A stress control method for plasma surfacing of ultra-high hardness materials

Abstract

The application discloses a stress control method for plasma surfacing of ultrahigh-hardness material, belongs to the field of welding additive manufacturing, and can solve the stress cracking problem that is prone to occurring in the surfacing process of the existing high-hardness material, and specifically relates to a self-heat-supply device for surfacing of rod shaft parts and a surfacing process control strategy, wherein the self-heat-supply device is distributed on both sides of the surfacing area, and heat transfer is used to control the solidification temperature of the arc-collecting section of the weld layer, so that the temperature of the arc-collecting section of the weld layer is homogenized and then slowly cooled; and the surfacing process control strategy mainly comprises the following steps: 1. cleaning, cleaning by using a cleaning agent, the cleaning area is 60mm outward from the surfacing position, and the cleaning agent cannot contain elements that can cause alloy embrittlement; 2. preheating temperature before welding is at least 350 DEG C or above; and 3. temperature control between layers is controlled in the range of 350 DEG C to 550 DEG C.

Description

Technical Field

[0001] This invention proposes a stress control method for plasma cladding of ultra-high hardness materials, belonging to the field of welding additive manufacturing. Specifically, it includes a self-heating device combined with a corresponding process control strategy, which is applicable to the plasma cladding process of various nickel-based high-hardness alloy powder materials with hardness exceeding 55HRC. Background Technology

[0002] Currently, plasma cladding technology for nickel-based high-hardness alloy powder materials is a commonly used method for designing localized performance gradients in rod and shaft parts. To meet the requirements of these parts, multi-layered heterogeneous structure plasma cladding is typically required. This involves melting metal powder at high temperatures using a plasma beam and depositing it layer by layer to form the final solid product. Generally, it is difficult to achieve the required dilution rate and powder melting quality during the plasma cladding process of high-hardness powder materials. Furthermore, the high alloying state of these materials results in significant thermal and grain boundary stresses during the melting and solidification processes, leading to a high scrap rate, primarily manifested as post-weld stress cracking. This stress cracking phenomenon is closely related to the stress state during the cladding process. In the plasma cladding process of high-hardness powder materials, the first layer of deposition is the fusion of the welding material and the base material. The subsequent cladding process is the fusion between the welding material, the base material and the previous layer of deposited metal, which makes the entire component present a complex stress state. In particular, there is a large temperature difference when the last layer is extinguished. Different materials generate uneven thermal shrinkage stress with large differences under huge temperature differences, which leads to direct cracking after welding or cracking during heat treatment / thermal shock test. The tendency of this cracking is very high. Summary of the Invention

[0003] To address the stress cracking problem in plasma welding of high-hardness powder materials described in the background art, this invention provides a stress control method for plasma welding of ultra-high hardness materials. This method effectively overcomes the post-weld stress cracking problem that cannot be solved by adjusting existing process parameters in plasma welding of high-hardness powder materials, enabling high-quality plasma welding manufacturing of high-hardness rod and shaft parts. Specifically, it includes a self-heating device and a welding process control strategy.

[0004] The self-heating device uses a square-section coil with rounded corners to form a closed loop when energized, achieving temperature uniformity on both sides of the welding area through heat transfer from both sides. It also controls the welding stress during the solidification process at the end of the arc. This device can rapidly raise the temperature to the set temperature. The plasma-welded workpiece is fixed on a chuck fixture, which is in turn fixed on a turntable. The turntable is driven by gears and has a displacement encoder. Heating is started and stopped based on different displacement positions, and the temperature is controlled during the heating process.

[0005] The square cross-section coil with rounded corners is made of conductive material, preferably copper. An insulating protective sleeve is added to the surface of the coil to avoid mutual interference between current loops. The coil has a cooling device inside to prevent coil oxidation, burn-through and other faults.

[0006] The self-heating device controls the solidification temperature of the weld layer at the end of the arc by heat transfer, so that the temperature is uniform and then cooled slowly to avoid the generation of gradient shrinkage stress caused by rapid temperature changes.

[0007] The aforementioned welding process control strategy, which controls the manufacturing process of rod and shaft parts within the most suitable process range for existing materials, mainly includes: 1. Pre-welding cleaning: using a cleaning agent to clean the area 60mm outward from the welding area, and the cleaning agent must not contain elements that cause alloy embrittlement; 2. Preheating temperature before welding: at least 350℃; 3. Interpass temperature control: 350-550℃.

[0008] This invention is applicable to plasma cladding of various nickel-based high-hardness alloy powders with a hardness of 55 HRC or higher, including Colmonoy 56 and Colmonoy 6. Different coil sizes can be designed according to different weld groove widths, resulting in more uniform heating and better control of stress generated during the cladding process. This invention uses medium-frequency coil heating, allowing for rapid control of heating temperature and rate. It is convenient, portable, replaceable, low-cost, and highly efficient, providing excellent stress control during the cladding process of high-hardness materials. Attached Figure Description

[0009] Figure 1 Schematic diagram of self-heating device

[0010] Figure 2 Schematic diagram of the coil location of the self-heating device (top view)

[0011] Figure 3 Pre-welding workpiece dimension drawing

[0012] Figure 4 Schematic diagram of cracked workpiece Detailed Implementation

[0013] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0014] refer to Figure 1The self-heating device in this specific embodiment is described below. First, the workpiece 1 is fixed by a fixture. The fixture 6 is fixed on the turntable 7 by clearance fit. The turntable 7 has a displacement encoder 8. The welding torch starts working under the existing welding process parameters. The welding torch 9 does not rotate. Welding is performed at the weld groove 3 of the workpiece by rotating the turntable 7. The square cross-section coil 2 and coil 4 with rounded corners are independently controlled and form a closed loop when energized. The temperature is uniformly treated on both sides of the weld overlay area by heat transfer from both sides. The weld overlay stress is controlled during the solidification process of the arc end. The square cross-section coil with rounded corners has a cooling channel 5.

