Molds, process, die-casting machines, and heat-treatment-free materials constitute the four core technologies of integrated die-casting [2-3]. Die-casting machines specifically refer to large-scale machines with clamping forces exceeding 60,000 kN [4]. Heat-treatment-free materials primarily used today are high-strength, high-toughness cast aluminum alloys [5], known for their high specific strength, excellent castability, moderate cost, and status as the primary material for current integrated die-cast automotive components [6-7].
Domestic large aluminum die-castings face the challenge of decreasing yield rates with increasing integration complexity. Key factors affecting qualification rates include:
1. Unstable quality at critical load-bearing points: Installation surfaces for shock absorber towers, subframes, and C-pillars demand high mechanical properties. These areas are often difficult to sample test, must not contain internal pores exceeding standards, and must be free of cold shuts externally. Notably, the C-pillar installation surface near the wheelhouse edge is prone to cold shuts.
2. Unstable dimensions at critical mating surfaces: Thin-walled side panel installation surfaces at the casting edge are susceptible to inward or outward deformation, or even twisting (front outward, rear outward). This compromises the stability after mating with counterpart parts and can cause pre-formed hole misalignment, leading to machining failure [8-15].
This study employs simulation to predict defects in an integrated die-cast rear floor component and optimizes the gating and overflow system to improve internal quality, aiming to provide a reference for designing similar large castings.
1 Structural Characteristics and Technical Requirements
The rear floor casting constitutes the floor section rearward of the passenger compartment, integrating components like left/right rear wheelhouses, rear longitudinal beams, crossbeams, floor connection plates, and inner beam reinforcements. The casting has overall dimensions of 1,630 mm × 1,624 mm × 666 mm, a mass of 63 kg, an average wall thickness of 3 mm, and a projected area of 23,000 cm². Due to its large size, thin walls, and the significant space requirements, long cycle times, and distortion risks associated with heat treatment, a heat-treatment-free aluminum alloy is mandated.
SPR (Self-Piercing Riveting) process is suitable for cold joining dissimilar steel-aluminum materials [8]. Consequently, the casting's front and rear ends connect to the front floor and rear floor assembly, respectively, via SPR. The left and right wheelhouses can also connect to the side panels via SPR. While these four edges are not primary load-bearing zones of the rear floor, they demand high sealing and connection integrity, corresponding to requirements for flatness and high strength-toughness of the material.
Appearance Requirements: Free from defects like cold shuts, cracks, and chips.
Material Performance Requirements (Pre-bake):
SPR Locations (Body Sampling): Tensile Strength ≥ 215 MPa, Yield Strength ≥ 115 MPa, Elongation ≥ 12%, Bend Angle ≥ 20°.
Rear Half of Wheelhouse (Slightly Lower): Tensile Strength ≥ 215 MPa, Yield Strength ≥ 110 MPa, Elongation ≥ 6%, Bend Angle ≥ 20°.
Other Areas: Elongation requirement between 6% and 12%.
Considering the inherent inhomogeneity of mechanical properties in die-cast body samples, achieving specified mechanical properties everywhere within designated areas is challenging. Therefore, bench tests are required to verify the key load-bearing performance of shock absorber towers and longitudinal beams [9]. Bench tests typically include durability and crush tests:
Durability and Z-direction Crush Tests: Simulate rear shock absorber loading. Durability test mean load is 11.5 kN. Under Z-direction crush, first-stage loading of 38 kN requires loading point deformation ≤ 3 mm; second-stage loading of 74 kN requires no cracking at the loading point.
X-direction Crush Test: Simulates longitudinal beam loading. Under unilateral loading force ≥ 206 kN, no cracking and deformation ≤ 3 mm must occur at the loading point.
2 Die-Casting Process Design
2.1 Gating System Design
The developed rear floor has a front cover plate connection window at its front end. However, its high aspect ratio (3.14) and edge location make center gating unsuitable. A single-side gating approach, typical for conventional die-castings, was adopted. Based on Magma flow analysis results, three runner designs (S1, S2, S3) were sequentially optimized:
S1 and S2 designs utilized a 70,000 kN die-casting machine.
S3 design utilized a 120,000 kN die-casting machine, incorporated minor structural optimizations to the casting body, and increased plunger diameter, number of ingates, and ingate area.
2.2 Filling and Solidification Simulation Analysis
Magma software simulated the rear floor die-casting process. Mold material was H13 tool steel; casting material was C611 high-strength, high-toughness aluminum alloy [1]. Parameters set: Melt temperature 680°C, Plunger temperature 200°C, Shot sleeve temperature 250°C, Mold temperature 180°C. Injection parameters varied per scheme.
Scheme S1 Analysis Results:
At end-of-fill, the wheelhouse edge position had the lowest temperature (~618.6°C) and solidified first (solid fraction ~1%). Actual casting requires higher melt temperature and focused monitoring of mold surface temperature in this region. Due to mold temperature variations, cold shut risk exists at the mid-section wheelhouse edge.
When melt reached the rear cavity half, the restricted flow area caused filling speeds up to 60 m/s. Two melt streams converged at the center of the end crossbeam. High speed caused melt swirling, creating a high risk of cold shuts and cracks, reducing mechanical properties.
A significant step difference and thicker wall near the ingate of the rear longitudinal beam caused large isolated air pockets on both sides. Machined holes in this zone make porosity defects detrimental to yield.
