**8. References**


5. The results show that base metal is characterized by a lower fatigue threshold than weld metal, i.e. a 3 to 4 times higher crack growth rate. This means that crack will initiate more likely in base metal, and that it requires fewer cycles to reach the critical size. Contrary to a typical welded joint, a surface welded layer is the safest place for

6. Values of fatigue threshold and crack growth rates corespond to initiation and propagation energies in impact testing. In the case of fatigue treshold and crack initiation energy, good correlation was achieved. Sample 1 has higher crack initiation energy (20 J) and higher Kth (9.5 MPa m1/2) than sample 2 (12 J and Kth =8.9 MPa m1/2). On the contrary, buffer layer didn't show decrease of initiated crack growth rate, as this effect is obvious in the case of toughness, i.e. crack propagation energy. Since the constructions from high-carbon steel are used at low temperature, and bearing in mind the extended working time, in modern surface welding technologies, the use of buffer

7. Testing results of base metal and surface welded layer represent typical behavior of two steel microstructure-pearlitic and bainitic, what is confirmed through microstructural investigation. It has been showned that, thanks to appropriate choice of filler material and welding technology, surface welding of damaged parts is not only a way of

The research was performed in the frame of the national project TR 35024 financed by

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*and Fracture of Engineering Materials and Structures,* Vol.26, No.10, 865-887, ISSN

research, *Proceedings of the 4th International Conference of contact mechanics and wear of* 

reparation, but and a way of improvement of starting properties

be kept in mind during design and exploitation.

crack initiation.

layer is not recommended.

Ministry of Science of the Republic of Serbia.

*International*., No.1, 25-28, ISSN 0373-5346

**7. Acknowledgement** 

**8. References** 

2001.

ISSN 0924-0136

8756-758X

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*rail/wheel systems*, Vancouver;1994

may be the reason for the absence of significant decrease of toughness and that should


**Study on Thixotropic Plastic Forming** 

*Department of Materials Processing Engineering, Nanchang University,* 

Magnesium alloys have a lot of advantages in mechanical and physical properties such as lightness, high specific strength, good thermal conductivity and damping. They are widely developed for automobile, spaceflight, electron, light instrument and so on. The damping capacity of materials plays a critical role in regulating the vibration of structure, decreasing the noise pollution and improving the fatigue properties of workpiece under circulating loading. Magnesium and its alloy have higher thermodynamically stability and aging stability as well as better damping capacity, whose applications are limited because of their poor mechanical properties. Creep is an important characteristic of mechanics behavior of metal at high temperature, which is a phenomenon for plastic deformation taken place slowly on the condition of constant temperature of long time and constant load. For the industrial application fields such as automobile industry and aviation industry, the creep is an important index to measure the good or bad property of a material at high temperature. The strength and creep resistance of magnesium alloys (AZ91D and AM60 alloys) are rapid decreased when the temperature is beyond 150℃. There are the disadvantages such as poor strength and toughness and poor creep resistance in magnesium alloy applied process, which limits its farther application. So it is an important to develop the magnesium matrix composites (MMCs) of high strength and toughness and good creep resistance and its forming technology. Specially, particle-reinforced magnesium matrix composites are characterized by low cost and simple process, which is a research focus of MMCs fields [Hai et al., 2004]. However, magnesium possesses low melting point, high chemical activity and ease of flammability, so preparing magnesium matrix composites is difficult in some extent. As a result, it is important to seek a better fabrication method for magnesium matrix composites [Zhou et al., 1997]. Powder metallurgy (PM) and casting are common methods for obtaining these composite materials. PM process needs complex equipments with higher expense, and can't fabricate large sized and complicated MMCs components. It has hazards such as powder burning and exploding. In contrast, casting method can produce large sized composites (up to 500kg) in industry at mass production levels with its simple process and convenient operation because of few investing equipment and low cost. So MMCs fabricated

by casting process are now investigated by many researchers [Kang et al., 1999].

The plastic formability of MMCs is poor, which need to introduce an advanced forming method. With the growing development of semi-solid forming technology, the thixoforming

**1. Introduction** 

**of Magnesium Matrix Composites** 

Hong Yan

*China* 

Zerbst, U., Madler, K., Hintze, H., (2005). Fracture mechanics in railway applications-an overview, *Engineering Fracture Mechanics,* Vol.72, No.2, 163-194, ISSN 0013-7944 **11** 
