Why semi-solid die-casting machining parts have become the first choice for manufacturing core structural components of humanoid robots

With the rapid landing of the humanoid robot industry, the mass production process of core structural components has become a key factor restricting large-scale delivery. In the first half of 2024, we collaborated with six humanoid robot manufacturers on joint shell projects, and four of them encountered the same problem: during the prototype stage, solid aluminum billets were used for five axis machining, which met performance standards but cost over 2000 yuan per part. After entering the small batch production stage, we switched to traditional die-casting blanks, reducing the cost to 300 yuan. However, after 100 hours of durability testing, 40% of the parts experienced stress cracking.

Pain points in the mass production process of humanoid robot structural components

The core structural components of humanoid robots, such as hip/knee/shoulder joint shells, large and small arms, and leg supports, face three rigid constraints simultaneously:

Mechanical properties: It needs to withstand more than 1 million alternating loads, with a yield strength of ≥ 240MPa;

Complexity: Built in wiring cavity, motor installation position, reducer positioning surface, multi curved irregular structure;

Cost: The comprehensive cost of a single component during the mass production stage needs to be controlled within 1/5 of the prototype;

Traditional casting, forging, and direct milling methods cannot simultaneously meet the three constraints, while semi-solid die-casting machining parts have become a recognized solution in the industry for mass production needs in the past year's production verification.

Comparison of adaptability of four types of forming processes for humanoid robot structural components

We have compiled four mainstream blank forming schemes and compared the core parameters for the structural component requirements of humanoid robots:

From the comparison, it can be seen that the semi-solid die-casting machining parts have achieved a balance in the three dimensions of performance, cost, and delivery cycle, and are also the preferred solution for the core structural components of humanoid robots under the current mass production scale of 1000-10000 pieces. The three mass-produced humanoid robot customers currently served by Huiwen Zhizao have all switched their joint structural components to the process route of semi-solid die-casting blanks+five axis precision CNC machining.

Three major technical difficulties and solutions for using semi-solid die-casting machining parts for humanoid robot structural components

The process principle of semi-solid die casting is not complicated, but to meet the high reliability requirements of humanoid robots, three core technical difficulties need to be solved. After more than 20 batches of process iterations, our team has formed a mature solution:

Difficulty 1: Internal shrinkage control of non-uniform wall thickness structures

The joint shell of humanoid robots is mostly a variable wall thickness structure of 3-12mm, and the cooling shrinkage rate of liquid aluminum alloy is 4% -7%. If the shrinkage of the thick wall position is insufficient, it is easy to produce internal shrinkage holes of ≥ 0.1mm, which will directly reduce the fatigue life of the parts by more than 40%. Our solution is:

1. The gate design corresponds to the maximum wall thickness position of the part, using a 12mm diameter straight gate, and directly supplementing the thick wall area during the pressure holding stage;

2. The holding pressure is set to 65~75MPa, and the holding time is set to 8~15s according to the wall thickness to ensure that the metal liquid is fully replenished under pressure;

3. The mold temperature is controlled at 220-260 ℃ to achieve sequential solidification from thin wall to thick wall, avoiding the formation of pores in local hot spots.

Through this solution, the semi-solid die-casting parts we delivered were subjected to X-ray inspection, with internal porosity controlled below 0.2% and no independent pores exceeding 0.05mm, fully meeting the requirements for robot alternating load use.

Difficulty 2: Post processing deformation caused by residual stress in the blank

When semi-solid die-casting parts are molded, the temperature difference can reach 150 ℃. If the residual stress inside is not released, the shape and position tolerance deformation of 0.05-0.12mm will occur after 1-2 weeks of processing, directly leading to excessive joint assembly clearance. Our solution is:

After the blank is molded, it immediately enters a hot air circulation furnace at 120 ℃ for 4 hours of insulation for low-temperature stress relief treatment, releasing more than 70% of residual stress;

Add secondary vibration stress relief treatment after rough machining to further release the stress generated during processing;

During the precision machining stage, the allowance is removed in layers, and the final machining allowance is controlled at 0.15~0.2mm to avoid excessive cutting stress;

The final joint shell we delivered had a positional tolerance variation of ≤± 0.008mm after 15 days of storage, fully meeting the high-precision requirements for joint assembly.

Difficulty 3: Process stability control for mass production

During the mass production stage of humanoid robots, the monthly demand for a single structural component can reach thousands of pieces. If the hardness and machining allowance of the blank fluctuate too much during mass production, it will lead to a decrease in the yield rate of five axis machining and a delay in the delivery cycle. Our control plan is:

Three pieces of semi-solid die-casting blanks are selected from each batch for hardness testing, with a hardness deviation controlled within HB8. Batches that exceed this range are directly scrapped;

Extract 10 pieces from each batch for 3D scanning retesting, with machining allowance deviation controlled within ± 0.2mm, to ensure that subsequent five axis machining parameters do not need to be frequently adjusted;

Each batch of raw materials for robot aluminum alloy parts undergoes material spectral testing to ensure that the alloy composition meets the 6061-T6 standard and avoid performance differences caused by material fluctuations

Through this casing control process, the batch processing yield of our semi-solid die-casting parts has remained stable at over 98.5%, with a delivery cycle error of no more than 1 day.

Huiwen Intelligent Manufacturing Semi solid Die Casting+Five Axis Processing Full Chain Delivery System

As a full industry chain service provider specializing in the field of robotics, Shenzhen precision machining service provider Huiwen Zhizao has formed a complete delivery capability from process evaluation to assembly verification for the core structural components of humanoid robots

After the customer provides the drawings for the pre process evaluation, our engineers will provide a process plan within 24 hours, including whether it is suitable for semi-solid die casting, the comprehensive cost of blank+processing, and the delivery cycle. For sample requirements of less than 100 pieces, we can also directly provide a solid blank five axis processing plan, which can be shipped within 3-7 days to help customers quickly complete prototype verification.

Our 10000 square meter production workshop is equipped with 16 five axis machining centers for integrated production control. The semi-solid die-casting blanks are directly processed after non-destructive testing and stress relief, without the need for cross factory circulation, fully meeting the high-precision requirements of robot reducer positioning surfaces and motor installation positions.

We strictly implement the IATF16949 quality management system for quality traceability throughout the entire process. The raw material inspection report, processing parameters, and factory coordinate inspection report for each batch of parts can be traced. Key dimensions are inspected using German imported Zeiss coordinate inspection to ensure stable delivery quality.

As a professional ODM service provider in the field of robotics with integrated design, manufacturing, and assembly capabilities, we can provide full process services from blank forming to finished product assembly, adapt to the full stage needs of humanoid robots from prototype verification to large-scale production, help customers shorten development cycles, control batch production costs, and ensure the delivery stability of core structural components.