[0015] The specific details of the workpiece and the implementation process are described below. This implementation case uses a typical rod and shaft part, namely a marine diesel engine valve, as an example.

[0016] The workpiece base material is 45Cr9Si3, the welding material is Colmonoy 56, the workpiece weld groove width is 16.6mm, the disc diameter is φ118mm, the disc circumference is 118π, and a square cross-section coil with a side length of 8mm is selected according to the weld groove width.

[0017] Before welding, it is necessary to control the influencing factors of the process, specifically including: 1. Cleaning: Use a cleaning agent to clean the workpiece area to be welded and its surroundings. The cleaning area should start from the weld overlay area and extend outwards by 60mm. The cleaning agent should not contain elements that can cause alloy embrittlement; 2. Preheating temperature before welding: 350℃. During the welding process, the interpass temperature of the weld overlay should be controlled, and the interpass temperature should be controlled between 350-550℃.

[0018] This implementation case compares the stress-control self-heating device and control method of plasma cladding with traditional cladding parameter loading schemes to control stress cracking. The same number of layers were clad under the same process parameters, including current, welding torch amplitude, turntable speed, feed rate, and shielding gas. After the same processing, a thermal shock test was conducted (i.e., the workpiece was placed in a heating furnace at 60°C, heated to 320°C within 4 hours, held at 320°C for 1 hour, and then rapidly immersed in 20-30°C water, followed by air cooling to room temperature after 5-8 seconds). High-hardness welding materials showed [further characteristics] after the thermal shock test following plasma cladding. Figure 4 The valve surface cracking problem shown is very high, and the pass rate of traditional processes is usually less than 50%. The main purpose of this invention is to improve the pass rate of products after welding to more than 85%.

[0019] In this implementation case, the self-heating device is set to a temperature of 850℃, a time of 3 seconds to rapidly rise to 850℃, and a setting to stop heating 3 seconds after arc termination.

[0020] The heating device achieves temperature uniformity on both sides of the weld overlay area through heat transfer from both sides, and controls the weld overlay stress during the solidification process of the arc-end segment.

[0021] When the last layer of welding material is deposited and the welding reaches half the circumference of the valve disc, the displacement encoder on the turntable sends a signal to automatically start the heating device, which heats the material to the set temperature within 3 seconds.

[0022] When the last layer of welding material is deposited and the welding reaches 3 / 4 of the circumference of the rod-shaped part, the temperature of the heating device is gradually and steplessly reduced to 500-550℃, and the heating device stops heating 3 seconds after the arc is extinguished.

Claims

1. A stress control method for plasma cladding of ultra-high hardness materials, used to solve the stress cracking problem that easily occurs in the existing high-hardness material cladding process, characterized in that... The invention includes a self-heating device for welding rod and shaft parts and a welding process control strategy. The self-heating device consists of a square cross-section coil with rounded corners forming a closed loop when energized. It achieves temperature homogenization and arc-end solidification process control on both sides of the welding area through heat transfer. The welding process control strategy includes clean area control, preheating temperature control, and interpass temperature control. The self-heating device, in which the plasma-deposited workpiece is fixed on a chuck fixture, which in turn is fixed on a turntable. The turntable is driven by gears and has a displacement encoder; the square coil with rounded corners has a side length H = 0.

4. 0.5h, where h is the width of the weld groove; The three main steps of welding process control are:

1. Cleaning: Use a cleaning agent to clean the area 60mm outward from the weld overlay. The cleaning agent must not contain elements that cause alloy embrittlement.

2. The preheating temperature before welding must be at least 350℃; 3. The interpass temperature must be controlled at 350℃. 550℃; when the final layer of welding reaches 1 / 2 of the circumference of the rod / shaft part, the displacement encoder on the turntable receives a signal and automatically starts the heating device, which needs to heat to the set temperature within 5 seconds; when the final layer of welding reaches 3 / 4 of the circumference of the rod / shaft part, the temperature of the heating device gradually and steplessly decreases to 500℃. At 550℃, the heating device stops heating 3 seconds after the arc ends.

2. The stress control method for plasma cladding of ultra-high hardness materials according to claim 1, characterized in that... The parameters of the heating device are set according to the melting point of the welding material and determined by the formula Ts=ζTf, where Ts is the set temperature, Tf is the melting point temperature, and ζ is a coefficient, ζ∈[0.8,0.9]. The high-hardness alloys applicable to the heating device include nickel-based high-hardness alloys with a hardness of 55HRC or higher, such as Colmonoy 56 and Colmonoy 6.

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