After melt entered the sill beam, casting pressure increased steadily to 30 MPa. Based on the casting body projected area (18,136 cm²), this required a clamping force of 69,000 kN. Considering a safety factor of 1.2 and including the gating system (estimated projected area ~25,000 cm²), the required clamping force reached 90,000 kN, exceeding the 70,000 kN machine's capacity.
Scheme S2 Analysis Results:
Adding a runner directly opposite the wheelhouse reduced wheelhouse fill time to 51 ms (vs. 59 ms for S1). Overall fill time was 86 ms.
Turbulence in both wheelhouses was more pronounced. The gas content was highest at the melt confluence point in the crossbeam at end-of-fill, creating high risks for porosity, cracks, and shrinkage defects [7].
The cold flow issue in the wheelhouse area was not effectively resolved.
Scheme S3 Analysis Results:
Optimizing the runner based on previous schemes, overflow wells were added at the wheelhouse edge center and end crossbeam center. Ingate area was increased (requiring higher injection force to maintain speed). Machine clamping force was upgraded to 120,000 kN.
Wheelhouse edge temperature was lower than S1/S2 but near the liquidus temperature. Melt reached the ingates at 305 ms (timing started from biscuit fill), with a max speed of 60 m/s. The cavity filled completely at 390 ms, taking 85 ms. Casting pressure was 40 MPa.
Based on the S3 gating system projected area (25,813 cm²), the maximum casting pressure the 120,000 kN machine could provide was 46.5 MPa, meeting the requirement.
Overflow wells added beside the wheelhouse improved air entrapment compared to S2. Proximity to the ingate also reduced porosity risk.
The S3 scheme was selected for mold manufacturing.
3 Test Methods and Results
3.1 Die-Casting Parameters and Test Methods
Production used a Lijin 120,000 kN die-casting machine. Alloy was C611 heat-treatment-free material (chemical composition met specifications). Compared to traditional AlSi10MnMg structural materials, heat-treatment-free alloys offer better as-cast toughness, beneficial for riveting. Melt temperature was 680°C. Dynamic and fixed mold vacuum was 10 kPa.
Process Flow: Spraying → Blow-off → Mold Closing → Pouring → Vacuum Evacuation → Injection → Local Squeeze → Direct Cooling/Spot Cooling → Mold Opening → Robot Extraction → Integrity Check → Water Quenching → Trimming & Straightening → Marking → Robot Handling → Casting Offline → Manual Deburring → Appearance & Dimensional Check → Transfer to Next Process.
Internal quality inspection used a Maice FSC heavy-duty 9-axis X-ray inspection machine. Tensile specimens were first cut from the casting body as small blanks (80-100 mm length, 15-30 mm width), then machined into standard tensile specimens with a 25 mm gauge length.
3.2 Internal Quality Inspection
X-ray inspection results showed no significant porosity defects at the ingate areas, rear crossbeam, or side wheelhouses of the rear floor casting. Internal quality met ASTM E505 Level 2 standards. Due to thicker walls, machining hole bosses were prone to porosity, requiring further checks for exposed pores and compliance with appearance standards. Load retention tests for threaded inserts or self-tapping screws were performed using a CMT5305 tensile testing machine.
3.3 Tensile Mechanical Properties from Body Sampling
Mechanical properties were tested at 39 locations on the casting body. Sampling points were symmetrically distributed (L: Left body side, R: Right body side), covering key areas:
Positions 1-10: Wheelhouse edge (side riveting edge).
Positions 11-20: Wheelhouse middle section.
Positions 21-23: Ingate area (rear floor assembly riveting edge).
Positions 31-34: Front cover plate connection edge.
Positions 35-37: Front floor riveting edge at the end-of-fill.
Results:
Tensile strength (TS) and yield strength (YS) were relatively stable across locations. Average TS was 237 MPa; average YS was 118.9 MPa.
Elongation varied significantly by location, averaging only 6.5%, with some points below 6%. The average elongation value is influenced by sampling location and quantity and serves only as a reference [9]. For comparison, another rear floor using the same material achieved an average elongation of 9%.
Based on the customer's initial development requirements, body properties (especially elongation at some locations) could not be fully met. Therefore, body sampling results alone cannot be the sole criterion for product qualification. The overall performance must be judged based on bench test and full vehicle validation results.
4 Conclusion
(1) Magma software was used to design and optimize the gating system for the C611 aluminum alloy rear floor casting. Simulation revealed that significant wall thickness variations in step regions, combined with low melt temperature flowing through these areas, create risks for air entrapment, cold shuts, and cracks. Analysis of filling pressure in the end crossbeam region indicated that a die-casting machine with a clamping force exceeding 90,000 kN is necessary for complete forming of the rear floor.
(2) Selecting a 120,000 kN die-casting machine for production, coupled with simulation-based optimization, effectively eliminated porosity and shrinkage porosity defects. However, cracks prone to occur in structural transition zones and areas with significant wall thickness variations impacted mechanical properties. Average yield strength, tensile strength, and elongation from specimens cut from the C611 rear floor casting body were 118.9 MPa, 237 MPa, and 6.5% respectively, essentially meeting the main design targets (TS ≥ 215 MPa, YS ≥ 115 MPa, Elongation ≥ 6%).
(3) Compared to traditional forming processes like riveting and stamping, the integrated die-cast rear floor achieved a weight reduction exceeding 10%. Future adoption of 200,000 kN die-casting machines holds promise for achieving short-cycle, low-cost, and high-strength/high-toughness manufacturing of integrated automotive body castings.
