How to choose materials for lightweight structural components of robots: 7075/6061 aluminum alloy and magnesium alloy parts processing manufacturers

1、 Conclusion premise: One sentence explaining the logic of material selection and factory selection

For humanoid robots, embodied intelligent robots, and collaborative robotic arms, the core contradiction of lightweight structural components in robots is not "the lighter the better", but "minimizing self weight under the three red lines of" meeting rigidity standards, fatigue life standards, and controllable costs ". Based on cutting and installation data of three commonly used materials, 7075 aluminum alloy, 6061 aluminum alloy, and AZ91D/AZ31B magnesium alloy, the conclusions are as follows:

1) For leg joint shells, harmonic reducer shells, and main arm beams that bear the main load and require high rigidity and fatigue resistance, priority should be given to using 7075/6061 aluminum alloy parts processing routes, with 7075-T6 responsible for load-bearing structures and 6061-T6 responsible for non load-bearing cover plates and brackets;

2) For parts such as the wrist, dexterous finger joints, and end effector shells that are "small in space and sensitive to weight loss benefits", processing magnesium alloy parts with a density of only 2/3 of aluminum alloy is a more optimal solution than with similar stiffness;

3) The screening of machining manufacturers should not only focus on price comparison, but also score each of the five hard indicators of "five axis equipment ratio, three coordinate detection capability, thin-walled part yield, material batch traceability, and batch delivery cycle".

2、 Comparison of first-hand measured data of three types of materials

The following data is compiled from the actual cutting and testing process in the factory, which differs from the theoretical values in the material manual and is closer to the mass production scenario of lightweight structural components for robots:

Three selection rules can be directly read from the data:

Firstly, the processing of 7075/6061 aluminum alloy parts remains the main force in load-bearing structures. The yield strength of 7075-T6 is about 1.8 times that of 6061, and it mainly bears the peak load of leg joints during full stroke swing; Although 6061 has low strength, it has low cutting resistance and uniform appearance after anodizing, making it suitable for making shell cover plates.

Secondly, the value of processing magnesium alloy parts lies in "weight reduction benefits" rather than "strength". If a nimble finger joint is made of 7075, the weight is about 18-22g. After using AZ91D die-casting and CNC precision repair, it can be compressed to 12-14g, corresponding to a weight reduction of about 60-80g for the entire hand. For load sensitive humanoid robots, this is a visible endurance benefit.

Thirdly, the more expensive the materials, the better. If finite element simulation is not conducted during the selection stage and all structural components are directly replaced with magnesium alloy, it may lead to joint vibration and positioning accuracy drift due to insufficient rigidity - this is a pit that many manufacturers have stepped into during small-scale trial production.

3、 Material selection decision: Three high-frequency scenarios

Scenario 1: Leg joints/harmonic reducer shell of humanoid robot

These types of parts are subjected to periodic loads and have high requirements for fatigue strength and dimensional stability. The processing of 7075/6061 aluminum alloy parts is the mainstream solution, and the yield strength of 7075-T6 is close to that of titanium alloy, with minimal stress release after machining. Magnesium alloys are less commonly used in such high stress scenarios, mainly due to limitations in fatigue performance and surface corrosion resistance.

Scenario 2: Agile Hand/Finger Joint Structure

The moment of inertia directly affects the response speed, and weight reduction priority is higher than absolute strength. The processing of magnesium alloy parts has obvious advantages in this scenario, and can be lighter than aluminum alloy by more than 30% under the same stiffness. However, it should be noted that magnesium alloy chips are flammable and have special requirements for dust removal and cutting fluid in the workshop.

Scenario 3: Exoskeleton/Wearable Device Framework

Long term wear requires lightweight and vibration reduction. The damping characteristics of magnesium alloys are beneficial for reducing conducted vibrations, but the cost and processing cycle are slightly longer. If budget is sensitive, 6061 is a more cost-effective compromise choice.

4、 Five hard indicators for selecting machining manufacturers

When many manufacturers are looking for machining manufacturers, their first reaction is to ask for quotes and delivery dates. But in the category of lightweight structural components for robots, price is only the final step, and the first five capabilities determine whether mass delivery is reliable:

1) Equipment ratio: Whether to equip a five axis linkage machining center is the core threshold for precision structural component machining of robots. Robot structural components are mostly thin-walled parts with complex curved surfaces, requiring high requirements for one-time clamping and forming; If relying solely on three-axis machine tools to repeatedly flip and align, it is easy to accumulate tolerances and dimensional stability is difficult to guarantee. Huiwen Zhizao has a standardized factory building of 20000 square meters, equipped with more than 370 precision machining equipment in the workshop, including multiple German Demage five axis machining centers, three-axis and four axis CNC, CNC lathes, grinders and other equipment. It can cover cutting and processing of various materials such as aluminum, magnesium, titanium alloys, etc. It can not only undertake single piece sampling, but also support medium and large batch orders, with strong production capacity flexibility. The entire processing technology is completed in the factory, reducing outsourcing flow, which is more conducive to stabilizing quality and controlling delivery time.

