Shock absorbing material

Abstract

A shock absorbing material comprising an Al-Mg-Si series aluminum alloy having high strength and showing excellent energy absorbing property when compressed in the axial direction of extrusion is obtained. The shock absorbing material of the invention has a hollow cross section, mainly comprises a fibrous structure and can be manufactured by press quenching just after extrusion followed by aging. In the press quenching, press quenching under air-cooling advantageous in view of the dimensional accuracy or the cost can be adopted. Further, the shock absorbing material of the invention has excellent cracking resistance to a compressive load in the lateral direction as well as in the axial direction. The shock absorbing material of the invention is suitable as side members or bumper stays in the frame structures of automobiles.

Description

[0001] 1. Field of the Invention[0002] The present invention concerns a shock absorbing material comprising an Al--Mg--Si series aluminum alloy extrusion material and having a function of absorbing impact loads when undergoing compressive impact loads. The shock absorbing material according to the present invention is suitable, for example, to side members, bumper stays or side frames in frame structures of automobiles.[0003] 2. Related Art[0004] In the frame structures for automobiles, it has been considered the use of aluminum alloy hollow extrusion materials in view of reduction of weight as shock absorbing materials such as side members or bumper stays. The shock absorbing materials undergoing axial compressive impact loads are required not to suffer from Eulerian buckling for the entire shape material (buckling in which the entire shape material is bent in an angled form) when undergoing loads in the axial direction of extrusion, to cause shrinking deformation in a bellows form...

AI Technical Summary

Benefits of technology

[0012] In view of the above, it is a second object of the present invention to provide an Al--Mg--Si series aluminum alloy extrusion material having high strength (yield stress) with excellent cracking resistance in the axial direction and energy absorbing property and, at the same time, excellent cracking resistance also in the lateral direction.

[0013] The present inventors have found an alloy composition and a structure having high strength (yield stress) and showing excellent energy characteristics when compressed in the axial direction of extrusion (showing high energy absorption with no occurrence of cracks) for the shock absorbing material of an Al--Mg--Si series aluminum alloy based on the premise of adopting press quenching under air cooling, which is advantageous in view of the dimensional accuracy and the cost.

[0017] On the contrary, the present inventors have found that Al--Mg--Si series aluminum alloy extrusion materials tend to cause cracks if the macrostructure thereof mainly comprises a recrystallization structure but occurrence of such cracks can be suppressed even at a high strength if it mainly comprises a fibrous structure (in a state where the fibrous structure after extrusion is left as it is with no recrystallization also during a heat treatment step subsequent to the extrusion step). On the other hand, it has also been found that the fibrous structure is loosened when the solution quenching treatment is applied in off-line and that formation of a surface recrystallization layer to a certain thickness is advantageous in order to obtain required characteristics by on-line press quenching.

Problems solved by technology

However, this results in a difference in the cooling rate along the cross section due to the difference of the cross sectional shape or the wall thickness of the extrusion materials, and makes the temperature distribution not uniform during cooling to cause distortion.

Accordingly, it results in problems that the accuracy is poor, thickness for the cross sectional shape is difficult to be reduced and the degree of freedom for the cross sectional shape is restricted if it is intended to prevent occurrence of such distortion.

There is also an additional problem that it requires higher cost compared with that in air-cooling.

However, since the cooling rate is limited, it results in a problem that no high strength (particularly, yield stress) can be obtained depending on the alloy compositions and, even when a high strength can be obtained, the energy absorption or the cracking resistance is poor.

Further, Al--Mg--Si series aluminum alloy extrusion materials involve a worry that spontaneous aging progresses to deteriorate the cracking resistance when exposed at a high temperature during use thereof, for example, as side members.

However, when strength is improved by T5, T6 treatment, it involves a problem of causing cracks when subjected to axial compressive deformation.

If cracks should occur, shrinking deformation in the bellows-form is inhibited failing to obtain stable energy absorption.

Further, the quenching sensitivity is generally increased as the amount of the additive elements increases and the amount of the precipitates at the boundaries also increases at a low cooling rate (for example, in air cooled press quenching which is advantageous in view of the dimensional accuracy or the cost), tending to cause cracks when crushed under large impact loads.

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