Foreword The bearing shell is a key part in the engine. Due to the high engine speed and heavy load, it requires a higher binding force for the coating.
Aluminum has high chemical activity and the surface is easily oxidized. Even if the surface oxide layer is removed, a new oxide film will be formed in a short time. Therefore, the aluminum alloy bushing plating is more difficult than the copper lead alloy bushing.
Plating process flow:
Degreasing→Pickling→Zinc Leaching→Nickel Plating→Pewter Tin Plating Aluminum alloy bearing pads are prone to defects in coating adhesion. The authors discuss the typical failures and solutions of poor plating adhesion encountered by aluminum alloy bushings and their peers.
1 Fault cases 1.1 Bad pre-treatment 1.1.1 Failure phenomena (1) The surface of the coating is blistering. After the binding force is tested, the blistering part of the coating is peeled off to expose the aluminum alloy substrate.
(2) The surface of the coating is smooth, but after the bond strength test, the coating is partially peeled off to expose the aluminum alloy substrate.
1.1.2 Causes and Solutions There are generally defects in poor adhesion of the plating, and the causes are all pre-treatment. When the concentration of caustic washing and pickling solution is not within the range of the process, or if the temperature of the degreasing solution is lower than the range of the process, or the concentration of hydrogen peroxide in the pickling solution is insufficient, the adhesion of the plating layer will be poor. The solution should be analyzed periodically and added in a timely manner to ensure that the solution components are within the process.
1.2 Nickel plating is not timely after zinc immersion 1.2.1 Failure phenomenon When the pipeline alignment is not accurate after zinc immersion, the nickel anode is clamped to the fixture, so that the workpiece can not be successfully placed in time to nickel, resulting in a large area of ​​pewter tin coating pitting. After the bond strength test, the plating was all peeled off.
1.2.2 Causes and Solutions (1) Since the surface of the aluminum alloy is easily oxidized, the nickel layer is not easy to be plated. Therefore, zinc plating must be used as a bonding layer before nickel plating, and nickel plating should be performed within 1 min after zinc plating, otherwise The zinc-impregnated layer will be removed, resulting in a large area of ​​pewter tinned and affecting the bond strength of the coating.
(2) Immersion zinc solution is too thick, too dilute or inappropriate proportion of components; or improper process conditions, zinc immersion layer uneven or crystal thick, too thick; or zinc plating not timely plating, will cause poor nickel-plating bonding.
The zinc dipping solution shall be adjusted or replaced in time, and the operation shall be strictly performed according to the process conditions. Nickel plating shall be carried out within 1 min after the zinc dipping.
1.3 Temperature Changes 1.3.1 Failure Symptoms From December to February of each year, the coating is prone to defects of poor adhesion. From June to September, the coating surface began to fluff. Microscopic bubbles can be seen on the surface of the coating through the microscope, but no peeling of the coating was found after the binding force test.
1.3.2 Causes and Solutions We discovered this phenomenon for two consecutive years. Check each process and found that when the temperature of the zinc dipping solution is lower than 15°C during the winter, the temperature of the zinc impregnation layer is not uniform and the summer temperature exceeds 35°C, and the zinc-impregnated layer is easily removed. Therefore, we added a temperature control device to the zinc bath to control the temperature of the zinc bath at 15 to 25°C. This problem was solved.
1.4 Poor pre-plating of tin-lead alloys 1.4.1 Failure phenomena After electroplating, the bond strength was tested and the partial plating was peeled off, but the nickel layer was well bonded to the substrate.
1.4.2 Causes and Solutions Before plating tin-lead alloys, they must be activated to make the nickel layer and lead-tin layer combine well. However, due to the presence of organic substances in the activation solution, the adhesion of the plating layer is deteriorated, and therefore it is required to treat it with activated carbon.
At the same time, in the case of electroplating of lead-tin alloy, the workpiece must be charged into the groove, otherwise, the combination of lead-tin layer and nickel layer will also appear poor.
2 Other failures 2.1 Traces of airflow on the surface of the coating 2.1.1 Symptoms After plating, a faint trace of air flow from the bottom to the top occurs on the surface of the coating.
2.1.2 Causes and Solutions (1) Contamination of organic substances in the bath requires continuous filtration of 1 to 2 shifts of activated carbon.
(2) After the machining process prior to electroplating, burrs, rough surfaces, and edge edges are left on the two planes and the mating plane of the bearing pad. Therefore, they should be inspected and removed before plating.
(3) In case of poor cleaning, it is necessary to extend the cleaning time or replace the cleaning water or check whether the air mixing in the cleaning tank is normal.
2.2 Faults caused by the quality of the bearing aluminum alloy material 2.2.1 Failure phenomena There are sporadic horizontal pinholes on the surface of the bearing plating after electroplating.
2.2.2 Causes and Solutions Due to the large amount of production, we believe that the nickel plating solution is caused by impurities. Therefore, the nickel plating solution is treated according to a conventional method. However, after two shifts were processed, horizontal pinholes still existed on the surface of the coating. Afterwards, we removed the defective bearing pad. After we removed the coating, we removed the 20 μm aluminum alloy layer and found that there were small pits on the surface of the aluminum alloy. We then tested the electroplating of the bearing bushes with pits. There were also obvious pits on the surface of the coating. Later we replaced the alloy material and solved this problem.
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