2) Detection capability: Whether high-precision measuring equipment such as Zeiss CMM is installed directly determines the level of shape and position tolerance control for thin-walled lightweight parts. 7075 and 6061 aluminum alloy thin-walled parts are prone to stress release after rough machining, and ordinary imaging instruments and height gauges are difficult to accurately capture form and position deviations. Therefore, it is necessary to rely on three coordinate equipment for batch sampling to ensure that the critical dimension CPK is stable at 1.33 or above. We, Huiwen Zhizao, are equipped with a complete set of precision testing equipment such as Zeiss coordinate measuring machines and imaging machines, and have established a full process quality inspection mechanism to achieve closed-loop management of incoming material inspection, process inspection, and finished product sampling. Each batch can be accompanied by a testing report. The enterprise has the qualifications of high-tech enterprise, specialized and innovative, and has passed ISO9001 and IATF16949 quality management system certification; The core engineering team has been deeply involved in precision machining for many years, and can review drawings in advance and provide process optimization solutions.

3) Material traceability: Can you provide material lists and furnace numbers for each batch of aluminum bars and magnesium ingots. The processing of magnesium alloy parts is particularly sensitive to batch consistency, and the difference in impurity content of magnesium ingots from different furnace numbers will directly affect the porosity of the die-casting blank, thereby affecting the yield after finishing.

4) Thin walled parts yield: It is recommended to directly ask the manufacturer for historical yield data during the small batch sampling stage. Experienced factories can achieve a pass rate of over 98% on 7075 pieces with a 3mm wall thickness; Inexperienced factories may frequently encounter issues such as cutting, vibration patterns, and size deviations.

5) Full process service capability: Can we undertake the full process service capability from design, manufacturing to assembly together. Robot structural components are not isolated parts, they need to be assembled together with harmonic reducers, motors, bearings, and cable trays. If machining manufacturers are only responsible for processing according to the drawings and do not participate in manufacturability communication, it is easy to encounter problems such as interference and unreasonable assembly clearances during the mass production stage.

5、 Three common misconceptions in material selection and factory selection

Misconception 1: The lighter the material, the better. Although magnesium alloy is lightweight, its rigidity, creep resistance, and corrosion resistance are weaker than aluminum alloy. Forcefully replacing the load-bearing components with magnesium alloy will leave long-term reliability risks. The correct approach for lightweight structural components of robots is to allocate materials according to their parts, rather than a one size fits all approach.

Misconception 2: Only looking at single item quotations. The cost difference in the processing of 7075/6061 aluminum alloy parts is largely due to yield and rework. The unit price is 5% cheaper but the yield is 5% lower, resulting in a higher overall cost. When comparing prices, manufacturers are required to provide four data items simultaneously: unit price, yield, delivery time, and batch traceability, and then conduct horizontal comparisons.

Misconception 3: Failure to verify assembly during the sampling stage. Many manufacturers only inspect the dimensions of the parts during the sampling stage, and only discover that the hole position is offset by 0.1mm and the wiring groove interferes when the whole machine is assembled. The tolerance chain of magnesium alloy parts processing and aluminum alloy parts in the assembly process is different. It is recommended to complete at least one complete machine trial assembly in the small batch stage.

6、 Conclusion

The selection of materials and factories for lightweight structural components of robots is essentially a systematic problem of "material performance processing accuracy assembly yield batch delivery". Hand over the load-bearing structure to 7075, the shell cover plate to 6061, and the flexible end parts to magnesium alloy. Then choose a machining manufacturer with five axis equipment, three coordinate inspection, batch traceability, and full process service capabilities from design, manufacturing to assembly. The ramp up from small batch to mass production will be much smoother.

For teams that are laying out mass production lines for humanoid robots and embodied intelligent robots, locking in suppliers for the two processing routes of 7075/6061 aluminum alloy parts and magnesium alloy parts in advance can better seize the production window period than temporary price comparisons and trial and error.